Discussing audio amplifier design -- BJT, discrete

[Paul E. Schoen]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:4ch8m516qf4n46fqm1uefefcc48hpb1i90@4ax.com...

Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

If I were designing an audio amp nowadays (which I certainly aren't)
I'd use mosfets with an opamp gate driver per fet. That turns the fets
into almost-perfect, temperature-independent, absolutely identical
gain elements. That's what I do in my MRI gradient amps, whose noise
and distortion are measured in PPMs.

ftp://jjlarkin.lmi.net/Amp.jpg

Why keep repeating a 50-year-old topology when you could have a little
fun?

Nice photo. But how about a schematic to show how you implemented the
design? I made a simulation of an amplifier design where I used a MOSFET
output stage similar to my other post, and I closed the loop with a single
op-amp and appropriate negative feedback.

I think the OP is looking for a basic learning experience using the
simplest components. It may be argued that a MOSFET is simpler than a BJT,
but experience with both is a good idea. If the object were just to make an
audio amp, there are single package designs and kits that will do the job
nicely.

I prefer making the circuit using LTSpice, but it can be a thrill to build
something with real components. There are usually some gotchas that cause
unwanted behavior not indicated in the simulation. But I need a real reason
to build something, other than practice with soldering and handling
components, so I rarely commit these designs to copper and silicon.

Paul

 

[Jon Kirwan]

On Fri, 29 Jan 2010 22:00:03 +0530, "pimpom"
<pimpom@invalid.invalid> wrote:

Jon Kirwan wrote:
On Thu, 28 Jan 2010 19:19:06 +0530, "pimpom"
pimpom@invalid.invalid> wrote:


Q2 needs about +/-50uA peak of base
current at full drive. At signal frequencies, R2 (plus the
much
smaller input impedance of Q1) is effectively in parallel with
the output.

R2 is connected from the output to an input, which
effectively doesn't move much after arriving at it's DC bias
point. As you later point out, the _AC_ input impedance is
lowish (near 600 ohms), so the 10k is pretty close to one of
the rails at AC, anyway. Is that a different way of saying
what you just said? Or would you modify it?

That's another way of putting it, yes.

Okay. Thanks.

The output swings by about 4V peak at max power,
which has 400uA of negative feedback current going back
through
R2. The input current requirement goes up by a factor of 9.
IOW,
a negative feedback of 19db. This is substantially better than
nothing and should significantly reduce distortion and improve
frequency response.

Okay. This goes past me a little (as if maybe the earlier
point didn't.) I'd like to try and get a handle on it.

Let's start with the 4V peak swing at max power.

Since you are discussing AC and converting it 400uA current
via the 10k, I would normally take this to mean 4Vrms AC.
Which in Vp-p terms would be 2*SQRT(2) larger, or 11.3V which
I know is impossible without accounting for the BJTs, given
the 9V supply. So this forces me to think in terms of
something else. But what? Did you mean 4Vpeak, which would
be 8Vp-p? If so, that would be about 2.8Vrms.

Yes. It's 4Vp, 8Vp-p and 2.8Vrms. I wanted to give you a mental
picture of how much the output voltage can swing. Each output Q
has about 4.4V of Vce available, and about 4V before hard
saturation is reached (these are all round figure values). That's
4V peak for a sinusoidal wave form.

Got it.

In that case,
wouldn't a better "understanding" come from then saying that
the negative feedback is closer to 280uA?

Yes, it's 280uA rms. But I was talking in terms of the maximum
amplitude of instantaneous change, which is why I used the terms
"swing" and "peak".

Understood.

The next point is on your use of "goes up by a factor of 9."
Can you elaborate more on this topic? Where the 9 comes
from? For volts, not power, I think I can gather the point
that 20*log(9) = 19.085), so I'm not talking about that
conventional formula. I'm asking about the 9, itself, and

Without feedback, the input transistor Q1 needs 50uA of AC input
signal to drive the output Qs to full power output (still talking
in terms of peak to avoid confusion). With NFB, we need an
additional 400uA to overcome the current fed back from the
output. That's a total of 450uA peak, which is 9 times the
original 50uA.

Okay. Let me put it in my own words. Since you chose to
stick a 10k in for the NFB, and since it supplies a peak of
400uA into the input node and since Q1 itself requires its
own 50uA (corrected below), then the signal source itself
must "comply" by supplying 450uA peak into the input node.
And that is where you got your 9. If that is it, I've got
it.

Actually, I made an error when I cited the 50uA figure. Q1 is
biased at Ic = 7.6mA, Ib = 50uA. But only 5mA peak is needed from
Q1's collector to drive the output transistors. Divide that by
Q1's hfe of 150 and you get 33uA (peak) of AC signal current
needed into the base of Q1. The corrected total needed from the
signal source is now 433uA. The gain reduction factor due to NFB
is now 13 instead of 9. That's 22db (feedback is usually given in
db).

Okay. I can compute the numbers. I think the problem I'm
having with this, as a mental concept, is that the feedback
is _from_ the complementary BJT outputs at their emitters
_backwards_ into the node at the base of Q1. Not outwards
from that node _towards_ the emitters.

Let me think a little about this from an AC point of view,
not DC. The output is supposedly running with about 4Vpeak
into an AC divider made up of what you've earlier described
as about 600 ohms to ground via Q1 and about 1k for R1. So
given that, we are talking about 1k from input to base node,
600 ohms from base node to ground, and 10k from 4V peak
output into base node. The 4V peak has the opposite sign as
the input because of the relationship of Q1's collector to
its base voltage. I'm just spouting things here without a
lot of understanding, so bear with me.

The change in Ic of Q1 for a change in the base voltage
(computing a transconductance of Q1) is 1 divided by re,
which you computed as 3.4 ohms earlier. So gm=294mSeimens,
assuming the emitter (small signal wise) follows the base
exactly. A 1mV input change at the base yields 1mV*294mS or
294uA change in the collector, ignoring the 100 ohm rbb you
earlier mentioned for now. Multiplied by the collector load
of about 566 ohms (your figure) produces 166mV change at the
collector. A voltage gain (base node to output) of -166.

This 166mV change is fed back via the 10k into an existing
divider composed of the 1k and the beta-multiplied 3.4 ohms
to ground (if I'm following you.) So with the rbb of 100,
about 600 ohms as you mentioned (still AC-minded.)

So back to the 1k from input, 600 to ground, 10k in from that
-166mV change in response to a +1mV change at the base node.
The 1k/600 divider means the input had to vary by about
2.66mV to achieve that node change. That means our voltage
gain wasn't really -166, but more like 62.5 -- audio input to
Q1 collector and then to output drive.

Am I going around the barn about right, so far? Here's what
worries me now. The postulated thevenin base node change of
1mV is through a thevenin of about 375 ohms (the 1k and 600
ohm splitter.) The 10k feeds into this from the other side
with -166mV there. If I imagine 1mV on one side via 375 ohms
and -166mV on the other side via 10k ohms, what is the node
itself at? Well, (1mV*10k+-166mv*375)/(10k+375) is -5mV or
so. This is a lot, isn't it? And it is more than enough to
oppose the postulated thevinin change at the input node of
+1mV.

So I follow the calculation of 22db. The problem I'm having
is with what kind of signal will result at Q1's collector.

Okay. Granted. I am sure my reasoning fails on some points
you will make clearer. I'm just trying to see this in a
variety of ways rather than just let you tell me stuff
without running through different thinking to see if I get to
the same place. So what did I do wrong here? I can't argue
with success and I know I'm not doing this right. But there
it is.

also your thinking along the lines of concluding that it
significantly reduces distortion.

The basic principle of NFB is that it reduces THD and extends
frequency response by a factor equal to the feedback ratio. So,
in our example, if you have 10% THD without feedback, it will
drop to 0.77% with the feedback factor of 13. But there are
caveats. E.g., phase shifts can cause undesireable effects,
especially with large amounts of feedback. I'm afraid a detailed
treatment of such things is really outside the scope of this
discussion - unless someone else is willing to take it up.

