Really? I'm listening to a very nice amplifier with no loop and no
degeneration. Where's the feedback in that?
Really? I'm listening to a very nice amplifier with no loop and no
degeneration. Where's the feedback in that?
Really, even including what can be internal to the semiconductors? Does the amplifier have voltage gain?
ES
I'll be sure to pick it up next paycheck, Bob. Can always use another good tech book for the library.
Great info for me, Bob. I guess you could say that, in a perfect world, there would be absolutely no reason for negative feedback, and everything would be stable as hell without the use of it. Actually, I can see this happening in the not-too-distant future, where nano-tech graphene chips are monitoring linearity, with circuitry making minute adjustments to make absolutely sure things stay that way, but that's a ways off yet
BTW, are u guys fired up about graphene, and have u considered the ramifications of it's use at audio? I can only imagine what a great amp is going to sound like in the next few years
Nice typing at ya, Bob, and best regards...
Randy
Hi Randy,
I think most things in nature have some sort of negative feedback if you look closely enough in the right way. It is certainly true that the process of driving a car involves negative feedback. I would hate to be on the road with someone who was driving open-loop.
Come to think of it, when someone is texting they are driving open-loop. We know what happens to some of them.
With respect to graphene, each new technology brings with it its own promises and its own challenges. When decent power BJTs came along we thought that SS amps would always win, hands down. Then we found out otherwise - they had their own set of problems.
Cheers,
Bob
The argument that there is no such thing as no-feedback contributes very
little. Perhaps the patent office really should have denied Black's patent
based on the idea that nature has already imposed loop feedback and
degeneration internal to gain devices.
While I'm at it, I will repeat my assertion that loop feedback and degeneration
are two different things. They share some common effects, but each has
unique attributes.
The argument that there is no such thing as no-feedback contributes very
little. Perhaps the patent office really should have denied Black's patent
based on the idea that nature has already imposed loop feedback and
degeneration internal to gain devices.
While I'm at it, I will repeat my assertion that loop feedback and degeneration
are two different things. They share some common effects, but each has
unique attributes.
Black did an interview where he explained the difficulty in getting the patent. It seems up until then everyone knew that feedback always resulted in oscillation!
No I haven't followed your definitions of feedback. The start of the reasoning was that it is impossible to build an amplifier without feedback. If you define that as a loop then the answer is a trivial of course you can and there are lots of examples. I thought you might actually have come up with a clever way of avoiding all versions.
The argument that there is no such thing as no-feedback contributes very
little. Perhaps the patent office really should have denied Black's patent
based on the idea that nature has already imposed loop feedback and
degeneration internal to gain devices.
While I'm at it, I will repeat my assertion that loop feedback and degeneration
are two different things. They share some common effects, but each has
unique attributes.
Hi Nelson,
I agree with you; I'm sure neither Black nor the patent office had local degeneration in mind. Its really all a matter of degree. Some rule out global NFB, some rule out local loops, some rule out DC servos - to each his own.
Personally, if I was going to market an amplifier that claimed to have no negative feedback, I would be OK with using emitter degeneration for sure.
However, for those who talk about re-entrant distortion (after Baxandall), it is important for them to understand that the re-entrant distortion mechanism exists just as much in emitter degeneration as it does in global feedback. The re-entrant distortion argument does not really hold in the real world anyway, so this argumemt is a bit academic.
Cheers,
Bob
Re - Negative Feedback
Another great book, with Lots of info/data/circuits etc, is Douglas Self's - Audio power amplifier design handbook
According to him in there,
Chapter 2 - Page 44 - Negative Feedback in Power Amplifiers
*
Susan Parker, yes SUSAN !has designed this.
Zeus Power Amplifier
http://www.susan-parker.co.uk/zeus-about.htm
*
So it appears that feedback does exist, in one form or another, whatever might be stated, and/or claimed !
Another great book, with Lots of info/data/circuits etc, is Douglas Self's - Audio power amplifier design handbook
According to him in there,
Chapter 2 - Page 44 - Negative Feedback in Power Amplifiers
*
Susan Parker, yes SUSAN !
has designed this.Zeus Power Amplifier
http://www.susan-parker.co.uk/zeus-about.htm
*
So it appears that feedback does exist, in one form or another, whatever might be stated, and/or claimed !
