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Old 4th April 2007, 08:09 PM   #1
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Default odd harmonics?

Do transistor amps produce odd harmonics all the time or only when over driven?
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Old 4th April 2007, 08:21 PM   #2
Limhes is offline Limhes  Orkney Islands
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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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Old 4th April 2007, 08:37 PM   #3
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A is even. B is odd.
D???? H??
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Old 4th April 2007, 09:02 PM   #4
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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.


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Old 4th April 2007, 09:33 PM   #5
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Quote:
Originally posted by janneman
In general, push-pull amps generate more odd order harmonics........
Only if they operate in Class A.
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Old 4th April 2007, 09:38 PM   #6
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Quote:
Originally posted by mikeks


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.

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Old 4th April 2007, 09:40 PM   #7
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Quote:
Huh?
I was just about to say that.
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Old 4th April 2007, 09:56 PM   #8
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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.

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Old 4th April 2007, 10:17 PM   #9
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Quote:
Originally posted by SpittinLLama
[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.

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Old 4th April 2007, 10:37 PM   #10
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by janneman
Huh? Push pull means symmetrical waveform means odd harmonics, no?

Sorry, Jan. You're absolutely right: I misread your post.

Push-pull action in fact only eliminates even order harmonics in Class A.
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