I can do phase shift calcs given simple cases and if I take
into account all the necessary parts (which, being ignorant
about all this, I'm unlikely to do without more thought
experiments to clarify my thinking.) So when I get to that
point where I can actually walk myself along better, I'll be
able to handle that (I hope.)

How does one decide how much is enough?

For one thing, how much distortion one is willing to put up with.
Another factor is input sensitivity, or IOW, how much gain is
needed. E.g., to drive the 1W amp to full output, we need 433uA
peak (306uA rms) from the signal source into 1k. That's 306mV
rms, plus some millivolts at the b-e junction. Say about 0.32V
rms total input voltage into about 1k input impedance.

To present the basic concepts, I've made several approximations.
E.g., I neglected the shunting effect of R2. Besides, the input
resistance of Q1 is constant at 600 ohms only for very small
signal amplitudes relative to the quiescent dc levels. This
dynamic input resistance changes significantly with large signal
swings and adds distortion while also complicating precise
calculations.

Okay. I'm going to leave things with the above "issue" laid
out for you. It's bugging me right now.

I am refusing, by the way, to attempt any simulation. I
don't want to be "told" something by a simulator or handed
things on some platter. I want to try and work through my
thinking, find the flaws, slap myself for them, get back and
try again, until I'm good from a paper-and-brain point of
view. _Then_ I'll go and check it out to see where the chips
fall in the simulator. Might bring up a good question at
that point. But until I can get the gross aspects down, that
won't really matter.

Thanks so much for your efforts so far. It's been helpful to
me, at least.

Jon

 

[Jon Kirwan]

On Fri, 29 Jan 2010 20:01:53 -0800, John Larkin wrote:

On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold
ggherold@gmail.com> wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

"Vbe multiplier."

Got it. Since that time, I've found "rubber diode" as
another term mentioned on wiki.

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

I've already expressed my concern about that.

Two alternates are:

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments. Bothe need negative feeback to kill
crossover distortion.

I need to get some basics down before I return to these. I'm
not there, yet. But I _do_ see an issue with the Vbe
multiplier if it isn't crafted carefully for the situation.

Or...

3. Use mosfets

No FETs.

Jon

 

[Jon Kirwan]

On Sat, 30 Jan 2010 06:52:01 -0800, John Larkin wrote:

On Sat, 30 Jan 2010 15:18:55 +1100, "Phil Allison" <phil_a@tpg.com.au
wrote:

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the output
stage.

Larkin has no idea how it is done - cos Larkin is bullshitting asshole.


1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

Zero bias can be done, but never so crudely as that.


2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them - SAE brand amps from the
late 1970s being one exception.


In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in use.


Both need negative feeback to kill crossover distortion.


** One thing that NFB is notoriously very poor at doing.



.... Phil


Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

So there's a need to refresh our minds about this. You
talked about FET designs, but before one can understand
whether or not they compare well to BJT designs it seems to
me that one needs to understand what can be done with BJTs
first. Just _stating_ (or making a premise based on what you
say is more history and authority that actual _best_ design
practices) that they would be better isn't enough, I suspect.

If I were designing an audio amp nowadays (which I certainly aren't)
I'd use mosfets with an opamp gate driver per fet. That turns the fets
into almost-perfect, temperature-independent, absolutely identical
gain elements. That's what I do in my MRI gradient amps, whose noise
and distortion are measured in PPMs.

Well, if I were opening the door to ICs there'd be no real
learning going on. An opamp doesn't teach one that much.
They are pretty close to ideal and what's learned by that?
Now designing one... that would be another case. But using
ICs with a FET tacked on the end teachs about as much about
deeper levels of analog design (getting closer to the physics
around us) as does using a Visual BASIC drop-down box teaches
about the Windows messaging layer. It's almost all hidden
from view in either case.

That doesn't mean a Visual BASIC drop-down isn't useful or
that people should make programs using them as a shortcut.
They should, and do. But if you want to know how to make
some new widget of your own... you may find yourself with
only one oar in the water... spinning in circles.

ftp://jjlarkin.lmi.net/Amp.jpg

Why keep repeating a 50-year-old topology when you could have a little
fun?

The fun for me is in digging closer into the physics. Just
as the programming fun comes from seeing how a c compiler
implements the resulting code on the machine instruction
level or how a coroutine may be implemented using a thunk.

Put another way, one can move from understanding one's own
backyard in two directions. (1) Towards seeing how plants
participate in the meadow or a woods and how those
participate within an Earth/air/ice/water/sun system (in
other words, reaching towards larger and larger abtraction
levels.) (2) Or else, delve deeper towards seeing how organs
function within the organism, how cells function within that,
how proteins work, how peptide chains are brought together
into those, how atoms work in making peptide chains, and so
on. In other words, there are two telescoping directions to
head.

With electronics, this can be towards higher abstraction
levels using ICs or towards smaller, more concrete levels
towards depletion regions and eventually QM events at the
particle interaction level.

Which is more interesting depends on your goals. Right now,
I want to focus on the BJT design level of abstraction. This
has nothing to do with making an amplifier. It has to do
with using an amplifier as an excuse to learn but also as a
well defined outcome that can then be measured and observed
using well-understood measurement criteria (and the ability
to experience the result as a basic, visceral thing to the
ear, too.)

Jon

 

[John Larkin]

On Sat, 30 Jan 2010 12:41:16 -0800, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Fri, 29 Jan 2010 20:01:53 -0800, John Larkin wrote:

On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold
ggherold@gmail.com> wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

"Vbe multiplier."

Got it. Since that time, I've found "rubber diode" as
another term mentioned on wiki.

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

I've already expressed my concern about that.

The basic tradeoff is to use big emitter resistors to prevent thermal
runaway, but that wastes power at large signal swings. Another
tradeoff is to use a small Vbe multiplier voltage (ie, small quiescent
DC drops across the emitter resistors) to reduce idle power and
heatsink temp at the cost of more crossover distortion.

Or change the rules. Semiconductors are cheap, heatsinks are
expensive.

Take a crack at calculating the thermal runaway situation of a typical
class AB output stage. It's interesting.

John

 

[Phil Allison]

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the
output
stage.

Larkin has no idea how it is done - cos Larkin is bullshitting asshole.


1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

Zero bias can be done, but never so crudely as that.


2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them - SAE brand amps from the
late 1970s being one exception.


In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in
use.


Both need negative feeback to kill crossover distortion.


** One thing that NFB is notoriously very poor at doing.



Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

** How the fuck would you know ?

Clearly you don't and just make things up to suit your wacky prejudices.

If I were designing an audio amp nowadays (which I certainly aren't)

** So shut the fuck up.

You clueless fucking wanker.


.... Phil

 

[John Larkin]

On Sun, 31 Jan 2010 09:54:01 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the
output
stage.

Larkin has no idea how it is done - cos Larkin is bullshitting asshole.


1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

Zero bias can be done, but never so crudely as that.


2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them - SAE brand amps from the
late 1970s being one exception.


In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in
use.


Both need negative feeback to kill crossover distortion.


** One thing that NFB is notoriously very poor at doing.



Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

** How the fuck would you know ?

Clearly you don't and just make things up to suit your wacky prejudices.


If I were designing an audio amp nowadays (which I certainly aren't)

** So shut the fuck up.

You clueless fucking wanker.


... Phil

I showed everybody an amp I designed, 17KW peak power out, a few PPM
noise and absolute analog accuracy.

Hey, Mr Audio, show us a power amp that you designed.

John

 

[Phil Allison]

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the
output
stage.

Larkin has no idea how it is done - cos Larkin is bullshitting
asshole.


1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

Zero bias can be done, but never so crudely as that.


2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely
any
effect.

Very few power amp designs have ever used them - SAE brand amps from
the
late 1970s being one exception.


In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in
use.


Both need negative feeback to kill crossover distortion.


** One thing that NFB is notoriously very poor at doing.



Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

** How the fuck would you know ?

Clearly you don't and just make things up to suit your wacky prejudices.


