[Amps] MOSFET amp filtering - was: auto-tune

[Manfred Mornhinweg]

Jim,

> AND -- there's another point that virtually EVERYONE who has commented 
> in this thread seem ignorant of.  ANY distortion mechanism produces BOTH 
> harmonics and INTERMOD.

Good point to keep in mind. At least in any audio circuit this is most 
certainly true. But is it in an RF amplifier?  Really?

What happens with a single-ended class AB amplifier, like all common 
tube amps? The tube basically conducts only half a sine wave, and cuts 
off the other half. The tank circuit restores the other half. Such an 
amp cannot preserve the RF waveform, but it does preserve the envelope 
waveform. It can, at least in theory, work at zero IMD, but not at zero 
harmonics. The tank circuit strongly attenuates the harmonics, but does 
not attenuate close-by IMD.

Any RF amplifier that causes envelope distortion will produce IMD. But 
will it necessarily also produce harmonics? I'm not sure right now... I 
would have to think it over when my head is clearer than now.

As I see it, in an RF amp that's amplifing an RF signal that has low 
frequency modulation on it, harmonic generation depends on the 
distortion of the basic Rf waveform, while IMD depends on distortion of 
the envelope and also of the phase. And of course harmonics can be 
attenuated by a low pass filter or a tank circuit, while close-in IMD 
cannot. This is what makes single-ended linear class AB amplifiers 
possible at all! Linear RF amps don't need to be linear regarding the RF 
waveform, only regarding the amplitude (envelope waveform) and phase.

What happens if you take a single transistor or tube, carefully bias it 
into class AB and apply negative feedback, achieving high amplitude 
linearity, and feed its output into a dummy load, without any filter? If 
my way of thinking is correct, this amplifier will produce a very high 
level of harmonics, but a very low one of IMD.

So I would think that IMD and harmonics do not necessarily go together 
in our RF amps, as they do go together in audio amps. I don't put my 
hands on fire for this, though... I prefer thinking it over for some 
time! :-)

Manfred


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