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Amplifier transistor matching?

R

Rich Grise

Jan 1, 1970
0
Hello Rich,


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

Ah.

Thanks,
Rich
 
R

Rich The Newsgroup Wacko

Jan 1, 1970
0
Kevin Aylward wrote:

Tastes Great!
A bi polar transistor is a current controlled device actually.
A small base current is used to control a larger collector current.
A FET is a voltage controlled device.

Less Filling!
--
Cheers!
Rich
------
"There was a young lady of fashion
Who had oodles and oodles of passion.
To her lover she said,
As they climbed into bed,
"Here's one thing the bastards can't ration!""
 
J

John Larkin

Jan 1, 1970
0
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. 90% of the time, a simple 0.6Vbe-plus-Beta model is the
easiest and *most reliable* way to design transistor circuits.

This is sci.electronics.design, not sci.device.physics. If I had to
analyze the physics of every component I use, I wouldn't get anything
done. What matters is how they behave.
What we have here is an enormous amount of erroneous waffle on the
bipolar transistor. Its quite amazing really. There is no point in
perpetuating this daft myth. It only leads to confusion.

Any *actual* *understanding* of the standard 3 junction pictorial
descriptions of basic transistor operation should leave people with no
doubt as to the correct operation of a bipolar transistor. Base current
don't "cause" the flow of collector/emitter current.

That's like saying that poison doesn't cause death because only heart
failure *really* causes death.

John
 
J

Joerg

Jan 1, 1970
0
Hello Jim,
My inclination is to a multiple-microphone version for later
expansion, since they do a lot of Flag Ceremonies with multiple
speaking parts.

The Sennheiser system does that nicely. Currently we have four but I can
see it going to 6-8. However, we'll only do that with AA battery powered
gear, no more 9V.

On our current gear the transmitters show their frequency on a little
LCD, same for the receivers. Receivers also show RF level and audio
level. The latter makes it really easy to see if a mike is active
without the mixer pot turned on.

Regards, Joerg
 
K

Kevin Aylward

Jan 1, 1970
0
John said:
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.

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?

Hint: Rbb' drops a voltage to Vibe, Ic = Io.exp(Vbi/Vt)
This is sci.electronics.design, not sci.device.physics. If I had to
analyze the physics of every component I use, I wouldn't get anything
done. What matters is how they behave.

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.

http://www.anasoft.co.uk/EE/bipolardesign1/bipolardesign1.html

To wit,

Av = RL/re = 40Vdc

Av max = Va/Vt.
That's like saying that poison doesn't cause death because only heart
failure *really* causes death.

No it isn't. Base current is not equivalent to poison, its equivalent to
the body stinking after the heart failure. i.e. its just a nuisance.

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.

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

Kevin Aylward

Jan 1, 1970
0
Roger said:
Most of us know how to design a simple bipolar transistor stage using
the current model.

People believe they do.
The design equations and explanations need just a few rows of text.

As do the voltage controlled equations.
How would you design the same transistor stage using the
voltage-controlled model?
http://www.anasoft.co.uk/EE/bipolardesign1/bipolardesign1.html


Can you show us how your way is faster, better, etc..?

It is the correct way. Once you get to really designing complex
transistor level circuitry, it will become obvious. All basic ac
understanding is based on gm, that is, vo=gm.vb
How does it work in real transistor stage design?

Its what real professional analogue designers do that actually
understand transistor level design. Unfortunately, they are many that
only believe they understand. They dont.

I have given the basics in the above link.

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

John Larkin

Jan 1, 1970
0
The voltage controlled model is the simple model, and the correct one.


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


Then I owe somebody about $200 million.

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

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

John
 
K

Kevin Aylward

Jan 1, 1970
0
John said:
Then I owe somebody about $200 million.

You must be switching a lot of relays;-)
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

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.
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
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
L

Laurence Payne

Jan 1, 1970
0
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.
 
J

Joerg

Jan 1, 1970
0
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
 
M

Mike Monett

Jan 1, 1970
0
Joerg said:
Hello Laurence,


That would be 30c US. Really?

Regards, Joerg

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
 
J

Joerg

Jan 1, 1970
0
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
 
W

Walter Harley

Jan 1, 1970
0
Joerg said:
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.
 
J

Joerg

Jan 1, 1970
0
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
 
B

BCTweaker

Jan 1, 1970
0
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
 
R

Rich Grise

Jan 1, 1970
0
There is no practical example on that web page, and it surely does not
look like a simpler or faster way to design a transistor stage.

I once slapped together a textbook common-emitter amp because I
needed a quick-n-dirty mic preamp. I used a constant .6V B-E
drop and picked bias resistors based on beta, albeit swamped
out my expected base current by about a factor of 10.

Sorry if I did it "wrong", but I got it done in 10 minutes, and
it worked. ;-)

Cheers!
Rich
 
T

Tom MacIntyre

Jan 1, 1970
0
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
 
S

Spehro Pefhany

Jan 1, 1970
0
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
 
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