[Amps] HV MOSFETs for RF

[Manfred Mornhinweg]

Kevin,

 > The days of the 12V SSPA in rigs are numbered, or should be.

I don't think so...!

12V 100W amplifiers do have their place - in mobile rigs. Although 
nowadays it does make sense to use a DC-DC converter to raise the 12V to 
something more convenient, the improvement may not be enough to warrant 
the additional cost - specially because mobile rigs running 100W into 
small, low, highly compromised mobile antennas just don't have much 
potential to QRM anyone.

The problem starts, of course, when massive numbers of hams buy mobile 
rigs and use them at home, often even driving a legal-limit amp with them!

I agree that given the components available today, 100W radios for fixed 
station use should run the finals from 50V. But at the same time I see 
nothing wrong about mobile stations using 12V 100W final stages.

> If I was designing a final section for one of these rigs it would be a 
> pair of 50V MRF-150's derated to 150W max out. They're plentyful, 
> comparatively cheap,

Almost twice as much as the commonplace RD100HHF1. If you were designing 
that radio for a company that has to stay competitive, then your boss 
would say "nope"! But it sure would work nicely in a radio designed for 
people willing to pay more for an cleaner output. In the US it would 
have some market, in Europe more, but still I think that most hams would 
buy the cheaper radio. So we should look for something that's both 
better and less costly.

If I was asked to design a 50V 100W final amplifier, I would look very 
closely at devices like the MRFE6VP5150NR1. It's a Gemini LDMOSFET, 
rated for 150W CW output, so it runs quite cleanly at 100W. It has a 
rated dissipation capability of nearly a kilowatt, so the thermal design 
should be rather easy when dissipating less than 100W. The capacitances 
are far lower than those of the MRF150, the gain is higher, and the 
price is just $36 at Digikey, in single quantity. That's about half as 
much as a pair of RD100HHF1, not to mention the comparatively 
super-expensive MRF150!

Note that the very low capacitances will contribute markedly toward a 
lower IMD. I haven't tried these devices, but I would fully expect them 
to deliver a cleaner output than the MRF150, thanks to much lower phase 
distortion.

The MRF150 was good in its time, but that was several decades ago! It 
has a strong market presence, which means it will remain available for 
quite a long time, and that's a plus, but in terms of both price and 
performance it just can't compete against modern devices! In my view 
it's a mistake to base new designs on such aged devices.

> So... that leaves me with a rig that will do -45 to -50 IMD and an amp 
> (AL-1500, Ameritron's number) that'll do -38. Hmmmm.

Hmm... yes... Better build a good amp directly into the rig. Offer your 
rig in two versions: 100W and 1500W. Or 200W and 1500W, or whatever fits 
  best. The two versions wouldn't differ by one having an _additional_ 
stage, but by having a _different_ final stage (and power supply, 
filters, etc, of course). The gain of modern devices is high enough that 
a 1500W final stage can be driven from a 4W or even less. So it would 
make sense to provide a 4W driver amp on the main RF board of the radio, 
and use that to drive either a 100W final (with some surplus drive 
power) or a 1500W one.

> Let the anecdotes fly.

Done herewith! ;-)

Manfred

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