Date: Sun, 23 Apr 2017 22:43:31 +0000
From: Manfred Mornhinweg <manfred@ludens.cl>
To: amps@contesting.com
Jim,
> ## why not use 4 x BLF188XR or 4 x MRFX1K80H ?
Cost is a powerful reason against that, in my opinion. It would be
expensive overkill. I do think that using 4 devices (or more) is a good
idea, but then these should be cheaper devices.
A few days ago I was looking at the IXZ210N50L2, which should be
suitable for a multi-device legal limit linear class AB amp. Each of
these individual (not Gemini) MOSFETs should be good for close to 200W
RF output in linear service, and they are 28 bucks apiece at Digikey, in
single quantity. An added benefit is that they run from 100 to 150V
instead of 50V, which should allow achieving better efficiency and
linearity.
> Is it even possible to use 2 x devices in parallel in each half of a
> push-pull amplifier ?
At 50V supply, the drain impedance for a single 1500W amp is so low that
it's really hard to handle properly. It's much better to make separate
lower power amp modules, and combine them.
> If it is possible, then the heat could be extracted over 4 x
> devices.
Yes. Or 8 or whatever. It makes for a much simpler, smaller, lighter
cooling solution.
> The TX imd, with 4 x devices run at 1.5 kw, on paper, should be good,
> since each device is running at 375 w pep output.
Don't let this fool you! If you put each of the 4 devices in the typical
circuit designed to saturate at 750W (1:9 transformer at the output),
then indeed you will get good IMD at 375W, but also you will get an
absolutely lousy efficiency. And if you instead use a lower ratio
transformer, to make each module saturate at 400W or so, then you are
back at square zero, with "normal" efficiency and IMD! The only thing
you gain that way is the easier heatsinking, and/or robustness against
overheating damage.
## I was afraid of this. We are back to square one. 50% eff and lousy imd,
or better IMD and lousy 35% eff..take your pick. My 1000-d and also
1000-MP-MKV
run 50%, when run at 200w out. back po down to 100watts..and eff drops to 35%
..but
imd improves a bunch.
## so say a pair of MRFX1K80H in PP. run at 750w out. Then an identical 2nd
pair in PP,
also run at 750w out. Then combine them for 1.5 kw out. Lousy eff and good
imd
...or normal 50% eff..and lousy imd. We just cant win.
## You may well be able to put 2 x 50-60 vdc switching supplies in series to
obtain your
100-120 vdc for your proposed
## baffles me how SS marine xcvrs all have superb IMD..and also run on 11-15
vdc.
Are they using loads of NFB ?
Jim VE7RF
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