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Old 13th September 2011, 02:37 PM   #15901
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Quote:
Originally Posted by gpapag View Post
[snip]What is this "a rather new alternative to precise low-distortion, wideband, voltage-mode amplification" ?
[snip][/URL]
... current-mode amplification...?

jan
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Old 13th September 2011, 02:38 PM   #15902
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Hi Bruno,

Quote:
Originally Posted by Bruno Putzeys View Post
Back to hiding, Jan Didden asked me to chip in so I hope this is more or less the answer he was looking for.
Before I let you go, you make one interesting statement in your reply to Menno Van Der Veen's LTE: "Conclusion: Negative feedback does not attenuate the harmonics of the open-loop nonlinearity, but of its inverse."

The math is heavy on an empty stomach and I disagree that discounting "distortion of distortion" is right.

Further, there are implications from your conclusions that are no doubt clear to many but I think it may be worth elaborating.

Ciao T
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Old 13th September 2011, 02:46 PM   #15903
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Scott,

Quote:
Originally Posted by scott wurcer View Post
I knew this post would come. Yawn, you couldn't be more wrong.
Wrong in what? That your results are NOT results a JC-3 would produce?

Quote:
Originally Posted by scott wurcer View Post
I will state, to be clear, the local feedback (2-1M resistors) shown in that circuit do nothing useful.
You mean in YOUR circuit they do nothing useful. What they do in the JC-3 can not be concluded from your sims...

As they say in German "getroffene hunde bellen" (dogs that have been hit bark").

Ciao T
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Old 13th September 2011, 03:08 PM   #15904
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Quote:
Originally Posted by ThorstenL View Post
I disagree that discounting "distortion of distortion" is right.
For intermediate values of loop gain "distortion of distortion" may only be "very nearly" negligible instead of entirely. The difference being exactly that between the curved lines and the straight asymptotes in my graph. And your point is?
Quote:
Originally Posted by ThorstenL View Post
Further, there are implications from your conclusions that are no doubt clear to many but I think it may be worth elaborating.
Occasionally not elaborating every implication spurs people on to think for themselves. That ability comes in handy sometimes.
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Last edited by Bruno Putzeys; 13th September 2011 at 03:21 PM.
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Old 13th September 2011, 03:18 PM   #15905
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Quote:
Originally Posted by ThorstenL View Post
Scott,



Wrong in what? That your results are NOT results a JC-3 would produce?



You mean in YOUR circuit they do nothing useful. What they do in the JC-3 can not be concluded from your sims...

As they say in German "getroffene hunde bellen" (dogs that have been hit bark").

Ciao T
WOOF! WOOF! (Or is it woffen woffen wackle wackle mit den hueften) No I meant the JC-3 circuit or for that matter any simple folded cascode gm gain stage like that (whice includes the AD844 and its ilk).
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Old 13th September 2011, 03:19 PM   #15906
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Hi,

Quote:
Originally Posted by Bruno Putzeys View Post
For intermediate values of loop gain "distortion of distortion" may only be "very nearly" negligible instead of entirely. And your point is?
First, given your original "conclusion" it would obvious what.

You note (I have not checked the math but I take it to be correct) that feedback produces an inverse function from original non-linearity of the circuit. What then happens when this non-linearity is applied to the original non-linearity.

Of course, we are back to what some call "re-entrant distortion".

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Old 13th September 2011, 03:26 PM   #15907
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Quote:
Originally Posted by ThorstenL View Post
First, given your original "conclusion" it would obvious what.
You are failing to state your case. I am not going to enter a discussion with someone who simply barks "don't agree" without making a counterpoint he is willing to defend.
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Originally Posted by ThorstenL View Post
Of course, we are back to what some call "re-entrant distortion".
And just what do you think is re-entrant distortion other than the fact that the inverse of a low-order function has an infinitely long series expansion and hence more harmonics? I just gave you the bloody general mathematical explanation of re-entrant distortion. What will you do next? Tell Newton his gravitational law is incorrect because stuff just falls down?
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Old 13th September 2011, 03:42 PM   #15908
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Bruno,

Quote:
Originally Posted by Bruno Putzeys View Post
You are failing to state your case.
Okay, more clearly, as the feedback's distortion reduction based on your math is not a linear function "distortion of distortion" becomes a significant term, not one to be swept under the carpet.

Quote:
Originally Posted by Bruno Putzeys View Post
And just what do you think is re-entrant distortion other than the fact that the inverse of a low-order function has an infinitely long series expansion and hence more harmonics? I just gave you the bloody general mathematical explanation of re-entrant distortion.
Not quite, you oversimplified. But yes, the mathematical foundation is there. However, have you ever attempted to go through enough iterations to get a precise result? Can the result even be made to converge?

Quote:
Originally Posted by Bruno Putzeys View Post
What will you do next? Tell Newton his gravitational law is incorrect because stuff just falls down?
Pullllease, Newton. Pshaw. Newton lived in a Universe that strongly diverged from the one we live in now. And even now, we do not know what gravity is.

Ciao T
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Old 13th September 2011, 03:46 PM   #15909
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Jan Didden (janneman) has a great library of famous papers and articles. here
Direct link to the Gilbert paper "Are Op Amps Really Linear?" here

S
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Old 13th September 2011, 03:50 PM   #15910
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Quote:
Originally Posted by Bruno Putzeys View Post
For intermediate values of loop gain "distortion of distortion" may only be "very nearly" negligible instead of entirely. The difference being exactly that between the curved lines and the straight asymptotes in my graph. And your point is?

Occasionally not elaborating every implication spurs people on to think for themselves. That ability comes in handy sometimes.
Back around 1975 there was an interesting article about distortion in cable television amplifiers. Sensitivity to distortion for televisions is not quite as high as it may be for audio, but the bandwidth is so wide that there is a much greater chance for it to occur.

The first example was the normal signal level out of a line amplifier is around 100 mV for each channel. As some systems may have 150 or more channels that could in theory require 15 VRMS out if all the channels were to hit peak voltage at the same time. As they were AM modulated and not in sync that would be unlikely to happen. As the power supply voltage was often 24 VDC the maximum output could only be about 8 VRMS. So there was a minimum power supply voltage required based on probability and VHF transistors of the day had quite low voltage limits. (The solution became bridged amplifiers.)

The second issue was that of IM distortion particularly in multi-stage amplifiers. As the channel spacing is fixed at 6 Mhz. there could be build up of cross products that would affect certain channels.

The third and interesting issue was that sometimes the distorted products of a single stage would have some of the products returned to the correct result by a second stage of amplification (W/ it's distortion).

In audio as you are aware 9th harmonic distortion is particularly objectionable. So where the third order product may be considered adequate at -40 db re the source a 9th order product may not be at -80.

When the error product is included with each stage then each stage's contribution to the final product is more obvious and allows additional design decisions.

Now to me the actual issue on applying classic control theory could be that audio reproduction may be more critical of the small errors than most other control systems. I suspect a gun turret does not require -80 db precision.

ES
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