Amplifier transistor matching?

[Kevin Aylward]

John Larkin wrote:

On Tue, 07 Jun 2005 18:37:56 GMT, "Kevin Aylward"
see_website@anasoft.co.uk> wrote:

John Larkin wrote:
Kevin has a hair up his butt about voltage control, a really long
hair ;-)

Indeed. It is they way they work. Only the illinformed consider
that the transister is current controlled.


No, being 'informed' doesn't make an engineer pick a more difficult
analytical approach to quantitative circuit design; quite the
opposite.

The voltage controlled model is the simple model, and the correct
one.

90% of the time, a simple 0.6Vbe-plus-Beta model is the
easiest and *most reliable* way to design transistor circuits.

Not at all. This is not true in the slightest.


Then I owe somebody about $200 million.

You must be switching a lot of relays;-)

Please explain, as discussed in this thread, from a beta perspective,
why a hfe mismatch of 2 can result in say, 10 times mismatch in
current (or more) for || devices. Why is it not just a factor of 2?

Because the beta model doesn't address current sharing of base
currents when transistors are paralleled. Discrete circuit designers
rarely (as in, never) hard-parallel bipolar transistors anyhow.

Thats still missing the point. No one is suggesting this.

If
they ever elected to do so, they would have to analyze the situation
properly.

But you cant do this without using the exponential nature of Ic verses
vbe, i.e. the inherent voltage controlled nature of bipolar transistors.
That's why the matching goes off tremendously. As I noted, 50ma at 5
ohms is 250mv. This is huge as the exponent of the exponential is
250/25.

Nobody can afford to design without regard to min/max beta specs. Not
even you.

That misses point as well. The voltage controlled model does not ignore
beta in the slightest, so no, I don't ignore it. I have already pointed
these interested to the relevant paper that shows how beta is included.

Indeed, in SS, worst case analyses is automatic with button presses. It
has decent defaults for all main parameter variations, including max/min
beta. http://www.anasoft.co.uk/WCSETUP.GIF

Its not about the physics is about understanding how to design
transistor circuits correctly. To do this, one must understand that
the transistor is a voltage controlled device. The why is
irrelevant. Sure, for a simply switch one might just work out the
base current needed to saturate the device, but for any design that
isnt trivial, the beta model is, essentially, useless. The first
order model for gain doesn't even have a beta term.

Well, maybe yours doesn't, but that's just the equations you prefer,
to make your point.

Its the way the transistor works, and its what you need to do if you are
designing non switching circuits.

This is really all getting a bit tiresome. I have explained this many
times. Base current is an effect caused by an application of voltage.
Without understanding this basic fact, it is impossible to *design*
an amplifier correctly. All one can do is piss about under the
illusion that there was a "design" performed. Period.

So, if A causes B, and B causes C, you conclude that A does *not*
cause C. OK, can't argue any more about that.

Wrong analogy. I have explained many times that is *electric field* that
causes charges to move. Period. It is why it is often referred to as an
*accelerating* potential. It tries to accelerate charges. Base current
does not in any way try to cause an acceleration of emitter charge. This
is so bloody simply. F=qE. End of story.

Look, this *is* how it is.

Apply a voltage to a diode. A current is generated, to wit:

Id = Io.exp(Vd/Vt)

This current will flow through that junction irrespective of how that
voltage gets to the junction.

Now add a junction for the collector. *Nothing* bloody well changes. The
voltage at the base is the same voltage as it was in the stand alone
diode, therefore that junction current will still be set by that
voltage. However, in this case, most of the emitter diode current gets
sucked up into the collector, not the base. The base terminal is just a
convenient way to impress a voltage across a diode junction, but without
actually supplying the current for that junction.

The above is how one really needs to think about basic transistor
operation.

Kevin Aylward
informationEXTRACT@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.

 

[Laurence Payne]

On Sat, 04 Jun 2005 17:39:31 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

That is one reason why we changed to NiMH. The other was cost
as 9V alkalines are really expensive. They rarely go on sale like AA
batteries sometimes do.

The theatre technicians here in London UK seem to be able to source
alkaline 9v batteries at about a third of the retail price of a
Duracell.

 

[Joerg]

Hello Laurence,

The theatre technicians here in London UK seem to be able to source
alkaline 9v batteries at about a third of the retail price of a
Duracell.

That would be 30c US. Really?

Regards, Joerg

http://www.analogconsultants.com

 

[Mike Monett]

Joerg wrote:

Hello Laurence,

The theatre technicians here in London UK seem to be able to source
alkaline 9v batteries at about a third of the retail price of a
Duracell.