If I were designing an audio amp nowadays (which I certainly aren't)

** So shut the fuck up.

You clueless fucking wanker.


I showed everybody an amp I designed,

** FFS that monstrosity is NOT any kind of audio amp.

YOU have no experience with any aspect if the subject.

YOU are nothing but a LYING PIECE OF SHIT.

FUCK OFF



..... Phil

 

[John Larkin]

On Sun, 31 Jan 2010 10:18:58 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the
output
stage.

Larkin has no idea how it is done - cos Larkin is bullshitting
asshole.


1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

Zero bias can be done, but never so crudely as that.


2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely
any
effect.

Very few power amp designs have ever used them - SAE brand amps from
the
late 1970s being one exception.


In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in
use.


Both need negative feeback to kill crossover distortion.


** One thing that NFB is notoriously very poor at doing.



Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

** How the fuck would you know ?

Clearly you don't and just make things up to suit your wacky prejudices.


If I were designing an audio amp nowadays (which I certainly aren't)

** So shut the fuck up.

You clueless fucking wanker.


I showed everybody an amp I designed,


** FFS that monstrosity is NOT any kind of audio amp.

YOU have no experience with any aspect if the subject.

YOU are nothing but a LYING PIECE OF SHIT.

FUCK OFF



.... Phil

Nothing to show, we see.

How about something you were allowed to repair?

John

 

[George Herold]

On Jan 29, 3:11 pm, Jon Kirwan <j...@infinitefactors.org> wrote:

On Fri, 29 Jan 2010 09:19:31 -0800 (PST), George Herold

ggher...@gmail.com> wrote:
Hi Jon,  I'm enjoying your posts.

Thanks.  I feel like I'm way behind some curves, but it's fun
taking a moment to think about things and it is fantastic
that anyone else is willing to help talk about things with
me.  That is priceless.  So the real thanks go to those who
are sharing their knowledge and experience here.

What's a pin driver?

Hmm.  I think I first heard the idea when talking about
testing ICs, to be honest.  But imagine instead a micro with
software to test some discrete part (could be an IC, too,
that that's more complex.)  For example, to automatically
derive some modeling parameters for a BJT.

Take a look at this datasheet, for an example of the features
one might support:

http://www.analog.com/static/imported-files/Data_Sheets/AD53040.pdf

I made a nice
switchable current source (10nA to 1mA) from a voltage reference,
opamp and switchable resistors.  (circuit cribbed from AoE.)

I'd require at least one that can either sink _or_ source to
the pin.  And that would be only one of the pin driver's
required features.  I think the datasheet mentioned above
provides some more.  But that part is expensive and not
readily available to us hobbyist types and doesn't teach me
anything about various trade-offs I might want to make or how
to design it at all, besides.

Jon

Wow, that's some chip.
Thanks,
George H.

 

[George Herold]

On Jan 29, 11:01 pm, John Larkin
<jjSNIPlar...@highTHISlandtechnology.com> wrote:

On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold

ggher...@gmail.com> wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

"Vbe multiplier."

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors. Two alternates are:

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments. Bothe need negative feeback to kill
crossover distortion.

Or...

3. Use mosfets

John

Cool thanks John, I tend to only use transistors when I need more
poop on the output and always have an opamp in the loop.

George H.

 

[George Herold]

On Jan 30, 6:14 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 31 Jan 2010 09:54:01 +1100, "Phil Allison" <phi...@tpg.com.au
wrote:





"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the
output
stage.

Larkin has no idea how it is done  - cos Larkin is bullshitting asshole.

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

  Zero bias can be done, but never so crudely as that.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them -  SAE brand amps from the
late 1970s being one exception.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in
use.

Both need negative feeback to kill crossover distortion.

** One thing that NFB is notoriously very poor at doing.

Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

** How the fuck would you know  ?

Clearly you don't and just make things up to suit your wacky prejudices.

If I were designing an audio amp nowadays (which I certainly aren't)

** So shut the fuck up.

 You clueless fucking wanker.

...  Phil

I showed everybody an amp I designed, 17KW peak power out, a few PPM
noise and absolute analog accuracy.

Hey, Mr Audio, show us a power amp that you designed.

John- Hide quoted text -

- Show quoted text -

Hee Hee, OK I'm not real proud of it, but we've sold several hundred
of them so at least it's paid for my ~week(?) of design and testing
time.

http://www.teachspin.com/instruments/audio/index.shtml

Hey, at least it's called an audio amplifier. It's powered by a 15V /
1A switching supply. And uses two power opamps. One sets the ground
and the other does the work.
(Well both have to work when there is significant ground current.)

George H.

 

[John Larkin]

On Sat, 30 Jan 2010 22:25:55 -0800 (PST), George Herold
<ggherold@gmail.com> wrote:

On Jan 30, 6:14 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Sun, 31 Jan 2010 09:54:01 +1100, "Phil Allison" <phi...@tpg.com.au
wrote:





"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the
output
stage.

Larkin has no idea how it is done  - cos Larkin is bullshitting asshole.

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

  Zero bias can be done, but never so crudely as that.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them -  SAE brand amps from the
late 1970s being one exception.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in
use.

Both need negative feeback to kill crossover distortion.

** One thing that NFB is notoriously very poor at doing.

Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

** How the fuck would you know  ?

Clearly you don't and just make things up to suit your wacky prejudices.

If I were designing an audio amp nowadays (which I certainly aren't)

** So shut the fuck up.

 You clueless fucking wanker.

...  Phil

I showed everybody an amp I designed, 17KW peak power out, a few PPM
noise and absolute analog accuracy.

Hey, Mr Audio, show us a power amp that you designed.

John- Hide quoted text -

- Show quoted text -

Hee Hee, OK I'm not real proud of it, but we've sold several hundred
of them so at least it's paid for my ~week(?) of design and testing
time.

http://www.teachspin.com/instruments/audio/index.shtml

Hey, at least it's called an audio amplifier. It's powered by a 15V /
1A switching supply. And uses two power opamps. One sets the ground
and the other does the work.
(Well both have to work when there is significant ground current.)

George H.

Things like this, laser drivers and gradient amplifiers and such, are
more interesting than actual audio, whose performance criteria are
fuzzy things like absurd power specs, "soundstaging", "microdynamics",
and similar fuzzy blather. They sell for a lot more, too.

John

 

[John Larkin]

On Sat, 30 Jan 2010 21:57:11 -0800 (PST), George Herold
<ggherold@gmail.com> wrote:

On Jan 29, 11:01 pm, John Larkin
jjSNIPlar...@highTHISlandtechnology.com> wrote:
On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold

ggher...@gmail.com> wrote:

"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"

"Vbe multiplier."

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors. Two alternates are:

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments. Bothe need negative feeback to kill
crossover distortion.

Or...

3. Use mosfets

John

Cool thanks John, I tend to only use transistors when I need more
poop on the output and always have an opamp in the loop.

Exactly. Opamps make gain and precision cheap, so every power
transistor deserves one.

John

 

[John Larkin]

On Sat, 30 Jan 2010 21:31:40 -0800 (PST), George Herold
<ggherold@gmail.com> wrote:

On Jan 29, 3:11 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
On Fri, 29 Jan 2010 09:19:31 -0800 (PST), George Herold

ggher...@gmail.com> wrote:
Hi Jon,  I'm enjoying your posts.

Thanks.  I feel like I'm way behind some curves, but it's fun
taking a moment to think about things and it is fantastic
that anyone else is willing to help talk about things with
me.  That is priceless.  So the real thanks go to those who
are sharing their knowledge and experience here.

What's a pin driver?

Hmm.  I think I first heard the idea when talking about
testing ICs, to be honest.  But imagine instead a micro with
software to test some discrete part (could be an IC, too,
that that's more complex.)  For example, to automatically
derive some modeling parameters for a BJT.

