John,
The engineer from
another institution who was next to me had made a compact 6 kW CW amp at
1.3 GHz using RFMD gallium nitride transistors, 63 % efficient,linear,
64 transistors, water cooled. Quite impressive.
That's very similar to what I'm doing, except that I'm doing it for HF,
1.2kW, and using ultra cheap silicon switching MOSFETs.
The biggest SSPA is in France - 180 kW CW at 352 MHz using DMOS. Some
newer versions are being built for other synchrotrons using LDMOS with
NXP devices. Even so, these amplifiers will fill the space of a garage
due to the combiners and splitters. Approximately 750 devices.
I don't feel comfortable with those many splitters and combiners. Things
get too complex. They do have advantages, of course, such as continued
operation on reduced power if some of the modules fail. But I like
simpler approaches, such as directly paralleling devices. Of course this
can be done only up to a certain limit.
My topic was the bad boy tetrode HPA at 200 MHz for 3.6 MW peak power,
420 kW average power. Its commercial now, first units coming out of
manufacturing. Try doing that with silicon.
I wouldn't do that with today's silicon. Maybe some years in the future,
but not now. Engineering is about using the best, least expensive method
to fulfill practical needs. For 420kW average on 200MHz, indeed I think
that tubes are the way to go, for now. For 1.5kW on HF, instead, I think
that the way to go is solid state.
Manfred
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Visit my hobby homepage!
http://ludens.cl
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