...food for thought...: 
http://www.tde.lth.se/home/mlz/paper02c.pdf
....and for dessert...
http://www.tde.lth.se/home/mlz/paper02a.pdf
http://www.tde.lth.se/home/mlz/paper02c.pdf
....and for dessert...
http://www.tde.lth.se/home/mlz/paper02a.pdf
BTW, jcx, I bought the Wambacq and Sansen book on your recommendation and am in the process of reading it now. It's the most expensive book I've ever bought! But it's excellent. I hadn't studied the Volterra series at all previously. It's a real eye opener, opening up the door to understanding of frequency-dependent nonlinearities. One thing I wish I had access to is the Schetzen text that the authors reference over and over. I tried to buy it at Amazon but it's unfortunately out of print.
I have read the Cherry articles, but there's something about him that gives me the creeps. The tone of his amplifier distortion article is often defensive, and he tends toward hand-waving. Perhaps these two traits go hand in hand? Wambacq and Sansen are anything but hand-waving, so I don't switch into my skeptic mode when reading them.
Books are described here for anyone interested:
http://www.amazon.com/exec/obidos/t...f=sr_1_4/103-2533527-8819043?v=glance&s=books
http://www.amazon.com/exec/obidos/t...f=sr_1_1/103-2533527-8819043?v=glance&s=books
I have read the Cherry articles, but there's something about him that gives me the creeps. The tone of his amplifier distortion article is often defensive, and he tends toward hand-waving. Perhaps these two traits go hand in hand? Wambacq and Sansen are anything but hand-waving, so I don't switch into my skeptic mode when reading them.
Books are described here for anyone interested:
http://www.amazon.com/exec/obidos/t...f=sr_1_4/103-2533527-8819043?v=glance&s=books
http://www.amazon.com/exec/obidos/t...f=sr_1_1/103-2533527-8819043?v=glance&s=books
Andy,
You raise an interesting point about Cherry.
Years ago I built his NDL amp and it was utterly unstable. I tried everything. A few years later I saw an article on fixing the amplifier. It had so many stabilizing bandaids, many of them inductors, it was unbelievable. He copped a lot of flack for that design.
A fellow I met five years back took out a patent on nested feedback loops, the so-called J-loop amplifier. As he put his patent to rest he went to meet Cherry. The good professor's attitude was duh, why didn't I think of this, then abject resentment towards the fellow who had. He had the impression he was a rather bitter man.
What was that amusing quote about academia? 'And the fighting was bitter, and malicious, and never ending because the stakes were so low'.
Cheers,
Hugh
You raise an interesting point about Cherry.
Years ago I built his NDL amp and it was utterly unstable. I tried everything. A few years later I saw an article on fixing the amplifier. It had so many stabilizing bandaids, many of them inductors, it was unbelievable. He copped a lot of flack for that design.
A fellow I met five years back took out a patent on nested feedback loops, the so-called J-loop amplifier. As he put his patent to rest he went to meet Cherry. The good professor's attitude was duh, why didn't I think of this, then abject resentment towards the fellow who had. He had the impression he was a rather bitter man.
What was that amusing quote about academia? 'And the fighting was bitter, and malicious, and never ending because the stakes were so low'.
Cheers,
Hugh
Spot on!!
....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!!
AKSA said:
....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!!
andy_c said:
Actually, i prefer Nordholt:
http://www.amazon.com/exec/obidos/t...f=sr_1_1/103-4802095-0116660?v=glance&s=books
andy_c said:
if you are refering to his Electronics World articles on distortion, then you do have a point in some specific areas......
...for instance, despite Cherry's persistent screams to the contrary, output stage inclusive compensation with power BJT's IS unstable...Period.
...Moreover, his 'modification' of Thiel's load stabilizing network invariably results in a grossly unstable global feedback loop...Period.
Re: Spot on!!
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 said:
....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
Re: Re: Spot on!!
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....
janneman said:
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....
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
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
janneman said:
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.
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