Bob Cordell Interview: Negative Feedback

Jump to Latest
#182
mikeks said:


No, you clearly do not ''understand you correctly''.

Your design is stable because you supplement lead compansation with shunt (lag) compensation.

This is transparently noted here and here.



I have proved this to my satisfaction. I'll leave re-examination to those who deem it necessary.
Click to expand...


Mike, this is really going around in circles. You need to tell me explicitly what "supplemental lag compensation" you are talking about. I discussed the role of C3 and R14 from the very beginning and many times thereafter, so I assume that they are not what you are talking about.

If you are referring to C3 and R14, I have pointed out over and over again that they are for the purpose of stabilizing the inner loop, not the overall amplifier.

Again, semantics. At least you are agreeing that my amplifier, as designed, is adequately stable; then we are apparently just arguing semantics on how it got that way. As I said earlier, call it whatever you want, it works, and it works very well.

Bob
 
#185
mikeks said:


I don't how many times i have to repeat the same thing. This is the last:

Your design is stable because you supplement lead compansation with shunt (lag) compensation.

There. Ciao.
Click to expand...


Mike, you did not answer my simple question. Are you referring to C3 or not?

Else, we will just have to agree to disagree.

You seem to vaporize when you are challenged to put up or shut up. It seems like you never deliver the goods. If you are going to insist on challenging what other people say, you need to learn to back it up, with specifics, what you say; else you are being irresponsible.

Bob
 
#187
Hi Mike, I see you are still so full of yourself that you don’t even consider reading carefully you own exact words, and then crow about counting coup on people actually trying to communicate complex ideas

Looking on the bright side, your abuse of the word “feedforward” and bizarre definition of “feedforward loop” below could gain you a position on a US presidential campaign’s speech writing staff:

mikeks said:


Hi Bob,
…
…By definition, therefore, your compensation cannot confer the same response since, contrary to your assertion that it ''surrounds both the input stage and the VAS stage'', your compensation loop essentially constitutes a feedforward path about the output stage.

Moreover, the input stage is not included in said feedforward loop, as the only active block so-enclosed is the non-inverting output stage.

For these reasons your assertion that is untrue indeed.
Click to expand...


The Big Lie theory of argument?

your use of feedforward in criticizing Bob above is either gibberish or a deliberate distortion

in control theory/amplifier design feedforward is a specific term of the art

feedforward refers to signal/information/energy propagating around a stage in a cascade, carrying information from a node/stage nearer the input to a node/stage nearer the output - but not past the output back to the input! (why else we have the contrasting term “feedback”?)

the only valid electrical engineering construction of your phrase "a feedforward path about the output stage" in a feedback amplifier discussion is that of the signal /energy from a earlier stage bypassing the output stage and directly driving the load

your use here of the term "feedforward" in reference to Bob's C4,R13 compensation components which connect to the NEGATIVE INPUT node of the amplifier as a "feedforward" around the output stage AND the feedback resistor R12 stands the meaning of feedforward on its head

more specifically your "argument" is equivalent to saying that a2 is b2 in the transfer function canonical form:

http://www.mathworks.com/access/hel...helpdesk/help/toolbox/fixpoint/ug/f13041.html
(yes I know it’s a discrete time diagram, the continuous time diagram looks the same, just round the corners on “z” and turn them around)

you are saying white is black and therefore Bob is wrong and you are right - and being obnoxiously arrogant about it in the bargain

I could pass over a colloquial usage of the word “feedforward” but not when you warp that colloquialism into the point of a argument that denies the existence/role of a local negative feedback loop –ie the “normal” view of the circuit as local negative feedback around the diff input/current mirror and vas through C4,R13

Since the compensation components are bilateral linear devices the unilateral block diagram gains don’t correspond to individual circuit branches, C4 and R13 do have both feedforward and feedback information flowing through them, you could even say for 2 different loops – one involving the diff pair/current mirror/vas and the other the output stage (through R12)

Of all 4 abstract feedforward/feedback “signals” I would rate the feedforward from the vas to the output load through C4,R13 and R12 around the output stage dead last in importance – if it could even be somehow disentangled from the degeneracy/codependence of the 4 abstract “signals” on the v,i in the C4,R13 branch
 
#190
jcx said:
Looking on the bright side, your abuse of the word “feedforward” and bizarre definition of “feedforward loop” below could gain you a position on a US presidential campaign’s speech writing staff.
Click to expand...


Hi John,

I see you've been feverishly examining the Control literature to justify your prepared diatribe. 🙂

Last minute examination of texts you're patently not intimately familiar with leads to quotes taken out of context and outright misinterpretation.

I will not demean myself by calling your misrepresentation ''complex ideas''.

If you think Bob's feedforward (YES!!!) PHASE LEAD (YES!!) compensation causes pole splitting, then by all means you're entitled to your religion.

There is much I could say about your post, but for now it is enough to say you really don't know what you're talking about.

By the way i was not ''criticizing Bob'', but merely disagreeing with his viewpoint.
 
#192
Hi Mikeks,
I would really like to read and understand your viewpoint on this and other issues.

But, you habitually refer, in as few words as possible, to statements that are likewise compiled.

Please elaborate and convince us to change our mistaken opinion.

Don't be afraid to use the English language, it was invented to help with discussion (among other things).
 
#193
mikeks said:
Try it.🙂
Click to expand...

Well... you've made me thinking, how to measure pole spliting then 😕
Preferably without sophisticated spectrum analyser?
Maybe it would be easier if you told me how pole splitting occurs in Miller compensation and how things go different compared to other types of compensation lowering output Z of VAS?
 
#195
jcx said:
"Pole splitting" is a heuristic description of the "motion" of the open loop gain poles as you introduce a compensation element

a test for "pole splitting" would be to plot the poles and zeros of the open loop response as the compensation element impedance is varied from infinity to the design value

at least one alternate viewpoint exists:

http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf
Click to expand...


This is a good paper. It shows, among other things, that the concept of pole splitting is just an alternative way to looking at the circuit formed by the Miller feedback capacitor as a minor feedback loop. This latter view is the way that I usually view the modified form of "Miller" compensation that I use in my amplifier: It is a minor ("inner") feedback loop whose closed loop response gives the overall open-loop response of the amplifier the desired Miller integrator frequency response with a dominant 6 dB per octave rolloff over many decades.

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