Class B & distortion harmonics

[carbonzit]

OK, another theory question for y'all:

Now, I understand that a class B amplifier (push-pull where each side conducts for only 180° of each cycle) only produces odd-numbered harmonics; even-numbered ones are cancelled out. (We could probably extend this to class AB, where each side conducts for a little more than 180°, but whatever.)

So my question is, why does such an amplifier only produce odd-numbered harmonics? What is the mechanism that makes this work? I have a feeling, intuitively, that it has something to do with the symmetrical arrangement of devices (transistors, tubes). But I don't know exactly why this is so.

Let's make this a little contest. Not really, but how about if the winner is the one who can explain this not only correctly, but clearly, without a lot of jargon or gobbledygook, and with only as much math as is absolutely required.

OK, go to it!

 

[Mikebits]

Well I found this tidbit of text.

Nonlinear amplifiers generate harmonics of the input signal along with an amplified copy of the input. These harmonics can be separated into even order - 2nd, 4th, 6th, etc, and odd order - 3rd, 5th, 7th, and so on. Due to the way harmonics are generated in amplifying devices, the even ordered harmonics do not retain the phase of the input signal. Therefore in a push pull or differential amplifier the two amplifying halves will both generate a spectrum of harmonics but the even order harmonics from each half will cancel each other since their phases are the same and the amplifier output is the difference of the halves. The input signal and the odd order harmonics will not cancel since their phases are opposite between the two circuit halves. Push pull/differential amplifiers typically have lower distortion than single ended amplifiers, due to this cancellation of even order harmonics.

Text obtained from: **broken link removed**

 

[audioguru]

A symmetrical waveform will never have even-order harmonics. A symmetrical waveform can be "squashed' symmetrically to produce odd-order harmonics.

A single vacuum tube and maybe a FET has a non-symmetrical waveform and produces even-order harmonics.

 

[killivolt]

A symmetrical waveform will never have even-order harmonics. A symmetrical waveform can be "squashed' symmetrically to produce odd-order harmonics.

A single vacuum tube and maybe a FET has a non-symmetrical waveform and produces even-order harmonics.

I see no gobbledygook in this explanation.

Maybe a couple "quacks" or two

 

[audioguru]

I see no gobbledygook in this explanation.

A symmetrical waveform has the top half exactly the same as the bottom half so there are no even-harmonics.
Even-harmonics are produced when one half is "squashed" which never happens in a symmetrical push-pull circuit.

Odd-harmonics are produced when the top half and bottom half of a waveform are "squashed" equally.

Understand now?

Maybe a couple "quacks" or two[/QUOTE]

 

[carbonzit]

A symmetrical waveform has the top half exactly the same as the bottom half so there are no even-harmonics.

Whoa. Stop right there. That ain't right.

I think you may be trying to say something different from what you actually wrote. The two halves of a wave being amplified by a class B amp are not necessarily "exactly the same" (except for 180° inversion, of course). In fact, they will rarely ever be identical. Sure, if you're testing the amp with a pure sine wave source, then they'll be identical, but any kind of amplified program material (music, speech) is unlikely to contain "identical" upper and lower wave halves, as anyone who has ever used a computer .WAV editor knows.

Now, it's true that the amplifier operation is (theoretically, anyhow) completely symmetrical, but that's not really the same thing, now, is it?

So while your answer is ruled by the judges not to be gobbledygook, it is pure quackery, as is so much of your pontification here.

 

[carbonzit]

Well I found this tidbit of text.

Nonlinear amplifiers generate harmonics of the input signal along with an amplified copy of the input. These harmonics can be separated into even order - 2nd, 4th, 6th, etc, and odd order - 3rd, 5th, 7th, and so on. Due to the way harmonics are generated in amplifying devices, the even ordered harmonics do not retain the phase of the input signal. Therefore in a push pull or differential amplifier the two amplifying halves will both generate a spectrum of harmonics but the even order harmonics from each half will cancel each other since their phases are the same and the amplifier output is the difference of the halves. The input signal and the odd order harmonics will not cancel since their phases are opposite between the two circuit halves. Push pull/differential amplifiers typically have lower distortion than single ended amplifiers, due to this cancellation of even order harmonics.

First of all, thanks, Mike, for a very relevant reply.

Not to knock that site or your selection of that text, but there are is something about the explanation that bother me, or at least that I have trouble wrapping my head around (which probably says more about my head than the explanation!). They say "the even ordered harmonics do not retain the phase of the input signal", but what does that even mean? Both waves start at zero and end at zero (when looking at one complete cycle of the fundamental), correct? When I look at the waveform of the fundamental vs. the 2nd harmonic (double the frequency), I see nothing that could be interpreted as any kind of phase relationship between these two waves. So what do they mean by the phase relationship of the two waves?

 

[audioguru]

A pure sine-wave (not music or speech) is used for distortion measurement because it has no harmonics, is continuous and is perfectly symmetrical.
That is what I was quacking about.

 

[carbonzit]

A pure sine-wave (not music or speech) is used for distortion measurement because it has no harmonics, is continuous and is perfectly symmetrical.
That is what I was quacking about.

OK, so explain how a class B amplifier could be good (= low distortion) for anything but a laboratory-produced symmetrical sine wave? Because that's not what our amplifiers are amplifying.

Or maybe you listen only to pure sine waves? Please do explain.

 

[audioguru]

A class-B amplifier produces horrible crossover distortion (look in Google).
A good quality class-AB amplifier produces extremely low distortion and no crossover distortion.

If the amplifier produces very low distortion with a sine-wave then it usually produces very low distortion with music and speech.

A pure sine-wave is used for distortion measurement because it has no harmonics. At the output of the amplifier being tested the sine-wave is removed by a notch filter and anything remaining is distortion and noise.

On an oscilloscope the sine-wave shows what kind of distortion. Crossover distortion is obvious, even harmonics are caused by one side being squashed and odd harmonics are caused if both sides are squashed.