Hello Uwe,
I have completed the 2nd part of the circuit diagram of my small
"linear" PA :
http://www.qsl.net/dl4yhf/mf_reception/MF_Reception.html#mf_linear_pa
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IMO, the efficiency for this power range (50 ... 60 watts in my case) is
adequate. It's simple, and even when "slightly overdriven" it retains
the amplitude ramp of the keying signal from the driving transmitter (IC
706 with 2 watts output on MF). When driven to the clipping point, which
is OK for CW, the amplifier drew just above 3 amps DC at 13.8 Volts to
produce 30 watts of RF. At that power level, a small heatsink ("CPU
cooler") is sufficient.
With more turns on the drain output transformer, and a larger heatsink,
even more power would be possible, possibly up to 100 watts (which I
don't need at DF0WD).
The DC overcurrent protection is simple but effective: With the two
transistors on the 2nd part of the circuit, the gate bias voltage is
simply "pulled down" so the drain current and power dissipation stays in
non-lethal limits (for the MOSFETs). The 30 milliohms resistor is just a
short piece of 'Constantan' wire. It is barely visible on the photo of
the board, near the "DC current sense / limiter".
All the best,
Wolf .
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