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Old 2nd December 2006, 11:33 AM   #1111
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Default Re: Re: Re: Re: The bam challenge

Quote:
Originally posted by ingrast


Extremely insightfull.

While A behaves, the loop sits quietly there unobstrusively.
The least deviation, it wields a big to put things back in short order.

Another attribute of ec. You cut the ec loop, and the amp happily continues to work as if nothing happened, albeit with less linearity. Try that with a convential global nfb amp!

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Old 2nd December 2006, 01:06 PM   #1112
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Quote:
Originally posted by traderbam
Your humour is very dry...

err.. I think it actually is called 'imagination' . You may wish to look that up

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Old 2nd December 2006, 01:27 PM   #1113
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Jan wrote:
Quote:
Another attribute of ec. You cut the ec loop, and the amp happily continues to work as if nothing happened, albeit with less linearity. Try that with a convential global nfb amp!
Cut the all the NFB and it will go nuts. Bob's output stage will fry itself. Bob's circuit is different from the pure Hawksford model - it two halves are cross-connected so there is common-mode NFB that sets the output stage bias. The linearity degrades when the differential feedback path is disconnected.

Believe what you want. But my question is...if you were convinced it was just NFB in disguise, would you choose to configure your output stage like this? Is this the best way you could devise for applying 30dB of NFB to an output stage?

The trouble is, if you really believe there is something more than NFB at work, you'll restrict your thinking. You'll insist on two feedback paths rather than one, you'll insist on having an adjustment you don't need and you'll insist on using a positive feedback loop to generate gain, which has the drawback of amplifying its own distortion.

I've demonstrated by commonly accepted maths that its theoretical function is that of a NFB system, no more, no less. No one has yet shown fault in my maths. Bob agrees with it too and it's his circuit.

I am seeking to be persuaded by practical evidence, simulation result or rational argument that implementing a 30dB NFB loop around a FET output stage is BEST implemented by this "EC topology" but at the moment I have my doubts.

I am eager to see any evidence from you or Rodolfo as to why you think a PFB loop around a unity gain buffer makes a superior gain stage in a real circuit. I think that would be a really useful thing to share.

Brian
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Old 2nd December 2006, 01:54 PM   #1114
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Quote:
Originally posted by traderbam
[snip]I am eager to see any evidence from you or Rodolfo as to why you think a PFB loop around a unity gain buffer makes a superior gain stage in a real circuit. I think that would be a really useful thing to share.

Brian

Brian,

I will share my circuit, but not yet. I have my reasons, and I hope you will accept that.

In the mean time, I think Bob has mentioned that the ec ONLY on his output stage did dramatically decrease distortion. And that is with using two transistors that were there anyway for the bias. Or at least one of them.

Is that not worthwhile?

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Old 2nd December 2006, 02:25 PM   #1115
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Quote:
I will share my cicuit, but not yet. I have my reasons, and I hope you will accept that.
Of course.
Quote:
In the mean time, I think Bob has mentioned that the ec ONLY on his output stage did dramatically decrease distortion.
By about 30dB.
Quote:
Is that not worthwhile?
Sure it is.
Is this the best way to implement NFB around a FET output stage? Maybe it was in 1984. I don't think it is now. I could be wrong, but I want to find out.

NFB around an output stage is not a new idea. Bob's version of Hawksford's circuit is interesting to me not least because it uses positve feedback and it doubles up the function of the bias transistors. What I don't really like on the face of it is that it needs 10 transistors, it needs adjusting, it isn't very symmetrical (electrically), and at its nominal feedback setting it has poor stability into capacitive loads - so it needs an output inductor.
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Old 2nd December 2006, 02:36 PM   #1116
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The counterargument:

Hawksford/Cordell error correction is a 2 degree of freedom linear feedback control system,

All expressions of 2 degree of freedom control systems are equivalent.

Therefore Hawksford/Cordell error correction is equivalent to conventional error feedback with a command prefilter

Therefore the output linearizing disturbance rejection effect is exactly equivalent to error feedback and has the same gain-bandwidth/stability limits of ordinary 1 degree of freedom error feedback

A little reading: (slow to load, ~ 500K )

Click the image to open in full size.

Click the image to open in full size.

[I've referenced this a few time before in this thread I.M. Horowitz, "Synthesis of Feedback Systems" 1963 - that mkes this a 40+ year old "controversy"]

I like Bob's circuit, I do think understanding it as a local feedback loop and tailoring its loop gain and frequency response is likely to be a good way forward
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Old 2nd December 2006, 02:49 PM   #1117
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Hawksford is a negative feedback loop without any positive feedback as shown here and here.

This was demonstrated above with the bare minimum of intuitive Control theory.

Therefore, Julian, I don't think further invocation of Control paradigms will convince Traderbam.
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Old 2nd December 2006, 02:51 PM   #1118
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By the way, Julian, who authored that reference?
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Old 2nd December 2006, 03:02 PM   #1119
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Quote:
Originally posted by jcx
The counterargument
It's not clear to me whose argument you're countering.
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Old 2nd December 2006, 03:09 PM   #1120
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Quote:
Hawksford is a negative feedback loop without any positive feedback. This was demonstrated above with the bare minimum of intuitive Control theory.
I think it is your maths that has made your diagrams incorrect. Your are manipulating non-linear elements as if they were linear.
I think if jcx wishes to correct any of my reasoning he is capable of doing so himself and I warmly invite it. I don't see any conflict with what I have said and what jcx has said.
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