Automatic bias for RF power fets

[LM21]

RF power fets need positive gate voltage to function well. That causes some drain current and idle power consumption, even without RF input.

How about rectifying some of the input RF power to get that positive bias voltage to gate. Less  or no power consumption when there is no signal. And of course some delay in power output.

[rfclown]

You could try that; I've not seen it done. An RF detector is usually used to switch the bias voltage. Response time can be less than 1 usec.

[radiolistener]

how you're planning to rectify RF voltage? The diode has some forward open voltage about 0.3-0.7V, so it's not easy to properly rectify it.

[mag_therm]

Wsjt-x emits PTT via Rig_Control, then, with {user adjustable delay, default 200 ms}, baseband starts.

I have homebrew Linear Amp with bias and antenna switching.
The FET driver has asymmetrical +/- supply rails so that zero cross is adjustable relative to gate threshold .
During receive, the  driver (+) rail (adjustable +9 to +13V),  is clamped to 1.2 V dc with Vg at -2.0 V .

The Hermes 14 SDR Txcvr has an open drain that pulls low at PTT.
Initiation of Tx sequence:
   Receiver reed relays (fast) open antenna and short the receive input.
   The 13.8V to 200 V drain converter is started and ramps up.
   The Tx contactor is energised, to close.
   The driver (+) rail clamp is released and ramps up in few ms.
   By the time the RF starts, the 200V supply has already ramped up (about 150 ms)

If you sense RF instead of using Rig Control PTT, I think it should work OK.
    Take care that the TX antenaa switching is before RF starts.
    The loss of 200 ms or so should not affect wsjt-x decoding, and should be OK for ssb too.
   CW might need some extra consideration...

[LM21]

This wouldn't work  on  a  low power transmitter/fet,  you need a certain input voltage to get some DC out of it. But  yes, using RF detection to turn on the bias, would work.

[Wallace Gasiewicz]

If you have a VOX Unit perhaps the output from that could be used to turn on the mosFET BIAS??? Somehow?