Take a look at this datasheet, for an example of the features
one might support:

http://www.analog.com/static/imported-files/Data_Sheets/AD53040.pdf

I made a nice
switchable current source (10nA to 1mA) from a voltage reference,
opamp and switchable resistors.  (circuit cribbed from AoE.)

I'd require at least one that can either sink _or_ source to
the pin.  And that would be only one of the pin driver's
required features.  I think the datasheet mentioned above
provides some more.  But that part is expensive and not
readily available to us hobbyist types and doesn't teach me
anything about various trade-offs I might want to make or how
to design it at all, besides.

Jon

Wow, that's some chip.
Thanks,
George H.

But $25 at 100 pieces, and rather slow.

HFA1130 is interesting as a pin driver, run open-loop and slamming
into its voltage-set limits.

John

 

[David Eather]

On Jan 30, 9:06 am, Jon Kirwan <j...@infinitefactors.org> wrote:

On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather

eat...@tpg.com.au> wrote:
On Jan 28, 12:51 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
On Thu, 28 Jan 2010 11:17:02 +1000, David Eather

Sorry Jon,

I'm stuck on google groups for a little while - I can't believe people
actually use it full time or that google could make an interface this
bad. (I suspect it is very fine for simple threads) anyway...

Cripes.  Google didn't even show the thread when I'd looked,
a day ago or so.  And it had been around for at least 24
hours by then.  Used to be the case that google groups would
show the posts within an hour or so.  Doesn't seem to be
true, anymore.  If not, there is no possibility of having a
discussion very quickly via google.  It would greatly
lengthen out the interactions.  Maybe that's on purpose, now,
to cause people to find some other solution?

The "Google Delay" is my delay. In a couple of days it will be back to
normal

eat...@tpg.com.au> wrote:
Jon Kirwan wrote:
On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
eat...@tpg.com.au> wrote:
snip

My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
and this is, I think, I great reason to go down this design path (what
you learn is applicable in the most number of situations). I should hunt
down a schematics of what I'm seeing in the distance (which can/will
change as decisions are made) - some of the justifications will have to
wait

I'm fine with taking things as they come.

As far as the class, I guessed that at 10 watts class-A would
be too power-hungry and probably not worth its weight but
that class-AB might be okay.

I have to warn you, though, that I'm not focused upon some
20ppm THD.  I'd like to learn, not design something whose
distortion (or noise, for that matter) is around a bit on a
16-bit DAC or less.  I figure winding up close to class-B
operation in the end.  But I'd like to take the walk along
the way, so to speak.

10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
a few detours to see what would help or harm that.

Hehe.  I'm thinking of some numbers I saw in the area of
.002% THD.  I hate percentages and immediately convert them.
In this case, it is 20e-6 or 20 ppm.  Which is darned close
to a bit on a 16-bit dac.  That's why I wrote that way.  I
just don't like using % figures.  They annoy me just a tiny
bit.

Sorry.

Don't be.  I was just explaining myself, not complaining
about your usage.

Regarding .1% to .05%, I'm _very_ good with that.  Of course,
I'm going to have to learn about how to estimate it from
theory as well as measure it both via simulation before
construction and from actual testing afterwards.  More stuff
I might _think_ I have a feel for, but I'm sure I will
discover I don't as I get more into it.

A little experience will get you into the right ballpark when
estimating what you could expect for distortion. It is basically the
same "rules" as you would see with op-amps - the more linear it is to
start with the better. Higher bandwidth stages generally mean you can
use more negative feedback to eliminate distortion - but the lower the
final gain the more instability is likely to become a problem. And bad
circuit layout can increase distortion (and even more so hum and
noise) easily by a factor of 10.

As for how low you need distortion to be one rule of thumb (I forget
the reference) is to be clearly audible the message must be 20db above
the background noise and to be inaudible distortion has to be 20db
below the background noise - which pretty much sets "low" distortion
for PA and similar uses at 1% or 10000 ppm. For HiFi the "message" has
a high dynamic range and you (allegedly) want a distortion figure at
least 20db below that. So a 60 db signal range   0.0001% (or 100PPM).
The you start getting into all kinds of trouble with power output /
dynamic range of the amp etc and you relies that it is all a
compromise anyway. You do the best you can within the restrictions of
the job description.

Understood.

But speaking from ignorance, I'm good shooting for the range
you mentioned.  It was about what I had in mind, in fact,
figuring I could always learn as I go.

The first step is to think about the output. The basic equations are

(1).....Vout = sqrt(2*P*R)

With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
actually +/- 12.7 volts with a split power supply.

If you don't mind, I'd like to discuss this more closely. Not
just have it tossed out.  So, P=V*I; or P=Vrms^2/R with AC.
Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
equation.  Which suggests the +/-12.7V swing.  Which further
suggests, taking Vce drops and any small amounts emitter
resistor drops into account, something along the lines of +/-
14-15V rails?

Or should the rails be cut a lot closer to the edge here to
improve efficiency.  What bothers me is saturation as Vce on
the final output BJTs goes well below 1V each and beta goes
away, as well, rapidly soaking up remaining drive compliance.

(2).....Imax = sqrt(2*P/R)

This comes out to 1.6 amps. You should probably also consider the case
when R speaker = 4 ohms when initially selecting a transistor for the
output 2.2 amps - remember this is max output current. The power supply
voltage will have to be somewhat higher than Vout to take into account
circuit drive requirements, ripple on the power supply and transformer
regulation etc.

Okay.  I missed reading this when writing the above.  Rather
than correct myself, I'll leave my thinking in place.

So yes, the rails will need to be a bit higher.  Agreed.  On
this subject, I'm curious about the need to _isolate_, just a
little, the rails used by the input stage vs the output stage
rails.  I'm thinking an RC (or LC for another pole?) for
isolation.  But I honestly don't know if that's helpful, or
not.

Mostly not needed, if you use a long tailed pair for the input / error
amplifier, but you might prefer some other arrangement so keep it in
mind if your circuit "motorboats"

Okay.  I've _zero_ experience for audio.  It just crossed my
mind from other cases.  I isolate the analog supply from the
digital -- sometimes with as many as four caps and three
inductor beads.  There, it _does_ help.

Are you OK with connecting mains to a transformer? or would you rather
use an AC plug pack (10 watts is about the biggest amp a plugpack can be
used for)? The "cost" for using an AC plug pack is you will need larger
filter capacitors.

I'd much prefer to __avoid__ using someone else's "pack" for
the supply.  All discrete parts should be on the table, so to
speak, in plain view.  And I don't imagine _any_ conceptual
difficulties for this portion of the design.  I'm reasonably
familiar with transformers, rectifiers, ripple calculations,
and how to consider peak charging currents vs averge load
currents as they relate to the phase angles available for
charging the caps.  So on this part, I may need less help
than elsewhere.  In other words, I'm somewhat comfortable
here.

Ah, then there are questions of what voltage and VA for a transformer..
So there are questions of usage (music, PA, PA with an emergency alert
siren tied in etc) and rectifier arrangement and capacitor size /
voltage to get your required voltage output at full load.

I figure on working out the design of the amplifier and then
going back, once that is determined and hashed out, with the
actual required figures for the power supply and design that
part as the near-end of the process.  Earlier on, I'd expect
to have some rough idea of how "bad" it needs to be -- if the
initial guesses don't raise alarms, then I wouldn't dig into
the power supply design until later on.  The amplifier, it
seems to me, dictates the parameters.  So that comes later,
doesn't it?

Yes and No. All the published circuits are made by people who want to
sell transistors,

A concern I care not the least about.  My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices.  One part.  That's
it.  Why?  Because I've got thousands of them.  

Literally.  Something like 22,000 of the bastards.  I give
them away like popcorn to students at schools.  Got them
_very cheaply_.  So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.

What to go there?  

