odd harmonics?

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Depending on their configuration, they produce either more odd than even harmonics or vice versa. Anyhow, they always generate distortion, since there is no linear part in the transconductance curves.

For example, a simple common base amplifier using only one transistor amplifies because of the Vbe-Id relation, which is in certain regions rather linear. However, it always stays an exponential curve, which has no 'straight line' in it, nowhere: Id ~ exp(q.Vbe/k.T)

Talking BJTs, ofcourse...
 
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In general, push-pull amps generate more odd order harmonics, SE generate more even order harmonics.

This is easy to understand. PP-amps have the same distortion on the pos as on the neg swing. So you get a sine with both tops flattened (or sharpened). Such a signal can be shown to consist of the sine itself and a smaller 3rd harmonic. You can draw a sine and it's 3rd harmonic and if you then add the two you see a symmetrically distorted wave.

A SE amp produces assymmetrical wave forms. If you draw a sine and it's 2nd harmonic and add them, you get a wave that is "skewed", assymetrical.


Jan Didden
 
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mikeks said:


Only if they operate in Class A.


Huh? Push pull means symmetrical waveform means odd harmonics, no? I know that it will not be a *perfectly* symmetrical wave, so there will also be an even order component. Nothing is perfect in this world. SE amps also generate some odd order in addition to the predominantly even order. But I stand by the general rule. Class A or AB.

Jan Didden
 
I am a little confused on the terminology, so don't yell at me. Are you saying PP is one class and SE another? If not then how do the harmonics in Class A differ to the harmonics in Class AB, assuming good design for both?

If 3rd harmonic dominates, what does this indicate is the problem with the design? Is is related to a few possible issues?

Thanks, still trying to learn about all this discrete stuff. Feel like I get deeper and deeper under water as I learn.

-SL
 
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SpittinLLama said:
[snip]If not then how do the harmonics in Class A differ to the harmonics in Class AB, assuming good design for both?
[snip]-SL


I don't think there is a difference between A and AB in principle, although there may be a difference in the relative ratio's of odd and even as there is between any different amps. Predominantly, PP generates odd orders.

PP-amps can be in A or AB, but the governing mechanism that determines the predominant harmonics is the PP principle.

SE predominantly generates even order. Of course, because of the topology, SE is always class A, or semi-class A like some of Nelson Pass's designs.

Jan Didden
 
So a well designed PP amp should have the 3rd harmonic dominate, right? The 2nd and other even harmonics should be lower and then any odd above 3rd should be dropping in amplitude. Granted, all amplifiers have different harmonic signatures so more of a thought exercise.

Thanks for the explanations so far.
-SL
 
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SpittinLLama said:
So a well designed PP amp should have the 3rd harmonic dominate, right? The 2nd and other even harmonics should be lower and then any odd above 3rd should be dropping in amplitude. Granted, all amplifiers have different harmonic signatures so more of a thought exercise.

Thanks for the explanations so far.
-SL

I think you are spot on.

Jan Didden
 
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