Operational Amplifier Design

[Paul Burridge]

On 7 Nov 2004 18:26:07 -0800, Winfield Hill
<whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Ken Smith wrote...

Winfield Hill
The LM324 output stage is severely in need of improvement...

If the OP is just trying to get a starting point, operation down
to the negative rail can be set aside. Yes the output stage sucks
but it is simple.

Simple, but pedagogically best as an example of how not to do it.

It can't be *that* bad. In that poll of "classic chips" we had last
year, the 324 was one of the most popular, ISTR.

--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Fred Bloggs]

Winfield Hill wrote:

Ken Smith wrote...

Winfield Hill

The LM324 output stage is severely in need of improvement...

If the OP is just trying to get a starting point, operation down
to the negative rail can be set aside. Yes the output stage sucks
but it is simple.


Simple, but pedagogically best as an example of how not to do it.

Non-sense, it's a great little opamp.

 

[Winfield Hill]

Jim Thompson wrote...

Winfield Hill wrote:
Ken Smith wrote...
Winfield Hill wrote...

The LM324 output stage is severely in need of improvement...

If the OP is just trying to get a starting point, operation down
to the negative rail can be set aside. Yes the output stage sucks
but it is simple.

Simple, but pedagogically best as an example of how not to do it.

On a bipolar process you don't have many choices. The vertical PNP
plus current sink _did_ get you _almost_ to negative rail ;-)

My complaint is not with the pulldown circuitry.

The biggest problem was brand variability... Motorola's copy had such
severe cross-over distortion as to make the part unusable for active
filters... which is why I put the Motorola LM324 on the GenRad
(OmniComp) do-not-purchase list.

To my knowledge, this problem was equally bad with '324 parts from
all the manufacturers. I suppose they chose *zero* class-AB current
to help insure the 50uA pulldown could reach closer to the negative
rail, but it's a great example of how *not* to do it.


--
Thanks,
- Win

 

[Rich Grise]

On Mon, 08 Nov 2004 10:25:55 -0700, Jim Thompson wrote:

While I never dissected an LM324, I suspect that some versions derived
the 50uA from mirroring via a replica of Q12 (National data sheet
reference designators).

Might be an interesting exercise to trace one out under a microscope:
If you look at the published schematic, the output stage cannot
function properly, it has a major problem... can you tell us what it
is?

The short across Q5, Q6, Q7/Rsc, and Q13?

Thanks,
Rich

 

[Rich Grise]

On Mon, 08 Nov 2004 10:25:55 -0700, Jim Thompson wrote:

While I never dissected an LM324, I suspect that some versions derived
the 50uA from mirroring via a replica of Q12 (National data sheet
reference designators).

Might be an interesting exercise to trace one out under a microscope:
If you look at the published schematic, the output stage cannot
function properly, it has a major problem... can you tell us what it
is?

I posted asking, The short across Q5,6,7,13, and Rsc? with no

link to the schem.
http://perso.wanadoo.es/chyryes/componentes/LM324.pdf

Thanks,

Rich

 

[Winfield Hill]

Jim Thompson wrote...

But I don't know of a single commercial amplifier that isn't set
up that way.

Doesn't adding a modest series resistor (along with an ESD network
after the resistor) give one a chance to both stabilize the loop,
and avoid ESD failure?


--
Thanks,
- Win

 

[Winfield Hill]

Jim Thompson wrote...

Winfield Hill wrote:
I assume you're referring to Q7's collector-base diode in parallel
with Q13's base-emitter (NSC diagram, Q12 and Q11 for Motorola),
during pulldown. We don't know the relative I_s values...

But we do. From that era we know that the B-C junction of the NPN is
the same doping (same process step) as the E-B junction of the PNP.
And the current limit resistor is probably 35 ohms. So, as-drawn, we
would have serious (effective) beta reduction when the B-C of the NPN
forward biased. My guess is that there is at least one diode (and
probably two) between the collector of Q7 and the base of Q13

I always put a diode in the current-limit transistor's collector (as
is standard in the power-amp crowd). If we assume that Q7's area is
dramatically smaller than Q13's area, does that make a difference?

What are you thinking of when you say more than one diode? Do you
think there's a class AB diode or two in there afterall?


--
Thanks,
- Win

 

[Winfield Hill]

Jim Thompson wrote...

Are you aware of the infamous "let's copy a 741 episode", where the
Fairchild published schematic was copied into silicon by (IIRC)
Raytheon, except that 'tain't the circuit ;-)

Do you know what the correct circuit is?


--
Thanks,
- Win

 

[Kevin Aylward]

Jim Thompson wrote:

On 8 Nov 2004 15:57:30 -0800, Winfield Hill
whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Jim Thompson wrote...

But I don't know of a single commercial amplifier that isn't set
up that way.

Doesn't adding a modest series resistor (along with an ESD network
after the resistor) give one a chance to both stabilize the loop,
and avoid ESD failure?

Actually, most compensation caps _do_ have a series resistor (to
introduce a zero).