Signal Splitting? Can you sketch out what your thinking?
The wikipedia type circuit can use a few n2222 - I count a max of 5.
Even if you could use only n2222 it would not be a good idea - making
the circuit stable would be more difficult. On the good side the n2222
is a good choice for Q1,Q2 and as active replacements for R5,R6 and
one other (optional) we haven't met yet. What makes it a good
transistor is the large current gain / bandwidth product and the flat
DC current gain over a wide range of viable bias currents. Both
contribute to low distortion.

http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF (page 3
graph)

compared to say 2n3904
http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF

where the flat portion of the DC gain curve is over a very limited
range.

not audio systems, power supplies or transformers.

Got it.

As a result the power supply is often assumed to be regulated, which
is not true in this case, or the power supply is treated in a very
perfunctory manner that is not at all compatible with good design.

In this case you have the voltage you need for the 10 watts, plus
voltage drop for the driver circuitry and output stage , plus ripple
voltage, plus whatever is required for transformer regulation and
mains regulation. When you add it all up you might find that a chosen
transistor/component is actually not at all suitable for the job. Back
to the drawing board. Change this change that recheck everything again
etc.

In this case, though, there is nothing particularly
remarkable about the rails.  Taken across the entire span,
even, doesn't exceed the maximum Vce of a great many BJTs. So
no real worry there.  But I see some of where problems may
arise.  Luckily, at this level I can side-step worrying about
that part and get back to learning about amplifier design,
yes?

I come up with a figure of 50 volts rail to rail no load voltage -
after picking out a common transformer with 15% regulation.

If you do the power supply first you have the figures needed for your
worst case already. It saves time and makes a better result (no
tendency to comprimise to save all the calculations already done).

Well, does this mean we should hack out the power supply
first?  I'm perfectly fine with that and can get back to you
with a suggested circuit and parts list if you want to start
there.  We could settle that part before going anywhere else
and I'd be happy with that approach, too, because to be
honest I don't imagine it to put a horrible delay into
getting back to amplifier design.  So I'm good either way.

I'm looking at some of your other posts and I don't think you need a
maths lesson from me. If you want to do a power supply great. Its a
small one so nothing much too it. If you don't want, I'm OK too.

I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.

I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on.  For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area.  I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well.  I also have a room set aside
for this kind of stuff, when I get time to play.

OK. Next serious project, I'm coming around to your place!

You come to the west coast of the US and I'll have a room for
you!

Your gear is
better than mine. I had to ask, rather than just assume just in case my
assumptions got you building something you didn't want to, and got you
splattered all over the place from the mains, or suggesting you choose
the miller cap by watching the phase shift of the feedback circuit - I
don't read a lot of the posts so I didn't know what you could do.

To be honest, I can do a few things but I'm really not very
practiced.  My oscilloscope knowledge is lacking in some
areas -- which becomes all too painfully obvious to me when I
watch a pro using my equipment.  And I'm still learning to
solder better.  It's one of a few hobbies.

Jon

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

Done.

The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.

What I first see there is the amplifier sketch at the top of
the page

I wasn't going to prompt, but it is close to the sort of thing, I
think, you should be aiming for . As someone has already noted (I
would attribute you if I wasn't on GG, I'm sorry) it has been drawn up
for a single supply, rather than a more common (for this size /
configuration) split supply.

I had assumed we'd be using a split supply.
I think that's very much the preferred way.

I had assumed a speaker would be hooked up via a cap to the
output, so DC currents into a speaker coil would be removed
from any concern.  But I was also holding in the back of my
mind the idea of tweaking out DC bias via the speaker and
removing the coupling cap as an experiment to try.  And if
so, I'd pretty much want the ground as a "third rail."

Exactly right! There are two common ways to reduce/remove any offset
from the output. Neither is shown on the wikipedia circuit. If you
have another split rail circuit it will certainly have one method -
both methods involved use the diff amp.

(Playing just a bit upon the Chicago parlance about the once
dangerous rail in their transit system.)

(I don't really care too much about arguing about
efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair.  First thing I remember about is that R4
shouldn't be there

Correct. Theory says it does nothing. I practice the theory but have
the occasional heretical belief about that.

Actually, I think I've read that theory says it is _better_
to be removed.  The reason seemed pretty basic, as it's
easier to get close to a balanced current split; and this, I
gather, lowers 2nd harmonic distortions produced in the pair
-- notable more on the high frequency end I suppose because
gain used for linearizing feedback up there is diminishing
and can't compensate it.

In other words, it's not neutral.  It's considered to be
better if I gathered the details.  Then even better, the
current mirror enforces the whole deal and you've got about
the best to be had.

Of course, mostly just being a reader means I have no idea
which end is up.  So I might have all this wrong.

No. Thats all correct. I'll show a different circuit latter

and better still both R3 and R4 should be
replaced with a current mirror.  

This would provide more differential gain.

_and_ improve distortion because the currents are forced to
be balanced in the pair, yes?

yes.

R5 should be a replaced with
a BJT, as well.  

In the right configuration it would reduce the common mode signal gain
of things like mains hum and supply ripple (you mentioned power supply
isolation before).

Yes, that's how I thought about it.

Also, from another (what do you call it branch? thread?) you were
discussing boot-strapping R6. This is not done so much as amplifiers
get bigger but a BJT configured in the same way as the replacement for
R5 is very common. I'm leaving the details to you - perhaps there is a
way to reduce component count without affecting performance. (I am
hoping this is what you wanted "nutting it out for yourself")

Yes!  I don't want things handed on a platter.  But I also
don't want to have to rediscover all of the ideas by making
all of the mistakes, either.  This is the kind of "pointer"
towards something that I like a lot.  It gives me a place to
think about something, but leaves me some reason to have to
do so and that helps me own it better.

One general truth about learning is that you don't present
someone with a problem so out of their depth that they have
no chance at it.  Doing that means they fail, they feel like
a failure, and it causes a student to just want to go away.
They lose motivation, usually, in cases like that.  On the
other hand, providing no difficulty at all merely means
repetition of what they already know and they grow bored from
that, too.  Finding the sweet spot where a student is faced
with interesting problems that are not already known, but
perhaps within reach of grasping at with some effort, is the
key.  Then it can be fun, educational, and motivate.

That's what you just did for me.

I assume the input impedance of that example
is basically the parallel resistance of R1 and R2, but if we
Yes.

Okay.

There is the parallel resistance of R5 x Beta Q1 as well, but this is
normally so high it won't affect the result. And if R5 is replaced
with an active device it can become essentially infinite.

use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point.  
Yes, but you should probably think of a whole passive network to
filter out low and high frequency - (think what happens if you amp is
operated near a source of RF)

Well, every trace picks up like little antennae.  All kinds
of trace voltages appearing here and there.  Not good.

So.  Can you make an audio amplifier that can withstand a
microwave oven environment and deliver good performance while
irradiated with 1kW banging around in there?  

If you can do that the military wants you to EMP harden all there
electronics. The input is a little different because some user always
want to stick a bloody gret big long wire onto it.

There's no
miller cap on Q3,

Depending on transistors layout etc it might not be needed, but more
often it is the size that is the question.

I was thinking it helped locally linearize the VAS section
and that such would be "good" most anywhere.  But I am just
taking things without having worked through them on my own.
So...

It sets the bandwidth of the VAS stage so you can use negative
feedback without the whole thing turning into smoke. Do you know of
control theory / bode diagrams. There is a minuscule amount needed
for this app.

I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)

Vbe multiplier...

Okay.  Thanks.

The feedback ... well, I need to think about that a little
more.  There's no degen resistors in the emitters of Q4 and
Q5.

Why would/should you use them?

I'm still thinking about that.  In general, I was thinking
about them because of the "little re" that is kT/q based in
each BJT, and varies on Ie.  Since Ie is varying around, I
was thinking about something fixed there to overwhelm it and
"make it knowable" for the design, I suppose.  Maybe that's
all wet, given your query. I'll toss the idea off the side,
for now.

Try working through the thermal stabilization. Just make a stab at the
transistor junction temperatures - it will be pretty hot (unless you
can afford mega bucks for heatsinking)

Um.. okay, I need to sit down and think.  Mind is spinning,
but I've not set a finger to paper yet and there is lots to
think about in that one.  I could be way, way off base.

