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Old 2nd April 2013, 04:22 PM   #791
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Quote:
Originally Posted by RNMarsh View Post
One of the reasons for a compl push-pull circuit or other totally symetrical topology is so that the distortion will cancel withOut gnfb.
This is only true of even order harmonics and only if your "complementary" devices are ideal. Odd order harmonics are simply reinforced and not cancelled.
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Old 2nd April 2013, 05:32 PM   #792
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Quote:
Originally Posted by michaelkiwanuka View Post
This is only true of even order harmonics and only if your "complementary" devices are ideal. Odd order harmonics are simply reinforced and not cancelled.
the residuals shown dont show that happened. how come? assuming ideal. Open loop -- just a liitle atten of even... not cancelled. why not? same values, same parameters on transistors, symetrical topology. where's the cancellation?

Last edited by RNMarsh; 2nd April 2013 at 05:35 PM.
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Old 2nd April 2013, 05:53 PM   #793
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only works for even order distortions - everything has odd too - eventually clipping at symmetircal rails

and its hard to get the evens to match when you are balancing all the differences between N and P type silicon swaping roles in the "complementary" devices

diff pair, Nelson's SUSY work because matching, cancelation with like polarity devices distortion is more robust
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Old 2nd April 2013, 05:57 PM   #794
RNMarsh is offline RNMarsh  United States
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This is a SIM.... the transistors are perfectly matched. Why not perfectly cancelled 2H?? Doesnt show perfect cancellation in a perfect symetrical amp. Why?
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Old 2nd April 2013, 06:18 PM   #795
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Hi Magnoman,

I should not have put the distorsion results. I have simulated the circuit with a push-pull input stage with good results but it complicated the comparison with a differential input stage.
My aim was not to compare the performances of CFA vs VFA.

I was interested how both circuits work. I think that the shown values of voltages and currents prove that the circuits are fundamentaly governed by exactly the same principle which has already been clearly explained using few but sufficient words in some previous posts.
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Old 2nd April 2013, 06:28 PM   #796
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Both camps are wrong. With CFB almost all odd harmonics are cancelled. The 5th harmonic is clearly visible, though THD figures, compared to a VFB stage, are enormously smaller.
THD CFB: 0.7ppm
THD VFB: 1100ppm

edit: the black curve is the residual of the CFB stage. Green is THD of the CFB stage, red is THD of the VFB stage.

Cheers,
E.
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File Type: png CFAvsVFA-harm.png (23.5 KB, 87 views)
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Last edited by Edmond Stuart; 2nd April 2013 at 06:32 PM.
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Old 2nd April 2013, 06:36 PM   #797
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Quote:
Originally Posted by forr View Post
...
I was interested how both circuits work. I think that the shown values of voltages and currents prove that the circuits are fundamentaly governed by exactly the same principle which has already been clearly explained using few but sufficient words in some previous posts.
Understand I just dont agree. I havent seen anything clearly explaining this.
My previous post (787) although wordy, and likely not entirely clear, contradicts this. Maybe we just aren't talking apples to apples (I'm usually a step behind in these discussions).

Even for the "odd" distortions currently discussed I would expect an ideal cfa to have less resulting distortion that an ltp for the same rationale.

Edmond: Didn't see your post was busy typing. You can go as far as replacing the input transistors with an pure third order term (plus some dc bias) and see the same effect (adding the feedback will return some of the distorted cfa collector currents but you still win when ideal and isolated)


Thanks
-Antonio

Last edited by magnoman; 2nd April 2013 at 06:44 PM.
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Old 2nd April 2013, 10:21 PM   #798
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Default CFB galore

Looking for something else, I stumbled on a five years old design of mine (never built, btw). It not only has a so called CFB input stage, but also a CFB output stage. TMC is also applied two times : at the input- and output stage.

Just another 2 cents.

BTW1, who says that a (so called) CFB amp is inferior?
BTW2, who says you can't apply TMC to a CFB IPS?
And that makes 4 cents.

Cheers,
E.
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Old 2nd April 2013, 10:26 PM   #799
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Quote:
Originally Posted by Edmond Stuart View Post
never built, btw
A good reason to claim 0.65ppm THD20 distortions.
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Old 2nd April 2013, 10:35 PM   #800
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Edmond, what are the networks in the bases of Q3 and Q6 doing?

I also found cascoding (Q8 and Q11) made a significant impact.
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