Ahmmm. I disagree. If they do, its often a poor design. This particular
resister usually fails to do what it is idealised to do. It can only
work if the stage is not running out of steam at its zero frequency.
This is not often the case when you want to use such a trick. The cap
does a pole split, i.e. moves other poles further away, e.g low output
impedance driving the load. As soon as the resister starts coming in,
the poles start to migrate inwards again getting you back to where you
were without the Miller cap at all. i.e. loads of poles all over the
place.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Tony Williams]

In article <6d3vo0pr4nd4ju27tlov1g4riitlf9ge6f@4ax.com>,

The biggest problem was brand variability... Motorola's copy had
such severe cross-over distortion as to make the part unusable
for active filters... which is why I put the Motorola LM324 on
the GenRad (OmniComp) do-not-purchase list.

Plus a tendency for a little burst of hf oscillation
from the output transistors at swapover. It was best
with the LM324 to think of it as delivering positive
output current only, with the negative current being
supplied by an external pulldown.

--
Tony Williams.

 

[Ken Smith]

In article <auqvo05n40o4gdj5fau9f7qnm57gcogdu7@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[... me ...]

The output section has about 3 diodes worth of crossover distortion and
the compensation does not enclose that distortion so at high frequencies,
it appears in the output. It also makes the output impedance rise quite a
bit for increasing frequencies and vary a lot with load current. This can
lead to trouble when you use them to drive a transistor buffer.



The "simplified schematic" is flawed... it can't be as-drawn... there
is a fundamental error. I tried to tease Win into _stepping_in_it_,
but he won't take the bait ;-)

I had assumed that the schematic was simplified but that the 3 diode cross
over issue was real because of what I had seen in in actual use.

And what's this statement, "...compensation does not enclose that
distortion..."?

Consider these oversimplifications:

ASCII art:

Not enclosed:
-----------
! !
-------+--------! Distorty !------ output
! ! ! stuff !
--- ! ! !
---Cc ! -----------
! !/
---+----!
!\ Q1
!
GND



Enclosed:

-------------------------------
! !
! ----------- !
! ! ! !
! --------! Distorty !---+-- output
! ! ! stuff !
--- ! ! !
---Cc ! -----------
! !/
---+----!
!\ Q1
!
GND


When the compensation capacitor takes the feedback from the far side of
the distortion, the gain of Q1 will be acting to make the whole section
act more linear. ie: its collector will slew more quickly through the
cross over distortion. This greatly lowers the cross overdistortion of
the circuit.

This method only works for about 1 transistor followed by something like a
complimentry emitter follower. If you try to enclose too many devices,
the local feedback system can become unstable for some loops gains.
Transistor stages running right near the cut off point have a much lower
frequency responce. As the output switches from the NPN to the PNP, the
frequency responce of that part sweeps down and then back up. The local
feedback loop must be made stable for all of the gain/pahse values the
circuit sweeps through. I assume that this problem is why the LM324 does
not enclose the non-linearities in the compensation loop.

--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <kd00p0pcrl5k2d4uqpi3ftm3ot6maobeg4@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[... compensation cap enclosing output section ..]

Yes. But I don't know of a single commercial amplifier that isn't set
up that way.

I assume you are still talking of IC. amplifiers with the compensation cap
as an internal part. IIRC most op-amps that have external
(over)compensation provisions the external cap runs to the output.




--
--
kensmith@rahul.net forging knowledge

 

[Winfield Hill]

Jim Thompson wrote...

Winfield Hill wrote:

Jim Thompson wrote...

Are you aware of the infamous "let's copy a 741 episode", where the
Fairchild published schematic was copied into silicon by (IIRC)
Raytheon, except that 'tain't the circuit ;-)

Do you know what the correct circuit is?

Yes ;-) At least I _think_ I remember it still.

...Jim Thompson

OK, we're all eyes and ears!


--
Thanks,
- Win

 

[Kevin Aylward]

Ken Smith wrote:

In article <auqvo05n40o4gdj5fau9f7qnm57gcogdu7@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[... me ...]
The output section has about 3 diodes worth of crossover distortion
and the compensation does not enclose that distortion so at high
frequencies, it appears in the output. It also makes the output
impedance rise quite a bit for increasing frequencies and vary a
lot with load current. This can lead to trouble when you use them
to drive a transistor buffer.



The "simplified schematic" is flawed... it can't be as-drawn... there
is a fundamental error. I tried to tease Win into _stepping_in_it_,
but he won't take the bait ;-)

I had assumed that the schematic was simplified but that the 3 diode
cross over issue was real because of what I had seen in in actual use.

And what's this statement, "...compensation does not enclose that
distortion..."?


