Regarding the SS amps question, I am a bit surprised Jon didn't jump in.
A very common technique for producing a solid state amp with very good
IMD is to intentionally select devices whose gain is very high and whose
upper freq limit is very high. By picking a high gain device, you can
wrap feedback around it and then overall amp gain is determined
primarily by the passive (and theoretically linear) feedback network and
thus is not so much affected by beta variations with collector current.
By picking a very high cutoff frequency, you've ensure that the device
capacitances are a tiny fraction of the capacitances in your matching
network. Device capacitances vary with the instantaneous RF voltages,
and thus can contribute to IMD.
Typical television transposer devices can have a gain of 30dB at UHF,
but we used feedback to drop the gain to 10dB. Also, the transistors we
used at 800 MHz typically had ft of 6 - 8 GHz. The result of these
choices was a push-pull 50 watt class AB stage with 3rd order IMD of
-60dBc. Yes, we did have to be careful about what kinds of impedances
the device saw out-of-band - and we put our "suppressor networks" in the
collector bias path. These were actually parallel RLC networks that
provided the device with a very low resistive load at all frequencies
except the operating frequency.
In FET amplifiers, you usually don't get the choice of using a FET
capable of higher frequency operation - power FETs above 225 MHz are
very hard to find. But you do use FETs capable of frighteningly high
gain. In a typical RF heater, we might use a pair of Motorola MRF150s.
With gate swamping ( like grid swamping in a grounded-cathode design )
and negative feedback, we'd make a 300 watt stage good to 100 MHz with
20dB of gain. That's without doing any impedance matching to the FET
gate - as I said, we just swamped it. Never had any oscillations with
those. I did a calculation once with those FETs. At 50 MHz, if I chose
to actually attempt to match the gate impedance, the stage gain could be
as high as 50 dB. I would not want to try to tame that beast!
Arlen
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