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Old 27th January 2005, 01:29 PM   #1
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Default Musings about negative feedback

Negative feedback is always a bit of a problem because in a real-world amplifier it takes time for the signal to go through the amplifier and appear at the output, and it is this slightly delayed signal that is fed back to the input to be subtracted from the input signal, the difference of the two then being fed to the amplifier input. That means that among other things, the (delayed) feedback signal is never exactly suitable for the input signal - it would only be so if you had an amplifier with *zero* delay in the signal path.

What occurred to me however is that when a sinewave is fed into an amplifier for a distortion measurement, the feedback may have more validity than first appears because of what I would call the *self-similarity* of a sinewave. A sinewave can be represented by a rotating vector with a sine and a cosine component. The delay in the signal path of the amplifier simply applies a phase-shifted feedback signal, which due to the self-similarity of a sine, does an ok job subtracting from the input sinewave. Vectorially speaking, it is just a bit further around the circle but still the same shape,

However... That is all fine and good when we have a sinewave as a test signal, but what if we apply some other shape of waveform? There would be no moment-to-moment self-similarity of the input waveform anymore - the vector of the wave doesn't draw as a circle anymore. (bad description, I know) You can't move several degrees around the circumference and expect the new shape to have a constant radius.

What I am getting at is this - an amplifier may measure very good distortion figures with a sinewave, but if it were possible to measure distortion with a complex waveform (how far it deviates from ideal rather than spectral analysis method) then that may prove to be very interesting. It may shed some light on why many amplifiers measure good but sound bad.

OK, all you guys with Phd's and whatnot, shoot me down in flames.
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Old 27th January 2005, 01:35 PM   #2
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I believe you are correct.
That is why many amplifiers with large amounts of inverse feedback show poor squarewave response.
Most often, you'll see overshoot on the leading edge of the squarewave and undershoot on the lagging edge.
It's a symptom of the time delay through the amplifier.
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Old 27th January 2005, 01:46 PM   #3
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Depends on how you define "time delay." The minimum phase delay caused by rolloffs is exactly what feedback theory accounts for. The nonminimum phase delay, caused by signal transit time, is pretty irrelevant unless you're worried about making an amp flat into the tens of megahertz range.

Think about this: when you drive, how do you stay in your lane?
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Old 27th January 2005, 08:01 PM   #4
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Quite correct, Circ. The phase shift is indeed constant in the amplifier and manifests itself as a high-frequency rise, as ringing on the leading edge of a squarewave.

The key to avoiding this is to build a quick amplifier (basically, use a good OPT) and compensate so that rather than the process of "ohp, input wants to be high... better raise the output... oops not that fast, that's overshoot...go down.. oh not that far! etc.", instead it just rises slowly "oh what? huh you say you want to go up? uhhh okay whatever...".

Incidentially, the phase shift does NOT cause any distortion, only the above frequency artifact. It does cause IMD, but this is generally attributed to distoring a distorted signal (creating higher harmonics) and is avoided by reducing distortion to inaudible levels with levels >15dB of NFB.

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Old 27th January 2005, 09:02 PM   #5
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Quote:
Originally posted by Sch3mat1c
...The key to avoiding this is to build a quick amplifier...
The key is to make sure the amplifier has sufficient phase margin. If the gain is too high when the feedback becomes positive, even if the amp is stable, it will cause ringing. If the gain is low enough, then the feedback is no longer trying to correct the output and ringing is avoided. Building a fast amp may make this easier, but it doesn't solve anything by itself.
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Old 27th January 2005, 09:18 PM   #6
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Frank Berry has a PhD. And I like his answer. Time delay is time delay no matter how you slice it, and a horse out of the gate is a horse out of the gate. How's that for fancy thinking?
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Old 27th January 2005, 09:58 PM   #7
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No matter how you wrap your mind about it, wondering how it can work when it can only correct in retrospect, the fact remains that negative feedback works, and works well. You could try to avoid the problem by building circuits without feedback, but it's impossible, given how even a simple emitter-follower effectively has negative feedback.

The ringing that Frank mentioned can be completely avoided, even in high feedback amps. The fact that many amps do exhibit ringing is because the designer either doesn't understand that a well damped amplifier is desirable, or wanted the specs to look good (i.e. wide bandwidth and high slew rate rather than good control).
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Old 27th January 2005, 10:15 PM   #8
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Quote:
Originally posted by Mr Evil
No matter how you wrap your mind about it, wondering how it can work when it can only correct in retrospect, the fact remains that negative feedback works, and works well. You could try to avoid the problem by building circuits without feedback, but it's impossible, given how even a simple emitter-follower effectively has negative feedback.
"Works well" is not fact in my experience and the phrase, in any event, does not describe what any given form of feedback actually does. I'm happy to go on record to say emitter-follower feedback is not the same as local feedback is not the same as global feedback in sonically important ways of differing. Nor am I convinced that emitter- or cathode-follower feedback is "feedback," and the issue, to me, is not merely semantic. A circuit, so far as I know, might work all as a piece, meaning there might be no delay between current exiting and current returning to an active device, meaning the time and phase delay aspects of local and global feedback form no part of what emitter/cathode "feedback" does.

I'm personally interested in knowing why feedback changes the sound of an amplifier in the way it does. We all know at least the more significant of the sonic and electrical benefits of feedback. What accounts for its sonic drawbacks which I, overall, prefer to do without?
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Old 27th January 2005, 10:22 PM   #9
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Konnichiwa,

Quote:
Originally posted by Mr Evil
No matter how you wrap your mind about it, wondering how it can work when it can only correct in retrospect, the fact remains that negative feedback works, and works well.
From Evil Anime Charater to the evil Mr.....

The above is HIGHLY dependant on the definition and interpretation of the phrase "it works".

I would argue that the way it is commonly implememted NFB does not "work", by my own definition of the terms "works".

I can live with the fact that for you "NFB works". Can you live with the fact that for me "NFB potentially works BUT a long list of quite narrow conditions apply under which NFB works, otherwise it doesn't."

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Old 27th January 2005, 10:27 PM   #10
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Konnichiwa,

Quote:
Originally posted by serengetiplains
I'm happy to go on record to say emitter-follower feedback is not the same as local feedback is not the same as global feedback in sonically important ways of differing. Nor am I convinced that emitter- or cathode-follower feedback is "feedback," and the issue, to me, is not merely semantic.
I agree. I hate to see local degeneration and looped inverse feedback circuits to be lumped into one. It is easy to call the "NFB" as there are some similarities but technically speaking they differ more than they are the same.

My personal usage of NFB is strictly in the sense of "looped inverse feedback". Degeneration is degeneration, feedback (positive or negative) is feedback.

The common use indicates muddy thinking and poor language skills.

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