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

To: Gary Schafer <garyschafer@largeriver.net>, amps@contesting.com
Subject: Re: [Amps] MOSFET amp filtering - was: auto-tune
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Thu, 15 Dec 2016 16:07:11 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Gary,

your post pretty much condenses this matter. I will just comment on a few of the points:

Inter modulation is just what the term implies. Two or more signals modulate
each other in the signal path. For this to happen there needs to be a non
linear device in the path to cause modulation to occur.

Right. My doubt was what kind of products, and at what intensities, a certain sort of nonlinearity causes. That's why I made my tests and simulations, and found what others surely have found long before: That cutting off an entire half of a signal does cause strong harmonic distortion, as expected, but causes strong IMD only at low frequencies, while the IMD products close to the tones are weak.

Instead a nonlinear behaviour over the remaining half wave does cause strong IMD products close to the tones, and thus is not acceptable.

Of course SSB contains more than one signal
(frequency) at any given time as does a two tone signal.

I like to always look at these things both in the frequency domain and in the time domain. Right, if you look in the frequency domain, with a sufficiently long sampling time, then you get a mix of many frequencies and amplitudes within a 2.4kHz bandwidth, for a typical SSB signal. And they vary all the time. But if you look at this SSB signal in the time domain, what you see is a single RF signal, with a clean sine shape, that varies in amplitude and in frequency. And these amplitude and frequency variations are rather slow, when compared to the carrier frequency. Except for one point: When the signal reachs zero, it can start increasing again very quickly - a fast inflection that contains higher frequencies in the envelope. And at that same point, the phase generally jumps by 180 degrees.

It's interesting to consider what each sort of nonlinearity does to this signal, as seen in the time domain. Seen over a few full RF cycles, the signal stays almost constant, that is, it's basically a single-tone signal, and thus a nonlinearity causes harmonics but no intermodulation. Seen over a longer time span, like thousands of RF cycles, the modulation in amplitude and phase becomes obvious, and so any nonlinearity causes intermodulation too - but at lower levels than the harmonics. Instead with signals that are modulated at high frequency, such as at RF frequencies, the intermodulation would be basically as strong as the harmonics.

If those signals are pre-distorted before they reach the amplifier that in
itself will not cause IMD. But if the distortion causes new and more
frequencies to be generated then the linear amplifier will have more signals
to deal with and can more easily be driven into overload which will cause
the amplifier to operate farther from linear and that will cause IMD.

I would like to see this in a simpler way: Take an amplifier that isn't fully linear, but make sure it does have some response at all amplitude levels - no dead bands allowed. Drive it from an exciter, compare its instant output amplitude to the instant drive amplitude, and either predistort the drive signal or control the gain of the amplifier or control the attenuation of an attenuator between the driver and the amp. If properly done, this will render the output amplitude a very precise copy of the undistorted drive signal amplitude. At this point, I would expect that the only significant distortion that can remain, within the bandwidth over which the correction system works, is that coming fro any phase distortion introduced by the amplifier. And I WOULD LOVE if anybody here could derive equations to calculate how much IMD of what order would be caused by what type and amount of phase modulation! This is one thing I'm missing dearly in my plans for a high efficiency linear amplifier, and it seems that I'm too mathematically challenged to derive these equations myself!

A class B amplifier doesn't cause IMD because it cuts off one half of the
cycle provided that it remains linear over that half cycle. It does however
generate high second harmonic energy.

That's what my simulation shows too.

Class B audio amplifiers are usually run in push pull to cancel the harmonic
distortion. But if they generated IMD because they were a class B amplifier
they wouldn't be very useful as an audio amplifier.

In audio work any nonlinearity causes both harmonic and intermodulation distortion in roughly the same proportion, since we are always dealing with complex waveforms, not clean sines. But in RF signals carrying audio modulation, things are a bit different. Usually at RF we get far more harmonics than IMD. This needs to be always considered when audio people start working in RF, or vice-versa.

Although a distortion free class B amplifier is hard to obtain, in reality
what is called class B is usually AB, which is not totally cutoff when the
lower half of the signal is present. This increases the linearity and allows
close to class B operation.

Yes. Basically the gain of all semiconductor devices drops when the current is low. The class AB bias and the resulting idling current reduce this effect, tending to linearize the response in the small signal area - but it never becomes fully linear. Negative feedback at RF further linearizes amplifiers up to a usable level. External envelope feedback through a high gain, medium frequency error amplifier would linearize them much better, but it seems that no commercial amplifier uses it. The only ones I have seen using it are homebuilt ones, and with good results.

Maybe I am missing something here?

What you are missing is just the more complex extensions to this, such as the phase modulation caused by voltage-variable capacitances in semiconductors. We tend to think about linearity as the ratio between output amplitude and input amplitude only, but in SSB work we do need to also look at the phase: The output phase should closely follow the input phase, not affected by amplitude changes. That is, the amplifier should be able to reproduce all signal amplitudes from zero to saturation, without changing the phase. And I suspect that this is the hardest part with semiconductors, and that it's far easier with tubes. And I lack the knowledge about how important it is in practice, that is, how much amplitude-to-phase-modulation is acceptable for a given, required IMD performance. If anybody can provide input on this, with firm numbers or with a way to calculate them, that would be very interesting!

Manfred

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