That would be 30c US. Really?

Regards, Joerg

http://www.analogconsultants.com

Also, try to develop a connection with local hospitals and clinics. A
friend of mine used to get cases of alkalines (9V, AA, whatever) free.

Apparently, for medical use, they must be discarded by law when they meet
the expiry date. They were still good as new.

Mike Monett

 

[Joerg]

Hello Mike,

Also, try to develop a connection with local hospitals and clinics. A
friend of mine used to get cases of alkalines (9V, AA, whatever) free.

Apparently, for medical use, they must be discarded by law when they meet
the expiry date. They were still good as new.

That is a good idea. Although nowdays they can have over 5 years of
shelf life. Yesterday I replenished the lab with AA and they expire in
May 2012.

Another source that was mentioned here or on a.b.s.e. are photo
processors. Apparently disposable cameras often contain an AA or AAA
cell with lots of life left in it.

Regards, Joerg

http://www.analogconsultants.com

 

[Walter Harley]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:E5Koe.161$Z44.37@newssvr13.news.prodigy.com...

That's right, except that 9V are a lot more expensive per Watt hour than
AA batteries. But the real concern I have with 9V is that even brand new
ones fail a lot. Happened again this week at church. After just a few
minutes a brand new battery went from 9V to zero. They are just too
fickle.

Agree. At the same gig where we've gone through literally tens of thousands
of AAs in wireless mics with not a single problem, we've had probably a 1%
failure rate in 9V batteries for the few wireless units we have that take
those. Same thing as what you describe - they test okay, and then drain
within minutes.

I assume it's because of the multi-cell internal construction of a 9V.

 

[Joerg]

Hello Walter,

Agree. At the same gig where we've gone through literally tens of thousands
of AAs in wireless mics with not a single problem, we've had probably a 1%
failure rate in 9V batteries for the few wireless units we have that take
those. Same thing as what you describe - they test okay, and then drain
within minutes.

I assume it's because of the multi-cell internal construction of a 9V.

That may well be the reason. The first time I looked at how they are
internally connected my confidence level dropped a lot. Statistically
the six cells of a 9V must have a higher combined failure rate than two
AA cells. But not by this much, there must be something else that causes
failures.

The contacts on a 9V are a pain as well. How many times has something
ripped out when disconnecting a 9V battery? A lot...

Regards, Joerg

http://www.analogconsultants.com

 

[Kevin Aylward]

Bob Urz wrote:

Kevin Aylward wrote:

Pooh Bear wrote:

Ol' Duffer wrote:


In article <42A0C72A.68DD168@hotmail.com>,
rabbitsfriendsandrelations@hotmail.com says...

I've never specified matched pairs in any design ( pro-audio btw
) in my entire life despite being responsible for some 10s of
thousands of amplifiers out there. It seems like poor design to
need matched pairs to me.

You should check a batch of power transistors on a curve tracer
sometime. The results may scare you. I routinely see a factor
of three variation in Beta within batches. Of course you can
use big, wasteful swamper resistors, or a bank of 5 unmatched
devices where 2 matched would be sufficient. Or you can let
the amps blow up and they I buy more transistors than I need and
select a good grouping from the middle of the range and fix them
so they don't blow up anymore...

These devices which I currently use for example are pre-graded by
the manufacturer. Worst case match is 2:1 in either gain grade.

http://www.profusionplc.com/cgi-bin/gex/pcatdtl?ipartno=2SC5200-O

A simple low value emitter ballast resistor overcomes the bulk of
beta mismatch anyway and I would never fail to use them. You can't
depend on paralled device temps being identical - in fact quite the
reverse - never mind thermal runaway !



Well, it might be useful to explain just why beta/hfe matching is
important, considering that that the bipolar transistor is a voltage
controlled device!

A bi polar transistor is a current controlled device actually.

Here we go, yet again...

I see that you have succumbed to the Bantam paperback popular
misconceptions Bob. A Bipolar transistor is indeed a voltage controlled
device.

For example, see
http://www.anasoft.co.uk/EE/voltagecontrolledbipolar/voltagecontrolledbipolar.html

and for a simple pictorial description of the actual physics, see:

http://www.mtmi.vu.lt/pfk/funkc_dariniai/transistor/bipolar_transistor.htm

There is simply no mechanism as how a flow of base charge can instigate
a flow of emitter/collector charge. Its applied voltage that that cause
the injection of charge.

The relevent simplified equation is:

Ic ~ Is.exp(Vbe/Vt)

Note the lack of a base current term.