Attachments
Last edited:
Well, it's physically true that some infinitesimal portion of me is hanging out
in the Andromeda galaxy, but we should be able to ignore this and have a
practical discussion about my present location.
I choose to ignore the tiny resistance of the emitter connection in a
discussion of degeneration because it beings nothing to the table. Similarly
the "internal loop feedback" represented by the gain dependence on Vce,
(although I personally find that very interesting).
I appreciate that it annoys technical people when audiophiles
unreasonably dismiss the use of feedback. At the same time, I don't think
it's reasonable to try to annoy them back by insisting that everything has
feedback and that all forms of feedback are the same.
Surely we are bright enough here to recognize that the issue is more
complex than that.
in the Andromeda galaxy, but we should be able to ignore this and have a
practical discussion about my present location.
I choose to ignore the tiny resistance of the emitter connection in a
discussion of degeneration because it beings nothing to the table. Similarly
the "internal loop feedback" represented by the gain dependence on Vce,
(although I personally find that very interesting).
I appreciate that it annoys technical people when audiophiles
unreasonably dismiss the use of feedback. At the same time, I don't think
it's reasonable to try to annoy them back by insisting that everything has
feedback and that all forms of feedback are the same.
Surely we are bright enough here to recognize that the issue is more
complex than that.
Well, it's physically true that some infinitesimal portion of me is hanging out
in the Andromeda galaxy, but we should be able to ignore this and have a
practical discussion about my present location.
I choose to ignore the tiny resistance of the emitter connection in a
discussion of degeneration because it beings nothing to the table. Similarly
the "internal loop feedback" represented by the gain dependence on Vce,
(although I personally find that very interesting).
I appreciate that it annoys technical people when audiophiles
unreasonably dismiss the use of feedback. At the same time, I don't think
it's reasonable to try to annoy them back by insisting that everything has
feedback and that all forms of feedback are the same.
Surely we are bright enough here to recognize that the issue is more
complex than that.
Hi Nelson,
Every incandescent lightbulb has negative feedback in it
.Just kidding.
BTW, I agree that it is silly of Self to characterize the dynamic emitter resistance of a BJT (1/gm) as negative feedback.
Cheers,
Bob
As both a technical person and an audiophile it doesn't annoy me in the slightest. It seems to me to be an instance of reification, a philosophical error of thinking.
Reification (fallacy) - Wikipedia, the free encyclopedia
For sure it is even possible to have both.
Would you consider this type of circuit to be labeled as a 'no global feedback' amplifer?
Hi CBS240,
The question of whether an amplifier that uses a DC servo can be considered a no-NFB amplifier is a good one.
I think that the purists would claim that a true no-NFB amplifier cannot employ a DC servo, since that is a global feedback loop, albeit one that functions primarily at very low frequencies.
Cheers,
Bob
Andrew and David,
Feedback is using a sample of the (an) output, sending it back to the (an) input to make the output a desired value.
A servo has to be negative feedback, otherwise the offset would be increased by the servo.
Whether you use opamps, chopper-stabilized, tubes or a fast operator with a potmeter, is immaterial. A servo is feedback and negative.
Does NOT need to be global. I have designed servo loops that are strictly local to the output stage. Works like a charm.
Edit: David I think I read your post wrong. I thought you meant a chopper-stabilized amp as a servo; you probably mean the process of chopper-stabilizing an amp. But, you bet it is (negative) feedback. With all the same stability issues a continuous servo has. Although, come to think of it, you probably can also use feedforward.
jan didden
Last edited:
In the early chopper stabilized amplifiers the inputs and outputs were switched together so that any offset was averaged out. That is not feedback. (I haven't looked at how they work for 40+ years so the technique may have changed.)
Nelson.
I apologize if there was any evil intent read into my question. I was curious if you were playing with the immutable force - immovable object concept. For example a current source feeding a follower through a transformer.
ES