Dick,
Got any suggestions as to which devices meet your criteria ?
Among the ones I tried, the best was the IXTP1R6N50P. In class AB, at decent IMD
levels, I could obtain a power output from 60 watt per pair on 160m, through 55W
on 20m, down to 40W on 10m. Above 20 meters it was slew-rate-limited. In a tuned
amplifier it works pretty well on 6m too. When I used a bad layout, it
self-oscillated around 160MHz, and that's a pretty good sign!
Another pretty good one is the AOT3N50. It's bigger, has significantly more
dissipation capability, but also it's slower. I would not recommend that one to
operate on bands higher than 20 meters. Instead from 40m down it's preferable to
the previous one, due to the higher power capability for a lower price.
Among older FETs, the IRF710 has been used by several people. They claim
operation to 10 meters, but I tried it and found the Ixys above to be better.
Some people also use the IRF820, which is bigger but much more
frequency-challenged than the 710. I wouldn't use it at all.
This is all with supply voltages around 100V.
There are many more cheap FETs worth testing for RF performance. The problem is
that switchmode data sheets are only slightly useful to infer linear RF
performance. I selected a few promising candidates from their data sheets,
ordered a few of each, and tried them. I also tried some larger ones, like the
AOT8N50, just for comparison. That one is worthless at RF, except on the two
lowest bands, but the samples I ordered sure are giving me good service in
switching power supplies! I used up most of them already in different projects!
After the experiments, I was pretty much determined to build a Chilean legal
limit (1200W), watercooled amplifier for 160-10m, using 2x25 of the Ixys FETs
with a 108V supply. On the highest bands the power would be lower, down to 900W
on 10m. I experimented paralleling a few FETs, and got promising results. But
then my enthusiasm dissipated. I mean, it's class AB, with a peak effiency below
60%, and an average efficiency like 30% depending on the mode, and I'm
energy-challenged in summer time, because my power comes from a small turbine in
a creek, and that creek nearly dries out in summer! So for me a high efficiency
amp means more than just a buzzy name. In mid and late summer it means the
difference between being able to use it, or not!
I have some parts of the water cooling system ready, I built the output
transformer and tested it in reverse using the NCL-2000, and a few more things.
The design of the amp block, and the power supply, are essentially complete, but
I still need to design the control circuit, and see how I implement the low pass
filters and protection. Capacitors for such low pass filters are a challenge to
find at reasonable price. And I don't know if I will ever do all the remaining
work. Making something in class D or E looks far more attractive, although also
far more difficult.
By the way, the power supply is a very simple off-line buck regulator.
And between thinking about class AB and class E, I have to run out two or three
times a day to drive the neighbor's cattle out of my property... It looks like I
need better fences before I can think about amplifiers! Or I need to eat one of
his cows, so _he_ fixes the fence! What's more work? Building a fence, or eating
a cow?
Manfred
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Visit my hobby homepage!
http://ludens.cl
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