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Old 12th February 2002, 10:53 PM   #1
mlloyd1 is offline mlloyd1  United States
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Question power amp driver block topology?

OK, I'm looking for your thoughts, comments, rationale, etc. Consider the two basic topologies below. They are one side of a complementary symmetric design (I wanted the image to be small so both sides not included). The left side uses a folded cascode (seen in Borbely, McIntosh, Mark Levinson, etc. products); the right side does not (seen in Leach, SAE, Bryston, etc. designs). Is either inherently "better" than the other as configured here (yes, I know there are other things you can do to enhance performance to both - like make Q1 & Q2 JFETs, add a cascode Q4, replace R3 with a constant current sink, etc. ). Assuming the same devices, same bias points, same compensation, same output stage, etc.?

How does the distortion spectra change (notice I'm NOT asking does one have lower THD than the other)? Maybe you folks with simulators have already played around with such a question.

Well, what do you think?

mlloyd1
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Old 13th February 2002, 06:35 PM   #2
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I would not consider the one on the left a folded
cascode. Q3 driving Q4 is where the action is,
and the folded cascode part of the circuit is
trivial, and as such there is very little difference
between the two designs.
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Old 16th September 2003, 08:13 PM   #3
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Quote:
Originally posted by Nelson Pass
I would not consider the one on the left a folded
cascode. Q3 driving Q4 is where the action is,
and the folded cascode part of the circuit is
trivial, and as such there is very little difference
between the two designs.

Actually, it is a folded cascode...somewhat contrary to intuition.....The common-base connection at Q4's base has a profound effect on open-loop gain; greatly reducing it beyond the ability of Q3 to effect much improvement......compared to the traditional approach on the right...
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Old 16th September 2003, 08:25 PM   #4
mlloyd1 is offline mlloyd1  United States
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Intriguing ... would you say more ....
mlloyd1

Quote:
Originally posted by mikek
....The common-base connection at Q4's base has a profound effect on open-loop gain; greatly reducing it beyond the ability of Q3 to effect much improvement......compared to the traditional approach on the right...
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Old 16th September 2003, 09:09 PM   #5
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Quote:
Originally posted by mikek



Actually, it is a folded cascode...somewhat contrary to intuition.....The common-base connection at Q4's base has a profound effect on open-loop gain; greatly reducing it beyond the ability of Q3 to effect much improvement......compared to the traditional approach on the right...

Quote:
Originally posted by mlloyd1
Intriguing ... would you say more ....
mlloyd1


Apologies....that should have been...'the common base connection at Q4's emitter....'....and not ...'at Q4's base...'.....cheers.
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Old 16th September 2003, 10:23 PM   #6
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Default Common base?????

"...'the common base connection at Q4's emitter....'....and not ...'at Q4's base...'.....cheers."

What common base connection at the emitter of Q4?????

For the circuit on the left:

Q4 is not a common base configuration the base is driven by the signal from R1.
The AC current through R2 is largely determined by Q4. R2 looks like emmitter
degeneration for Q4.

In the circuit on the right R6 provides the emitter degeneration for Q4. I don't think it should come as a suprise to anyone that Mr. Pass knows what he is talking about. Well maybe maybe for one confused person here......
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Old 16th September 2003, 11:07 PM   #7
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Default Re: Common base?????

Quote:
Originally posted by Fred Dieckmann
"...'the common base connection at Q4's emitter....'....and not ...'at Q4's base...'.....cheers."

What common base connection at the emitter of Q4?????

For the circuit on the left:

Q4 is not a common base configuration the base is driven by the signal from R1.
The AC current through R2 is largely determined by Q4. R2 looks like emmitter
degeneration for Q4.

In the circuit on the right R6 provides the emitter degeneration for Q4. I don't think it should come as a suprise to anyone that Mr. Pass knows what he is talking about. Well maybe maybe for one confused person here......
Q4's emitter is driven by Q2's collector....which make the whole a folded cascode-effectively a single-stage design as far as open-loop gain is concerned.

.......Now mr. Pass, (who i am sure doesn't need Fred Dieckman pontificating ill-informed rubbish on his behalf), suggests that the double-ended drive to the cascode transistor by the emitter follower, Q3, renders the the common-base conection to Q4 irrelevant, which implies that the circuits characteristics are now indistinguishable from those of the circuit on the right.

This is not true.
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Old 17th September 2003, 01:29 AM   #8
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"This is not true."

Is this the best go can do for a technical discussion? Respectfully, I would go look up the meaning "common base." Even better, I would go plug some resistor values in a Spice model and compare the gains. Stop acting like a child and discuss things in actual technical terms, instead of throwing temper tantrums. Are you going to call the next person to discuss this circuit or disagree with you an idiot? If you want to analyze circuits instead of trying to intimidate people, I am all ears. If you want to throw fits like an insecure little boy, go elsewhere. I believe SY made a suggestion along these lines in another thread. Make your case in engineering analysis if you are convinced of your viewpoint. Attacking others credibility is doing nothing to increase yours. There is an old saying..... "put up or shut up." We are all waiting............

Fred


PS Perhaps Mr. Lloyd has some particular transistors and resistor values in mind for a more detailed comparison of the two circuit approaches. It is a simple enough circuit that any number of people on the forum could run a Spice simulation to gain some specific insight to the circuits.
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Old 17th September 2003, 12:34 PM   #9
DrG is offline DrG  South Africa
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Now I'm the kinda guy who likes a good verbal tet-a-tet but Freddie takes the cake. Typical of a loud-mouthed weenie protected by the separations of cyberspace.

Debate ISSUES, not PEOPLE and try to do so without INSULTING or being a general DIECK-HEAD, Mr Dieckmann...

With apologies to our moderators...
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Old 17th September 2003, 12:43 PM   #10
Bricolo is offline Bricolo  France
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Quote:
Originally posted by DrG
Now I'm the kinda guy who likes a good verbal tet-a-tet but Freddie takes the cake. Typical of a loud-mouthed weenie protected by the separations of cyberspace.

Debate ISSUES, not PEOPLE and try to do so without INSULTING or being a general DIECK-HEAD, Mr Dieckmann...

With apologies to our moderators...

Tête à tête
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