Bob Cordell's Power amplifier book

Jump to Latest
#4,927
I've been lucky to have worked with many talented EE's and SSP PhD's over the last 30 odd years. Very, very few of them were arrogant. Confident, yes.

You don't make a same for yourself in STEM by hammering those less educated than yourself, or less competent. You do it by making or writing things whose value is recognized by your peers and those in the wider community.
 
#4,930
Bonsai said:
IIRC it's buried somewhere in the CFA vs VFA thread, but I could be wrong.
Click to expand...
What is IIRC? Is CFA and VFA current feedback amp and voltage feedback amp?

I saw a topology suggested by Accuphase that looked like the circuit for those current feedback by the former Comlinear and Elantec amplifiers. I meant to read the article but just too caught up with the tube stuffs lately.

You guys have special acronyms in audio field I don't understand!!!
 
#4,934
Bob Cordell said:
Good point,

I have found this to be helpful in some cases as well. I have often used a Zobel-like R-C combination with R in the range of 100 ohms. Output Triples like to be driven by a resistive-looking source of reasonably low impedance at high frequencies. The low output impedance at high frequencies naturally provided by the shunt feedback of Miller compensation is often sufficient, but not always, since any feedback-created output impedance will usually ultimately rise with frequency and look somewhat inductive. The C shunt or R-C shunt tends to take over from the VAS output impedance at higher frequencies, mitigating this concern.

Cheers,
Bob
Click to expand...

Spot on explanation.

Bob Im not sure about "the some cases", triples are mostly used by Japanese designers and frankly I havent come across one of their commercial designs that doesnt include some shunt compensation for this very purpose - lowering of impedance. In contrast I see many here complaining about stability issues with triples.
 
#4,936
Waly said:
Such a cap has little to nothing to do with stabilizing the triple, but with compensating the Miller loop which, in particular if a beta enhancer is used, may have stability issues. If you disagree, I would be happy to listen to any proof you may have. I am in particular curious how do you analyze the triple stability and estimate the stability margins 😛.

Even so, simply a shunt cap is an unnecessary brutal Miller loop compensation method; a lead lag network (that is, a RC series) will save precious loop gain, while providing a very similar stability margin.
Click to expand...

This was already discussed in another thread.

I also gave you the best reference on the subject, Willy C M Sansen. In case you dont know who he is :

"Willy Sansen received a PhD degree from U.C.Berkeley in 1972. Since 1980 he has been full professor at the Catholic University of Leuven, in Belgium. From 1984 to 2008, he has headed the ESAT-MICAS. He has been supervisor of sixty-four PhD theses and has (co)authored more than 650 publications and 15 books among which the Powerpoint slide based book “Analog Design Essentials” (Springer 2006). He was Program chair of the ISSCC-2002 and President of the IEEE SSCS in 2008-2009. He received the D.O.Pederson award in 2011. He is a Life Fellow of the IEEE. "

Do yourself a favor and read up on his analysis of shunt compensation and why it can be so useful when applied correctly.

BTW youre a fan of TPC. What else is it but basically two shunts to ground ??

See D Self s new book where he touches on this subject as well.
 
Last edited:
#4,938
manso said:
This was already discussed in another thread.

I also gave you the best reference on the subject, Willy C M Sansen. In case you dont know who he is :

"Willy Sansen received a PhD degree from U.C.Berkeley in 1972. Since 1980 he has been full professor at the Catholic University of Leuven, in Belgium. From 1984 to 2008, he has headed the ESAT-MICAS. He has been supervisor of sixty-four PhD theses and has (co)authored more than 650 publications and 15 books among which the Powerpoint slide based book “Analog Design Essentials” (Springer 2006). He was Program chair of the ISSCC-2002 and President of the IEEE SSCS in 2008-2009. He received the D.O.Pederson award in 2011. He is a Life Fellow of the IEEE. "

Do yourself a favor and read up on his analysis of shunt compensation and why it can be so useful when applied correctly.

BTW youre a fan of TPC. What else is it but basically two shunts to ground ??

See D Self s new book where he touches on this subject as well.
Click to expand...

You never provided any reference, other than dropping Prof. Sansen name in the discussion (and that name was guessed by myself based on your vaguely spelled name http://www.diyaudio.com/forums/solid-state/240712-cfa-topology-audio-amplifiers-588.html#post3902276). Shunt compensation is your pet peeve, however this doesn't make any anywhere optimal when it comes to audio amplifiers.

Your interpretation of TPC shows exactly where you are in understanding frequency compensation. No, I'm not a fan of TPC, I'm a fan of optimal solutions. Shunt compensation, at least in the case of audio amplifiers, is not one. This was discussed ad nauseum by Mr. Self, Mr. Cordell, etc...

You desperately need to understand Miller compensation and why the Miller loop may need some compensation as well (and yes, the OPS input impedance may play a role in this). Using a pure shunt cap at the VAS output is nothing but a waste of otherwise perfectly good and useful loop gain, a lead-lag network makes sense.
 
#4,939
manso said:
Spot on explanation.

Bob Im not sure about "the some cases", triples are mostly used by Japanese designers and frankly I havent come across one of their commercial designs that doesnt include some shunt compensation for this very purpose - lowering of impedance. In contrast I see many here complaining about stability issues with triples.
Click to expand...

Good point. I used such a VAS output Zobel on the power amplifier example I showed in my Linear Systems LSK489 dual monolithic JFET application note here:
http://www.linearsystems.com/assets/media/application_notes/LSK489_Application_Note.pdf

I have found that the extent to which it is needed does depend on the details of the VAS and even sometimes the details of the current mirror load of the IPS in front of the VAS. If that current mirror is implemented with a third EF "helper" transistor, I have sometimes noted an increased need for the Zobel.

Cheers,
Bob
 
You must log in or register to reply here.
Top Bottom