power amp driver block topology?

mlloyd1

Member
Paid Member
2001-02-25 7:10 pm
Northern Iliinois
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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mikek

Disabled Account
2002-05-15 1:12 pm
Nelson Pass said:
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...
 

mikek

Disabled Account
2002-05-15 1:12 pm
mikek said:



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...


mlloyd1 said:
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.
 
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......
 

mikek

Disabled Account
2002-05-15 1:12 pm
Re: Common base?????

Fred Dieckmann said:
"...'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.
 
"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.
 

DrG

Member
2003-08-18 7:36 pm
Pretoria
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...
 
Keeping it Technical

"who i am sure doesn't need Fred Dieckman pontificating ill-informed rubbish on his behalf"

You mean a Technical disscussion like that?

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npncb.html#c1

http://www.odyseus.nildram.co.uk/Cascode_BJT_Circuit.htm

A cascode uses a common base transistor to minimize the voltage varations across the other transistor in the cascode. It doesn't have a signal driving the base of the transistor that provides the cascode. All the temper tantrums in the world are not going to change the definition of a cascode or a common base circuit. Pick some resistor values and compare the circuits with Spice. Go back to couple of text books and read what cascode circuits are designed to do. I would like to discuss this circuit but I am not going to put up with temper tantrums because someone does not understand a circuit and would rather resort to diversionary tactics than actual disscuss the circuit in any detail. I have been to many design reveiws and have never seen the design choice dictated on the merit of who could shout the loudest. You guys are bitching at the wrong guy..........

BTW is there anybody actually trying to disscuss the circuit but me at this point?
 
I agree with Nelson Pass on this. I have used that connection since 1973 or even before. The folded cascode connection is mainly trivial, but it doesn't appear to hurt anything and probably gives a little more gain. The way to test this would be to make a Spice simulation and then add a very large cap from R(1) to ground. Compare with and without the added cap.
 
Fred, what you say is correct, 100% on the money. No, I am never 100%....

If people will focus on content instead of personalities we would not have to go down this road. I have been reasonable and still been abused. It seems that who's signature is at the bottom matters more than the content. I am thrilled to have Mr. Curl comments as I have seen this topology before and wondered about it relative benefits. I will offer a preemptive apology for the real possibility a Mr. Know-it-all will be along to tell him he doesn't know what he is talking about as well. If the guy is smarter than Nelson he must by smarter the John too. I go on about cascodes since the subject of what constitutes a real cascode with its benefits in bandwidth and linearity. I was even talking to JAM on the phone about cascodes on the phone yesterday. No name calling or hurt feelings were involved. Ask him. Once again....... you guys are bitching at the wrong guy.