"Albert" bravely wrote to "All" (06 Jun 04 22:28:43)
--- on the heady topic of "using mosfets as rectifiers?"

One method is to use a low power cmos opamp configured as an ideal AC
full wave rectifier and powered by a square 9V batt. If you don't want
to use a battery then you can easily get a little power from the pc by
using either the game port (+5V at pins 1,8,9,15), printer connector
(+5V 4.7Kohm pin 35, or data), and rs-232 (enable any handshake line).

Don't forget you need a time delay circuit in order to hold the key
down otherwise it will be switching on the peaks only, as you are
experiencing now BTW.

A*s*i*m*o*v


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

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

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

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

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

Al Thanks,

Al Al

.... Back when I was a boy, we sucked the air out of our own vacuum tubes!