Not at all.

Thanks for that.  I'm just glad to be able to talk to someone
about any of this, at all.  So please accept my thanks for
the moments you are offering.

Is there a way you could post a schematic of where your thinking is
and what you would like to discuss - there is no need for a complete
circuit.

Yes.  I can use ASCII here, for example.  But before I go off
into the wild blue with this, do you want to focus on the
power supply first?  Or just jump in on the amplifier?

I don't mind. Earlier I put a stab at a no load worst case voltage,
you can use that if you want to. Until you get to output stage power
dissipation that is all you need.

Jon

 

[George Herold]

On Jan 31, 10:47 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Sat, 30 Jan 2010 22:25:55 -0800 (PST), George Herold





ggher...@gmail.com> wrote:
On Jan 30, 6:14 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Sun, 31 Jan 2010 09:54:01 +1100, "Phil Allison" <phi...@tpg.com.au
wrote:

"John Larkin is lying IDIOT

The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors.

** Done correctly, either way produces a stable bias situation in the
output
stage.

Larkin has no idea how it is done  - cos Larkin is bullshitting asshole.

1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.

** Guarantees serious x-over distortion.

  Zero bias can be done, but never so crudely as that.

2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.

** No need to ever use emitter resistors of more than 1 ohm.

With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.

Very few power amp designs have ever used them -  SAE brand amps from the
late 1970s being one exception.

In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments.

** Vbe multipliers always need adjustment to suit the actual devices in
use.

Both need negative feeback to kill crossover distortion.

** One thing that NFB is notoriously very poor at doing.

Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.

** How the fuck would you know  ?

Clearly you don't and just make things up to suit your wacky prejudices.

If I were designing an audio amp nowadays (which I certainly aren't)

** So shut the fuck up.

 You clueless fucking wanker.

...  Phil

I showed everybody an amp I designed, 17KW peak power out, a few PPM
noise and absolute analog accuracy.

Hey, Mr Audio, show us a power amp that you designed.

John- Hide quoted text -

- Show quoted text -

Hee Hee,  OK I'm not real proud of it, but we've sold several hundred
of them so at least it's paid for my ~week(?) of design and testing
time.

http://www.teachspin.com/instruments/audio/index.shtml

Hey, at least it's called an audio amplifier.  It's powered by a 15V /
1A switching supply.  And uses two power opamps.  One sets the ground
and the other does the work.
(Well both have to work when there is significant ground current.)

George H.

Things like this, laser drivers and gradient amplifiers and such, are
more interesting than actual audio, whose performance criteria are
fuzzy things like absurd power specs, "soundstaging", "microdynamics",
and similar fuzzy blather. They sell for a lot more, too.

John- Hide quoted text -

- Show quoted text -

Yup, I'm often making audio frequency stuff, but driving things other
than speakers. One difference is that we tend to care about DC.

"They sell for a lot more, too."

I was a little surprised when I scrolled down on my link and checked
the price. I think this started out selling for about $200. It's
probablly got about $100 worth of stuff in it so $300+ is a better
price.

George H.

 

[Jon Kirwan]

On Sun, 31 Jan 2010 12:38:49 -0800 (PST), David Eather
<eather@tpg.com.au> wrote:

On Jan 30, 9:06 am, Jon Kirwan <j...@infinitefactors.org> wrote:
On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather
snip

Yes and No. All the published circuits are made by people who want to
sell transistors,

A concern I care not the least about.  My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices.  One part.  That's
it.  Why?  Because I've got thousands of them.  

Literally.  Something like 22,000 of the bastards.  I give
them away like popcorn to students at schools.  Got them
_very cheaply_.  So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.

What to go there?  

Signal Splitting? Can you sketch out what your thinking?

Yeah, I think so. Something like this:

: | |
: \ |
: / R2 |
: \ |
: / |
: | |
: | |/c Q2
: +---------|
: | |>e
: | |
: |/c Q3 |
: -------| +-----
: |>e |
: | |
: | |/c Q1
: +---------|
: | |>e
: | |
: \ |
: / R1 |
: \ |
: / |
: | |

The "signal splitter" here is Q3. It's also providing gain,
too, though. The emitter and collector move in opposite
directions and the signal "splits" at Q3. (The emitter
follows the base, the collector inverts the base.)

If I read with any understanding about these things, properly
biasing Q3 is a pain, the Q3 gain varies with the load itself
as well as its bias, and compensation issues are complicated
a bit.

The wikipedia type circuit can use a few n2222 - I count a max of 5.
Even if you could use only n2222 it would not be a good idea - making
the circuit stable would be more difficult.

Yes, ignorant as I am still of the details, I think that's
very true. The splitter has significant signal voltage on
its input and I've read that pole-splitting methods for
improving stability are harder to apply here.

On the good side the n2222
is a good choice for Q1,Q2 and as active replacements for R5,R6 and
one other (optional) we haven't met yet. What makes it a good
transistor is the large current gain / bandwidth product and the flat
DC current gain over a wide range of viable bias currents. Both
contribute to low distortion.

http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF (page 3
graph)

compared to say 2n3904
http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF

where the flat portion of the DC gain curve is over a very limited
range.

Interesting point to consider. Something that had slipped by
me, so far.

not audio systems, power supplies or transformers.

Got it.

As a result the power supply is often assumed to be regulated, which
is not true in this case, or the power supply is treated in a very
perfunctory manner that is not at all compatible with good design.

In this case you have the voltage you need for the 10 watts, plus
voltage drop for the driver circuitry and output stage , plus ripple
voltage, plus whatever is required for transformer regulation and
mains regulation. When you add it all up you might find that a chosen
transistor/component is actually not at all suitable for the job. Back
to the drawing board. Change this change that recheck everything again
etc.

In this case, though, there is nothing particularly
remarkable about the rails.  Taken across the entire span,
even, doesn't exceed the maximum Vce of a great many BJTs. So
no real worry there.  But I see some of where problems may
arise.  Luckily, at this level I can side-step worrying about
that part and get back to learning about amplifier design,
yes?

I come up with a figure of 50 volts rail to rail no load voltage -
after picking out a common transformer with 15% regulation.

Okay. This is going to force me to sit down with paper and
work through. I was stupidly imagining +/-18V max, or 36V
rail to rail. I haven't considered the details of the output
section yet, driving a load from rails that run up and down
on capacitors that charge and discharge at 1A-level currents
into the load, and perhaps I need to spend some more time
there before moving on.

There are so many ways to cut this. Start at the input and
that's one focus that may work okay. Start at the output
stage and that provides important power supply information,
though. So maybe I should start at that end?

If you do the power supply first you have the figures needed for your
worst case already. It saves time and makes a better result (no
tendency to comprimise to save all the calculations already done).

Well, does this mean we should hack out the power supply
first?  I'm perfectly fine with that and can get back to you
with a suggested circuit and parts list if you want to start
there.  We could settle that part before going anywhere else
and I'd be happy with that approach, too, because to be
honest I don't imagine it to put a horrible delay into
getting back to amplifier design.  So I'm good either way.

I'm looking at some of your other posts and I don't think you need a
maths lesson from me. If you want to do a power supply great. Its a
small one so nothing much too it. If you don't want, I'm OK too.

I still haven't been down the path on my own, yet. So I
don't have strong opinions about this. It's like going to
Disneyland for the first time. Which land should I go to,
first? Later, after being there a few times, I may look at
the flow of people and decide that "Adventureland" is the
best first start. But first time out? Who knows? I'm open
to guidance. Everything is new.

I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.

I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on.  For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area.  I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well.  I also have a room set aside
for this kind of stuff, when I get time to play.

OK. Next serious project, I'm coming around to your place!

You come to the west coast of the US and I'll have a room for
you!

Your gear is
better than mine. I had to ask, rather than just assume just in case my
assumptions got you building something you didn't want to, and got you
splattered all over the place from the mains, or suggesting you choose
the miller cap by watching the phase shift of the feedback circuit - I
don't read a lot of the posts so I didn't know what you could do.