I would suggest obtaining good academic semiconductor physics text book
if you want more details on the correct operation of bipolar
transistors.

Kevin Aylward
informationEXTRACT@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.

 

[Mr.T]

"Anno Siegel" <anno4000@lublin.zrz.tu-berlin.de> wrote in message
news:d8411o$4hj$1@mamenchi.zrz.TU-Berlin.DE...

Mr.T <MrT@home> wrote in sci.electronics.design:

"Jim Thompson" <thegreatone@example.com> wrote in message
news:m3u9a1dl28qjegf4qeshodsjc49mfhnekg@4ax.com...
Be prepared!
That's the Boy Scout marching song
Be prepared!
If you should meet a Girl Scout who is suitably inclined
Be prepared!

What, no attribution for Tom Lehrer?

Perhaps because Tom Lehrer's text is different:

Which word?

MrT.

 

[Rich Grise]

On Sat, 04 Jun 2005 22:08:06 +0000, Joerg wrote:

Hello Rich,

I really want to discourage our church to buy any more 9V gear. It just
doesn't hold up well enough. But Ban's hint regarding the handheld is
great, too, because we also use handhelds and have the same issues there
(battery quits or just fails). Also, Sennheiser should have no problem
designing a similar transmitter for lapel use. I mean, it's not rocket
science. I have built transmitters that worked off 2.4V decades ago.
They could run for days.

Uh, stupid questions department here - why not just use that one?

Because that one was for ham radio and the stuff for secondary user UHF
needs FCC blessing.

Ah.

Thanks,
Rich

 

[BCTweaker]

A good reason is because a 9V can't source as much current as even a single
AA cell for the same duration due to their much smaller individual cells.
There are six seperate 1.5V cells connected in series to produce the 9
volts. The cells internal electrodes have significantly less surface area to
interact with a much reduced quantity of electrolyte. If you still need 9
volts for a circuit try ganging six AA's in series. If weight is a concern,
even AAA cells aranged in a battery of 9 volts will outlast most 9V's.

Regards,
Chris

 

[Tom MacIntyre]

On Sun, 05 Jun 2005 23:00:52 +0100, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

Jim Thompson wrote:

On Fri, 3 Jun 2005 23:22:52 -0700, "Walter Harley"
walterh@cafewalterNOSPAM.com> wrote:

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:d14oe.24267$J12.18509@newssvr14.news.prodigy.com...
BTW, since you seem to be an audio guru: Is there any truly digital
wireless mike system with a reasonable battery life (like >5hrs for the
lapel mikes)? Preferably with AA and not with 9V batteries.

Dunno about digital. The Shure analog UHF packs, with lav mics, last more
than 6 hours with a pair of alkaline AAs. One of my gigs uses a dozen or
more channels of them; we put fresh batteries in at 4:30pm, and at 10:30pm
when the show ends they're usually still showing three or four out of five
bars on the battery life indicator. We replace them every night anyway - if
we went for two nights, by the end of the second night we'd be too nervous.
As jak said, the price of batteries is small compared to the price of the
show going down.


Can I interject and ask some advice?

My wife has lots of Girl Scout speaking presentations, but she's a
walker... walks away from the podium and the microphone.

Did it again last week with me frantically waving, "Go back to the
microphone."

These presentations are usually in not-very-well or anciently equipped
locations... last week was in an old Catholic Church Parish Center.

What should I buy in the way of a wireless microphone, with facility
to plug the receiver into almost any PA equipment I might encounter?

About time to start a new thread ?

It all depends. Depends on your budget and the quality you're looking for.

I'm guessing that budget is low in your case. You *can* get cheap 'voice quality'
radio mics but these aren't a patch on the Sennheisers that Joerg is using.

You get what you pay for for the most part. The receiver should have no trouble
interfacing with any kind of PA gear btw.

I'm tempted to suggest looking on ebay for a cheap unit.

I remember reading years ago that the spoken human voice could be
intelligible at a quite high distortion level. I think the amplifier
in question was 30% or somesuch, but that sounds high even to me now.
Depending on the venue, even room acoustics may outweigh any
distortion issues, I'd guess. Think of the US National Anthem sung at
a sports venue...


"for th or e la and e nd of...you get my drift.

Tom

Graham

 

[Spehro Pefhany]

On Fri, 10 Jun 2005 19:32:17 GMT, the renowned Tom MacIntyre
<tom__macintyre@hotmail.com> wrote:

I remember reading years ago that the spoken human voice could be
intelligible at a quite high distortion level. I think the amplifier
in question was 30% or somesuch, but that sounds high even to me now.
Depending on the venue, even room acoustics may outweigh any
distortion issues, I'd guess. Think of the US National Anthem sung at
a sports venue...


