Amps
[Top] [All Lists]

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

To: amps@contesting.com
Subject: Re: [Amps] MOSFET amp filtering - was: auto-tune
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Mon, 12 Dec 2016 22:18:46 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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


========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>