To be honest, I can do a few things but I'm really not very
practiced.  My oscilloscope knowledge is lacking in some
areas -- which becomes all too painfully obvious to me when I
watch a pro using my equipment.  And I'm still learning to
solder better.  It's one of a few hobbies.

Jon

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

Done.

The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.

What I first see there is the amplifier sketch at the top of
the page

I wasn't going to prompt, but it is close to the sort of thing, I
think, you should be aiming for . As someone has already noted (I
would attribute you if I wasn't on GG, I'm sorry) it has been drawn up
for a single supply, rather than a more common (for this size /
configuration) split supply.

I had assumed we'd be using a split supply.

I think that's very much the preferred way.

I feel more comfortable assuming it, too.

I had assumed a speaker would be hooked up via a cap to the
output, so DC currents into a speaker coil would be removed
from any concern.  But I was also holding in the back of my
mind the idea of tweaking out DC bias via the speaker and
removing the coupling cap as an experiment to try.  And if
so, I'd pretty much want the ground as a "third rail."

Exactly right! There are two common ways to reduce/remove any offset
from the output. Neither is shown on the wikipedia circuit. If you
have another split rail circuit it will certainly have one method -
both methods involved use the diff amp.

Thanks.

(Playing just a bit upon the Chicago parlance about the once
dangerous rail in their transit system.)

(I don't really care too much about arguing about
efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair.  First thing I remember about is that R4
shouldn't be there

Correct. Theory says it does nothing. I practice the theory but have
the occasional heretical belief about that.

Actually, I think I've read that theory says it is _better_
to be removed.  The reason seemed pretty basic, as it's
easier to get close to a balanced current split; and this, I
gather, lowers 2nd harmonic distortions produced in the pair
-- notable more on the high frequency end I suppose because
gain used for linearizing feedback up there is diminishing
and can't compensate it.

In other words, it's not neutral.  It's considered to be
better if I gathered the details.  Then even better, the
current mirror enforces the whole deal and you've got about
the best to be had.

Of course, mostly just being a reader means I have no idea
which end is up.  So I might have all this wrong.

No. Thats all correct. I'll show a different circuit latter

Okay. I'll enjoy the moment when it happens.

and better still both R3 and R4 should be
replaced with a current mirror.  

This would provide more differential gain.

_and_ improve distortion because the currents are forced to
be balanced in the pair, yes?

yes.

Okay. So I am picking up details not too poorly, so far.

R5 should be a replaced with
a BJT, as well.  

In the right configuration it would reduce the common mode signal gain
of things like mains hum and supply ripple (you mentioned power supply
isolation before).

Yes, that's how I thought about it.



Also, from another (what do you call it branch? thread?) you were
discussing boot-strapping R6. This is not done so much as amplifiers
get bigger but a BJT configured in the same way as the replacement for
R5 is very common. I'm leaving the details to you - perhaps there is a
way to reduce component count without affecting performance. (I am
hoping this is what you wanted "nutting it out for yourself")

Yes!  I don't want things handed on a platter.  But I also
don't want to have to rediscover all of the ideas by making
all of the mistakes, either.  This is the kind of "pointer"
towards something that I like a lot.  It gives me a place to
think about something, but leaves me some reason to have to
do so and that helps me own it better.

One general truth about learning is that you don't present
someone with a problem so out of their depth that they have
no chance at it.  Doing that means they fail, they feel like
a failure, and it causes a student to just want to go away.
They lose motivation, usually, in cases like that.  On the
other hand, providing no difficulty at all merely means
repetition of what they already know and they grow bored from
that, too.  Finding the sweet spot where a student is faced
with interesting problems that are not already known, but
perhaps within reach of grasping at with some effort, is the
key.  Then it can be fun, educational, and motivate.

That's what you just did for me.

I assume the input impedance of that example
is basically the parallel resistance of R1 and R2, but if we
Yes.

Okay.

There is the parallel resistance of R5 x Beta Q1 as well, but this is
normally so high it won't affect the result. And if R5 is replaced
with an active device it can become essentially infinite.

Okay. I've got that detail from other discussions, too. So
yes, understood. Also, I mentioned replacing R5, I think. In
replacing R5 with active parts, I'm thinking of two BJTs in a
usual form that seems to work pretty well over supply
variations.

use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point.  
Yes, but you should probably think of a whole passive network to
filter out low and high frequency - (think what happens if you amp is
operated near a source of RF)

Well, every trace picks up like little antennae.  All kinds
of trace voltages appearing here and there.  Not good.

So.  Can you make an audio amplifier that can withstand a
microwave oven environment and deliver good performance while
irradiated with 1kW banging around in there?  

If you can do that the military wants you to EMP harden all there
electronics. The input is a little different because some user always
want to stick a bloody gret big long wire onto it.

I actually _do_ work on low-mass, direct-contact temperature
measuring devices designed to work within a microwave
environment. (But no electronics or metals inside.)

But you brought up the microwave environment, so I hope you
don't mind the teasing about it.

There's no
miller cap on Q3,

Depending on transistors layout etc it might not be needed, but more
often it is the size that is the question.

I was thinking it helped locally linearize the VAS section
and that such would be "good" most anywhere.  But I am just
taking things without having worked through them on my own.
So...

It sets the bandwidth of the VAS stage so you can use negative
feedback without the whole thing turning into smoke. Do you know of
control theory / bode diagrams. There is a minuscule amount needed
for this app.

I am familiar with _some_ closed loop control theory,
sufficient to get me by with PID controls (using _and_
writing code for them.) Bode diagrams are something I have
not used, though I've seen them. My math is adequate, I
suspect. But I will have to read up on them, I suppose.

For Laplace analysis, I'm familiar with complex numbers,
poles and zeros, partial fraction extractions, and so on.
Just inexperienced in the "short cuts" that many use to get
(and think about) answers.

I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)

Vbe multiplier...

Okay.  Thanks.

The feedback ... well, I need to think about that a little
more.  There's no degen resistors in the emitters of Q4 and
Q5.

Why would/should you use them?

I'm still thinking about that.  In general, I was thinking
about them because of the "little re" that is kT/q based in
each BJT, and varies on Ie.  Since Ie is varying around, I
was thinking about something fixed there to overwhelm it and
"make it knowable" for the design, I suppose.  Maybe that's
all wet, given your query. I'll toss the idea off the side,
for now.

Try working through the thermal stabilization. Just make a stab at the
transistor junction temperatures - it will be pretty hot (unless you
can afford mega bucks for heatsinking)

I need to understand the output configuration a little better
before I do that.

Including thinking more closely about swinging one end of an
output cap around so that 1Amp rms can pass through it at
20Hz. I = C dv/dt, but V=V0*sin(w*t), so I=C*w*V0*cos(w*t).
Assuming max current at the max slew rate for a sine at phase
angle zero, the w*t is some 2*PI*N thing, so cos(w*t) goes to
1. That makes I=w*C*V0. But w=2*pi*20, or about 126 or so.
So I=126*C*V0. So with I=1A, C=1/(126*V0). With V0=15V, I
get about 530uF for the output cap. That's an amp peak only
at the right phase, too. It'll be less elsewhere. To make
that an amp rms, the cap would need to be still bigger.

Peak current via the cap will take place right about the time
when the two BJTs's emitters are at their midpoint. One of
the BJTs will be supplying that. Not only that, but also
depending upon class mode of operation, supplying current to
the other one as well. How much is important to figuring out
the wattage.

I need to sit down with paper, I suspect. But if you want to
provide some suggested thinking process here, I'd also be
very open to that, as well. I'll take a shot at it either
way, but it helps to see your thinking, too. If you can
afford the moment for me.

Um.. okay, I need to sit down and think.  Mind is spinning,
but I've not set a finger to paper yet and there is lots to
think about in that one.  I could be way, way off base.

Not at all.