"for th or e la and e nd of...you get my drift.

Tom

Years ago, I heard a demo of the human voice with 1-bit resolution. It
was understandable, but not at all pleasant.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Tom MacIntyre]

On Fri, 10 Jun 2005 15:53:01 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

On Fri, 10 Jun 2005 19:32:17 GMT, the renowned Tom MacIntyre
tom__macintyre@hotmail.com> wrote:



I remember reading years ago that the spoken human voice could be
intelligible at a quite high distortion level. I think the amplifier
in question was 30% or somesuch, but that sounds high even to me now.
Depending on the venue, even room acoustics may outweigh any
distortion issues, I'd guess. Think of the US National Anthem sung at
a sports venue...


"for th or e la and e nd of...you get my drift.

Tom


Years ago, I heard a demo of the human voice with 1-bit resolution. It
was understandable, but not at all pleasant.

So I think our point is the same, or at least similar...the human
voice, when spoken, possibly doesn't require the same low distortion
for reasonable intelligibility.

Tom

Best regards,
Spehro Pefhany

 

[Joerg]

Hello Spehro,

Depending on the venue, even room acoustics may outweigh any
distortion issues, I'd guess. ...

In a sports arena it will also have to do with the amount of booze that
was consumed.

Regards, Joerg

http://www.analogconsultants.com

 

[Rich Grise]

On Sat, 04 Jun 2005 18:42:56 +0000, Joerg wrote:

Hello Graham,

I also found this one sold as a 'kit' with a clip on mic - but it's back to 9V batteries.

I really want to discourage our church to buy any more 9V gear. It just
doesn't hold up well enough. But Ban's hint regarding the handheld is
great, too, because we also use handhelds and have the same issues there
(battery quits or just fails). Also, Sennheiser should have no problem
designing a similar transmitter for lapel use. I mean, it's not rocket
science. I have built transmitters that worked off 2.4V decades ago.
They could run for days.

Uh, stupid questions department here - why not just use that one?

Thanks,
Rich

 

[Richard Crowley]

"Tom MacIntyre" wrote ...

So I think our point is the same, or at least similar...the human
voice, when spoken, possibly doesn't require the same low distortion
for reasonable intelligibility.

And cell-phone manufacturers regularly plumb the depths of
low-res to see how much we will put up with!

 

[Joerg]

Hello Richard,

And cell-phone manufacturers regularly plumb the depths of
low-res to see how much we will put up with!

Yes, it seems so. It could be the carriers since their currency is
kb/sec. Yesterday a long time friend called me on his cell phone. I
could not even recognize who it was until several seconds into the
conversation, and initially only by what he was talking about.

Regards, Joerg

http://www.analogconsultants.com

 

[Rich Grise]

On Fri, 10 Jun 2005 20:25:31 +0000, Joerg wrote:

Hello Spehro,

Depending on the venue, even room acoustics may outweigh any
distortion issues, I'd guess. ...

In a sports arena it will also have to do with the amount of booze that
was consumed.

The human ear + human brain synergy is an amazing tool for picking

information out of noise. Hasn't just about everybody been in a
crowded area with lots and lots of background noise, and yet been
able to figure out what the other person is saying, by sort of
mentally focusing on the subject?

And who amongst us isn't old enough to remember SSB? ;-) You can
pick voice information out of an incredible amount of noise!

Cheers!
Rich

 

[Mr.T]

"BCTweaker" <no.spam@orfrom.you> wrote in message
news:mz3qe.7046$_A5.4114@newssvr19.news.prodigy.com...

A good reason is because a 9V can't source as much current as even a
single
AA cell for the same duration due to their much smaller individual cells.
There are six seperate 1.5V cells connected in series to produce the 9
volts. The cells internal electrodes have significantly less surface area
to
interact with a much reduced quantity of electrolyte. If you still need 9
volts for a circuit try ganging six AA's in series. If weight is a
concern,
even AAA cells aranged in a battery of 9 volts will outlast most 9V's.

Of course, since all of your above comments still apply.

Now try ganging 6 alkaline watch batteries together instead.
There is no mystery here. If one uses exactly the same battery technology in
each case, the larger battery will have more storage capacity.
(packaging excepted of course, and in the case of six separate cells, this
will usually increase)
So the answer may be to use a larger 9V battery. Unfortunately these aren't
common any more. Most people prefer to use the smaller size, and change it
more often.

MrT.