Re: using mosfets as rectifiers?

From: starfire (starfire151_at_cableone.net)
Date: 06/08/04 

Date: Mon, 7 Jun 2004 19:12:10 -0600

If you use a MOSFET in a circuit to function similar to a diode in the
conducting direction, remember most MOSFETs have the built-in body diode
which will conduct in a direction opposite to what you want to rectify, so
this may not be a desired result.

If you say you're only about 0.5V away from something useable, why don't you
just change out your silicon diode for a germanium and regain about 0.4V?

Dave

"Albert" <pass@pass.com> wrote in message
news:mbk7c0dga3981f5rmd347r1kjve4av44vh@4ax.com...
> 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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