'Local Feedback and Nonlinearity of Multistage Feedback Amplifiers'

[mikeks]

Originally posted by AKSA
'And the fighting was bitter, and malicious, and never ending because the stakes were so low'.

Lovely!

[mikeks]

Originally posted by AKSA
.....A fellow I met five years back took out a patent on nested feedback loops, the so-called J-loop amplifier......

..More info. please??

[jan.didden]

Originally posted by mikeks


....actually, the design in Cherry's patent is utterly unstable.....

...if you are going to publish a nested feedback amp. (or indeed anything!!), surely one is entitled to expect that the feedback loops in said amp. are each shown to be stable, and that in fact the whole is stable with any transducer.

....at least two of the minor-loops, AND the global feedback loop are demonstrably unstable.....(for sound technical reasons).

...Should have done your homework Prof. Cherry!!

Mike,

I don't want to claim to understand this 100%, but the way I read Cherry is that the nested thing is stable as a system, but not that each loop in itself necessarily must be stable. Is that not so?

Jan Didden

[mikeks]

Originally posted by janneman

Mike,
....the nested thing is stable as a system, but not that each loop in itself necessarily must be stable......?
Jan Didden

Hi Jan,

Here is a good article which demonstrates why all loops, (internal or external) in a nominally linear amp. should be stable....:

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

...alternatively, get a copy of Roberge's excellent book....(out of print i fear!)

To answer your specific question, a global feedback loop can quite easily posses adequate stability margins, while some internal loop is merrily oscillating like a banshee...

This will undoubtedly show up in the amp's. step response, which will be utterly compromised....

[jan.didden]

Mike,

One of my (actually, my only) masterpiece was a poweramp with an internal POSITIVE feedback loop, definitely unstable! But with the outer loop nfb closed, the thing was tame as a lamb, no overshoot, perfect.
The problem was that at clipping, the nfb loop went (of course) inoperative, but the pos fb loop which had more dynamic range merrily chugged away. The output pegged to one of the supplies (usually the pos) and stayed there, until power down.

I managed at one time to blow two woofers within 3 minutes until my nickel dropped.

I read your reference; interesting paper, but says nowhere that all loops in itself must be stable. In fact, I can come up with a few articles that prove the opposite.


Jan Didden

[mikeks]

Originally posted by janneman

The problem was that at clipping, the nfb loop went (of course) inoperative, but the pos fb loop which had more dynamic range merrily chugged away. The output pegged to one of the supplies (usually the pos) and stayed there, until power down.

I managed at one time to blow two woofers within 3 minutes until my nickel dropped.

Jan Didden

Precisely....

Good reason to avoid Linsley-Hood's 1983(WW) design methinks...

[jan.didden]

..and it proves precisely my point, contrary to yours, I thought, that each loop doesn't have to be stable as long as the system is stable. Right.

And now I am confused, because in your post # 14 you say both...?

Jan Didden

[mikeks]

Originally posted by janneman
..and it proves precisely my point, contrary to yours, I thought, that each loop doesn't have to be stable as long as the system is stable. Right.

And now I am confused, because in your post # 14 you say both...?

Jan Didden

Not really....nominally stable ( in 'small-signal' terms) major feedback loop does not mean internal loops are stable....

..which is why executing a large signal step response is usually neccesary to ensure that all is well when the global feedback loop is disabled by clipping for instance....as you have so eloquently pointed out.

[mikeks]

...read section 6 here:

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

[andy_c]

Originally posted by mikeks
if you are refering to his Electronics World articles on distortion, then you do have a point in some specific areas......

I was referring to the article jcx cited,

"Estimates of Nonlinear Distortion in Feedback Amplifiers".

I have the pdf, so if anybody's interested just shoot me an email.