Albert wrote:
It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Albert, take a high beta BJT which needs only 750mV and maybe 25uA to switch
lets say 15mA of collector current. You can also double the voltage of the
transformer (if it has isolated windings).
___
o----. ,---+-|--------+----|___|--+
)|( | | + 39k |
)|( | ### | +--------o
o----' '-+ | --- | |
| | |47u | |/
+-)-----------+ +-|
| | + |
| ### |
| --- |
| |47u |
+--|-------+---------------+--------o
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
Maybe you have to use another resistor to discharge the caps, if the
transistor doesn't turn off.

--
ciao Ban
Bordighera, Italy