Consider these oversimplifications:

ASCII art:

Not enclosed:
-----------
! !
-------+--------! Distorty !------ output
! ! ! stuff !
--- ! ! !
---Cc ! -----------
! !/
---+----!
!\ Q1
!
GND



Enclosed:

-------------------------------
! !
! ----------- !
! ! ! !
! --------! Distorty !---+-- output
! ! ! stuff !
--- ! ! !
---Cc ! -----------
! !/
---+----!
!\ Q1
!
GND


When the compensation capacitor takes the feedback from the far side
of the distortion, the gain of Q1 will be acting to make the whole
section act more linear. ie: its collector will slew more quickly
through the cross over distortion. This greatly lowers the cross
overdistortion of the circuit.

Not in my opinion. I first evaluated this 20 years ago when deigning a
0.002% amp, i.e. I actually measured it. I have also been re-evaluating
this recently in spice.

"There is no such thing as a free lunch".

If you move the cap to the outside of the follower things start
collapsing. When the cap is connected as usual, you get a low impedance
voltage driving the Distorty stuff. Without the cap the impedance is all
over the place, meaning that any variation in input impedance of the
following stage creates distortion.

Secondly, you end up with a response that doesn't roll off cleanly at
HF. Again, try it in spice.

In summary here, the "no free lunch" is something to take to heart. Many
configurations that look, different are essentially, topologically
equivalent, such that there is often not much to chose them apart.

Designs undergo Darwinian selection. Most designs that survive are there
because the obvious alternatives don't work.

This method only works for about 1 transistor followed by something
like a complimentry emitter follower.

It doesn't work at all. Try doing some SuperSpice on it, noting that SS
does small signal distortion and LTSpice dosnt

If you try to enclose too many
devices, the local feedback system can become unstable for some loops
gains.

Yes.

Transistor stages running right near the cut off point have a
much lower frequency responce. As the output switches from the NPN
to the PNP, the frequency responce of that part sweeps down and then
back up.

Yes.

The local feedback loop must be made stable for all of the
gain/pahse values the circuit sweeps through. I assume that this
problem is why the LM324 does not enclose the non-linearities in the

Yes, and other reasons, some of which are noted above.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Ken Smith]

In article <cmp15a02tuc@drn.newsguy.com>,
Winfield Hill <whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Jim Thompson wrote...

But I don't know of a single commercial amplifier that isn't set
up that way.

Doesn't adding a modest series resistor (along with an ESD network
after the resistor) give one a chance to both stabilize the loop,
and avoid ESD failure?

Yes, adding a resistor in series with the compensation cap can add a zero
to the transfer function. If this zero lines up with the pole from, lets
say, the input pair, then stability can be improved.

--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <kd00p0pcrl5k2d4uqpi3ftm3ot6maobeg4@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[... compensation cap connected to output pin ...]

Yes. But I don't know of a single commercial amplifier that isn't set
up that way.

LT1364 and LT1814 do that.


--
--
kensmith@rahul.net forging knowledge

 

[Jim Thompson]

On Wed, 10 Nov 2004 00:52:09 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

In article <t29kd.10112$tV4.7591@fe2.news.blueyonder.co.uk>,
Kevin Aylward <salesEXTRACT@anasoft.co.uk> wrote:
[snip]
It turns 3
potential poles into one dominant one.

Yes
This extra zero idea only works
in practice if one is using a gain stage that is much nearer the ideal,

Or where the low impedance to the input section is otherwise assured.

Ken, Just plonk Kevin... keeps the heartburn down ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Ken Smith]

In article <iup2p0l8g8odi2rchjqkeecm7gav2jnmoo@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[...]

Ken, Just plonk Kevin... keeps the heartburn down ;-)

I had just unplonked[1] him. You may be right about the heartburn though.

[1] is that a word?
--
--
kensmith@rahul.net forging knowledge

 

[PaulCsouls]

I stand corrected.

Paul C


On Sun, 7 Nov 2004 22:24:50 +0000 (UTC), kensmith@green.rahul.net (Ken
Smith) wrote:

In article <h7oso0tolgmdo8h55dul9466hin60ssojj@4ax.com>,
PaulCsouls <paulcsouls@worldnet.att.net> wrote:
On Sun, 7 Nov 2004 13:00:24 -0000, "Leon Heller"
leon_heller@hotmail.com> wrote:


It's quite a popular project on degree courses; a former colleague of mine
had to do something similar. IIRC, his SPICE simulation worked OK, but the
actual circuit built from BJTs etc. didn't.

Leon


Op Amps require matched components. You'll have a lot trouble trying
to build it with discretes.

No, some designs of op-amps require matched components. Other designs do
not. They only need the parts to be within the normal process span.

You can trim the offset voltage out out with a trimmer. The offset
bias current can often be ignored. The output stage bias point can be
made value independant enough if you are willing to trade off gain per
device and swing. Lots of op-amps got built before ICs.



--

 

[Rich The Philosophizer]

On Wed, 10 Nov 2004 02:00:18 +0000, Ken Smith wrote:

In article <iup2p0l8g8odi2rchjqkeecm7gav2jnmoo@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[...]
Ken, Just plonk Kevin... keeps the heartburn down ;-)

I had just unplonked[1] him. You may be right about the heartburn though.

[1] is that a word?

It is now. ;-)

;^j
Rich