[Amps] Solid state amp high SWR distortion

[Manfred Mornhinweg]

John,

> I'm converting from a tube amp to a solid state amp. I've seen 
> references to increased distortion caused by a solid state amp when the 
> SWR is high, not high enough to cause reduced (foldback) power. Can 
> anyone point me to hard numbers (not folklore) on this? 

I don't think anybody can put a hard number to the change in distortion 
  caused by a a specific SWR. Too many factors are involved. Starting 
with the fact that a specific SWR can be caused by an infinite number of 
different load impedances.

Probably the strongest increase in distortion will happen when the 
amplifier is underloaded. For example, imagine the amplifier being set 
up injto a 50 ohm dummy load, with the drive being adjusted so that it 
operates at nominal output power and distortion, and then this amplifier 
is operated into an antenna that happens to have such an impedance that 
the amplifier proper (before the low pass filter) is loaded with 60 ohm 
instead of 50.

Since the drive remains the same, and transistors are controlled current 
sources, the transistors will try to push the same current into that 60 
ohm load. This would result in a 20% higher output voltage, which would 
  make the amplifier clip the signal, and thus generate heavy distortion.

Almost all solid state amplifiers employ negative feedback, which tends 
to counteract this voltage increase, but never completely. So in 
practice some light clipping will result, with some light increase in 
distortion.

Note that this is exactly the same with tube amplifiers. If you 
correctly load a tube amplifier into a dummy load and then connect it to 
that non-1:1 antenna, the tube will clip the signal at least as badly as 
the transistors. The advantage of tube amps in this regard is simply 
that ALL of them have the antenna tuner built in, in the form of a Pi 
tank. You need to load a tube amp into the actual antenna, not a dummy 
load, and on the free frequency nearest to your intended operating 
frequency, to get correct performance. If you do the same with the 
antenna tuner between a solid state amp and the antenna, you are on 
equal grounds and don't need to worry about this issue.

There is one difference between tubes and transistors, in terms of 
distortion: It's the fact that transistor capacitances change strongly 
with instant voltage. This can add additional effects and distortion 
with transistors operating under non-perfect SWR conditions. But modern 
solid state HF amplifiers tend to use transistors rated up to VHF and 
sometimes UHF, and their capacitances are low enough to reduce the 
practical effect of the capacitance modulation. Basic linearity problems 
such as saturation and inaccurate biasing cause far more trouble, and 
these are the same for tubes and transistors.

So, an equivalent to a tube amp with Pi output is a solid state amp with 
built-in antenna tuner. A solid state amp without antenna tuner has no 
real, practical equivalent in the tube world, due to the impossibility 
of making broadband impedance matching circuits for the high and very 
reactive impedances of tubes.

If anyone wants to run a normal solid state linear amplifier into 
practical antennas without using some sort of antenna tuning, the 
amplifier should use a drive control system that doesn't act on forward 
power, but separately on transistor voltage and current. The other (and 
better) option is to use a nonlinear (saturated) amplifier with supply 
voltage modulation and current monitoring. This allows maintaining very 
low distortion and high efficiency into variable loads.

If you use a commercially made solid state linear amp without antenna 
tuner, I would suggest to watch the SWR and reduce the drive power as 
the SWR rises. And even better than that, if you have a monitor scope 
watch the waveform for clipping and adjust the drive accordingly, or if 
you have some sort of spectrum analyzer (such as SDRs offer) watch your 
  IMD sidebands and adjust drive to keep them low enough.

Manfred

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