Thanks for that.  I'm just glad to be able to talk to someone
about any of this, at all.  So please accept my thanks for
the moments you are offering.

Is there a way you could post a schematic of where your thinking is
and what you would like to discuss - there is no need for a complete
circuit.

Yes.  I can use ASCII here, for example.  But before I go off
into the wild blue with this, do you want to focus on the
power supply first?  Or just jump in on the amplifier?

I don't mind. Earlier I put a stab at a no load worst case voltage,
you can use that if you want to. Until you get to output stage power
dissipation that is all you need.

Maybe I'd like to focus on understanding different output
pair configurations, first. I frankly don't like the "haul
the output pair around with a collector on one side and a
resistor on the other with a rubber diode in between to keep
them biased up" approach. It's smacks of heavy-handedness
and I simply don't like the way it looks to me. Everything
tells me this works, but it is indelicate at the very least.

However, it is crucial that I understand it in detail before
deciding what I really think about it. For example, I might
want to replace the resistor with a current source. But
without apprehending the output stage more fully, its time
domain behavior over a single cycle for example, I'm not
comfortable with hacking it here and there, ignorantly.

Jon

 

[Jon Kirwan]

On Sat, 30 Jan 2010 01:11:28 +0530, "pimpom" wrote:

snip
I like the biasing scheme mentioned by Jon and use it for all my
designs except the early ones using germanium transistors, though
I don't know the name either. The biasing transistor can be
mounted on the output transistors' heatsink for temperature
tracking.
snip

Okay. I'm giving this a little more thought -- as it applies
to temperature variation. The basic idea is that the two
bases of the two output BJTs (or output BJT structures) must
be separated a little bit in order to ensure both quadrants
are in forward conduction. With a "Vbe multiplier" in place
and with its own BJT tacked onto the same heat sink, the idea
is that the the Vbe multiplier's own voltage separation will
shrink as temperature rises, exactly in some proportion
needed to maintain the designed forward conduction
relationship of the output BJTs.

To be honest, this designed forward conduction mode may not
be critital. It might move a class-AB around a little within
its AB operation, for example, if the voltage tracking with
temperature weren't flawlessly applied. And that may be
harmless. I don't know. On the other hand, if tweaked for
class-A I can imagine that it might move the operation into
class-AB; if tweaked for lower-dissipation class-AB it might
move the operation into class-B; and if class-B were desired
it could move it into class-C with associated distortion.

There are several parts of the basic Shockley equation. One
is the always-in-mind part that includes a kT/q part in it
and relates that to Vbe. The other is the Is part and Eg is
the key there. So one thing that crosses my mind is in
selecting the BJT for the "rubber diode" thingy. Unless it's
Vbe (at 27C and designed constant current) and its Eg are the
same, even though it is a small signal device, doesn't that
mean that the variations over temperature will be two lines
that cross over only at one temperature point? In other
words, basically matches nowhere except at one temperature?

It seems crude.

I've seen this as a modification. In ASCII form:

: A
: |
: ,---+---,
: | |
: | \
: | / R3
: \ \
: / R2 /
: \ |
: / +--- C
: | |
: | |
: | |/c Q1
: +-----|
: | |>e
: \ |
: / R1 |
: \ |
: / |
: | |
: '---+---'
: |
: B

We've already decided that R1 might be both a simple resistor
plus a variable pot to allow adjustment. The usual case I
see on the web does NOT include R3, though. However, I've
seen a few examples where R3 (small-valued) exists and one of
the two output BJTs' base is connected at C and not at A.

The above circuit is a somewhat different version of the Vbe
multiplier/rubber diode thing. The difference being R3,
which I'm still grappling with.

But does anyone know, before I go writing equations all over
the place, why R3 is added? Or is R3 just some book author's
wild ass guess?

This is all pressing me into studying the output structure
more, I guess. It basically looks simple when I wave my
hands over it, but I suspect the intimate details need to be
exposed to view. On to that part, I suppose.

Jon

 

[pimpom]

Jon Kirwan wrote:

On Sat, 30 Jan 2010 01:11:28 +0530, "pimpom" wrote:

snip
I like the biasing scheme mentioned by Jon and use it for all
my
designs except the early ones using germanium transistors,
though
I don't know the name either. The biasing transistor can be
mounted on the output transistors' heatsink for temperature
tracking.
snip

Okay. I'm giving this a little more thought -- as it applies
to temperature variation. The basic idea is that the two
bases of the two output BJTs (or output BJT structures) must
be separated a little bit in order to ensure both quadrants
are in forward conduction. With a "Vbe multiplier" in place
and with its own BJT tacked onto the same heat sink, the idea
is that the the Vbe multiplier's own voltage separation will
shrink as temperature rises, exactly in some proportion
needed to maintain the designed forward conduction
relationship of the output BJTs.

To be honest, this designed forward conduction mode may not
be critital. It might move a class-AB around a little within
its AB operation, for example, if the voltage tracking with
temperature weren't flawlessly applied. And that may be
harmless. I don't know. On the other hand, if tweaked for
class-A I can imagine that it might move the operation into
class-AB; if tweaked for lower-dissipation class-AB it might
move the operation into class-B; and if class-B were desired
it could move it into class-C with associated distortion.

There are several parts of the basic Shockley equation. One
is the always-in-mind part that includes a kT/q part in it
and relates that to Vbe. The other is the Is part and Eg is
the key there. So one thing that crosses my mind is in
selecting the BJT for the "rubber diode" thingy. Unless it's
Vbe (at 27C and designed constant current) and its Eg are the
same, even though it is a small signal device, doesn't that
mean that the variations over temperature will be two lines
that cross over only at one temperature point? In other
words, basically matches nowhere except at one temperature?

It seems crude.

You lost me for a while with the Eg term. You mean the emitter
transconductance?

Perhaps a short diversion into my own background may be
appropriate here. Shortage of funds and scarcity of good books
even for those who could afford them in a technologically
primitive environment kept me from delving deeply into
semiconductor physics when I started teaching myself electronics
over 40 years ago. I had advanced Math in college, but lack of
practice has made me very rusty. You're probably much better at
that.

Over the years, I developed my own shortcuts and approximations
using mostly basic algebra, trigonometry and bits of calculus
here and there, blended with empirical formulas.

In any case, the Shockley equation seems to hold fairly well in
practice for the purpose of bias regulation within the
temperature range normally encountered. Temperature tracking with
simple circuits like diodes in series or a Vbe multiplier cannot
be more than approximate. Such a device can sense only the
heatsink temperature and,.except under long-term static
conditions, that temp will almost always be different from Tj of
the output devices. That Tj is what needs to be tracked and when
the output transistors are pumping out audio power, that
difference can be tens of degrees.

I've seen this as a modification. In ASCII form:

A
|
,---+---,
| |
| \
| / R3
\ \
/ R2 /
\ |
/ +--- C
| |
| |
| |/c Q1
+-----|
| |>e
\ |
/ R1 |
\ |
/ |
| |
'---+---'
|
B

We've already decided that R1 might be both a simple resistor
plus a variable pot to allow adjustment. The usual case I
see on the web does NOT include R3, though. However, I've
seen a few examples where R3 (small-valued) exists and one of
the two output BJTs' base is connected at C and not at A.

The above circuit is a somewhat different version of the Vbe
multiplier/rubber diode thing. The difference being R3,
which I'm still grappling with.

I've seen R3 used in that position too, but never gave it much
thought until you brought it up. Offhand I still can't see a
reason for it either. Maybe for stability against a local
oscillation? Perhaps taking some time to think about it will
bring some revelation. Or someone else can save us the trouble
and enlighten us.

But does anyone know, before I go writing equations all over
the place, why R3 is added? Or is R3 just some book author's
wild ass guess?

A possibility. But I wouldn't go out on a limb and call it that

This is all pressing me into studying the output structure
more, I guess. It basically looks simple when I wave my
hands over it, but I suspect the intimate details need to be
exposed to view. On to that part, I suppose.

Jon