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Old 4th November 2008, 03:55 AM   #11
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Quote:
Originally posted by leapcat
Question
Would a fast amp lower the Im distortion of a complex
signal?
No. The stuff I was talking about is somewhat independent of the
bandwidth of the amp. Of course bandwidth limitations would limit
the ability to pour on feedback, so of course it becomes relevant in
that context.

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Old 4th November 2008, 04:03 AM   #12
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Quote:
Originally posted by GRollins
One thing that I've always bounced back and forth in my mind is the idea that both the mathematics and real life measurements indicate different curves for the different gain devices.
Absolutely, and of course that means different techniques to maximize
gain and minimize distortion. Alas, some of those ideas remain on the
table.

I thought about it, but then it occurred to me - "Screw it. I'll use these
to make a million dollars!"

Muuuuuhahahahahahahahaaaa........

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Old 4th November 2008, 04:31 AM   #13
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Hi, Mr. Pass,

Is there a special way to reduce IM distortion? Are we stuck with active device gain linearity here, or is there a method special for overcoming IM distortion?

I see fig.13, the peak of IM distoriton (15V) is 3.75x the peak of the signal (4V).
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Old 4th November 2008, 02:09 PM   #14
leapcat is offline leapcat  United States
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Ok Thank you!
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Old 4th November 2008, 03:19 PM   #15
andy_c is offline andy_c  United States
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Hi Nelson,

Thanks for publishing that article.

One quibble, if I may. You state, "If you see a curve with distortion levels climbing as the output goes down, it implies crossover distortion caused by the gap between the two push-pull gain elements. This implies high-order harmonics."

For the often seen THD+N measurement, and for a very low-distortion amplifier, this kind of measurement can be due to decreasing S/N as the signal level decreases. A guy named Samuel Groner did a nice paper with lots of op-amp distortion measurements. That paper is here (warning: big download). In figure 2.4 (page 17 on the right) he shows the residual distortion of the AP System 1. You can see the increasing residual distortion as the signal level decreases. That's why I hate THD+N. Some people will point to such measurements and say they're due to crossover distortion of the unit under test. Of course, amps with high, but monotonic distortion, will look fine in this test because the distortion is always well above the noise. Amps with very low and monotonic distortion may falsely show indications of non-monotonic distortion in many cases.
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Old 4th November 2008, 05:27 PM   #16
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Where does a differential amp, or stage (i.e. SOZ) fit in here? Push-Pull?
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Old 4th November 2008, 06:31 PM   #17
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Quote:
Originally posted by lumanauw
Is there a special way to reduce IM distortion? Are we stuck with active device gain linearity here, or is there a method special for overcoming IM distortion?

I see fig.13, the peak of IM distoriton (15V) is 3.75x the peak of the signal (4V).
No special method - IM is just the same old nonlinearity looked at with
multiple tones.

And yes, I looked at Fig 13 for quite a while before deciding it was correct.

Keep in mind that some of this is amplitude changes to the original frequencies
as well as the addition of new tones.

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Old 4th November 2008, 06:36 PM   #18
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Quote:
Originally posted by andy_c
One quibble, if I may. You state, "If you see a curve with distortion levels climbing as the output goes down, it implies crossover distortion caused by the gap between the two push-pull gain elements. This implies high-order harmonics."

For the often seen THD+N measurement, and for a very low-distortion amplifier, this kind of measurement can be due to decreasing S/N as the signal level decreases. A guy named Samuel Groner did a nice paper with lots of op-amp distortion measurements. That paper is here (warning: big download). In figure 2.4 (page 17 on the right) he shows the residual distortion of the AP System 1. You can see the increasing residual distortion as the signal level decreases. That's why I hate THD+N. Some people will point to such measurements and say they're due to crossover distortion of the unit under test. Of course, amps with high, but monotonic distortion, will look fine in this test because the distortion is always well above the noise. Amps with very low and monotonic distortion may falsely show indications of non-monotonic distortion in many cases.
Absolutely correct. At one point I added a sentence mentioning this as a
possible source of confusion, but edited it for brevity, the feeble excuse
being that I said distortion and not THD+N.

Fortunately (or perhaps unfortunately, the egregious examples of Class B
are pretty obvious in power amplifiers, where the distortion gets nasty well
above the noise level.

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Old 4th November 2008, 06:38 PM   #19
SCD is offline SCD  Canada
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Default IM distortion

Well now I am really thinking. Is there a way to invert the IM distortion and use it as feed back.
If I understand the concepts behind John Broskie's Aikido topology correctly he developed a way to take the power supply noise, invert it and inject it at two points so it would cancel.
This concept sounds desirable. Can the concpet be applied to IM distortion?
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Old 4th November 2008, 06:46 PM   #20
andy_c is offline andy_c  United States
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Thanks for the clarification Nelson!
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