A fraction of an ohm resistor in the source lead would provide a negative bias with increasing device current.
There will be a small drop in output power I guess. The resister can be bypassed with a cap.
Never tried this with FET but just a thought!
Raj, vu2zap
P.S explanation for some:
If you have say 4V gate bias, thats the voltage between gate and source connected to ground.
with a source resistor, when the current increases in IRF - there will be a voltage drop across this
source resistor (say 1V) and that would subtract from gate voltage and ground thereby reducing the
gate-source bias to 3V. This will reduce IRF current IMHO.
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At 16-02-2017, you wrote:
Monitoring temperature in a PA (usually with a diode) works by regulating the drive voltage
as temperature varies. It has little to do with SWR. It reduces the drive as the finals get hotter.
Speaking of which, there is a great difference between the impedance at the drain of the
MOSFET and the end of the LPF which connects to the antenna. One of the functions of
the output transformer/LPF combination is to transform the drain impedance to 50 ohms for
the antenna. Usually it is too big a job which is why there is a lot of fiddling with transformer
ratios and capacitance at the drain output. If it is too far off, the device will oscillate..and it
doesn't take very much to be too far off. Keep this in mind when fiddling with things. The
main trouble is the high drain capacitance. BJT's have a much lower output capacitance and
are far easier to match. Most of the oscillating is in the VHF region but it can occur at any
frequency. The best solution to suppress it appears to be with a 10 ohm resistor right at the gate
between the gate and the drive circuit. Another MOSFET quirk. And if you think it is easy, this was
the first and most fundamental problem with the BITX20A, which was very well engineered and
pre-tested.
Reports and solutions for this oscillation are many and a lot of them are in the files section.
I wouldn't recommend a fast-blow fuse as a solution to the current problem. AS Raj notes,
any of them are really too slow for MOS devices. They are fine for vacuum tubes (and even TTL)
but CMOS is way too fast for them. The reaction time should be in the microsecond range.
john
AD5YE
Raj vu2zap