I have an extremely low power application (recovering a small amount
of dc power from the audio output of a PC soundcard). I am trying to
key a transmitter when the soundcard puts out audio. It's actually a
self powered VOX, deriving all it's needed power from the audio output
of the soundcard. Right now, I'm almost succeeding, but need another
half volt or so to make the keying reliable.

I am using my laptop computer, which has only speaker out jacks. I am
feeding the speaker output into the low impedance side of an audio
transformer and taking the stepped up voltage from the secondary (8
ohm to 1K ohm impedance transformer). I am feeding the secondary into
a small signal schottky diode and filtering the rectified output to
get my dc power.

As it is now, it's barely usable. I have to really crank up the laptop
audio in order to get reliable keying.

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??

Thanks,

Al



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