[Laser] Beginner preamp questions

[John Matz]

Hi
If I recall the K3PGP circuit, the PD has a ~22 meg resistor as a bias/load.
The detected signal goes to the gate of an MPF102 I think.  The situation is
that in high ambient light conditions the PD has a lot of photo-current thru
it, producing a large voltage on the load resistor ... 1/2 ua = 11 volts =
saturation on an 11 volt supply ... less supply, it saturates earlier too.
The solution K3PGP had was to use a lower-value resistor ... like 22k ... so
you can handle 1000 times more light, etc.  But the single stage gain drops
by 1000 too.  Another ac-coupled post-amp stage will compensate, but not for
S/N though.
See ya
John Matz KB9II
----- Original Message -----
From: "steve kavanagh" <skavanagh73 at yahoo.ca>
To: <laser at mailman.qth.net>
Sent: Friday, March 18, 2005 9:35 PM
Subject: Re: [Laser] Beginner preamp questions


> Here's a thought...how about a K3PGP preamp with a
> potentiometer replacing the JFET emitter resistor so
> the device can be rebiased for the same drain current
> at night or in daylight ?  Does that make sense ?
>
> Steve VE3SMA
>
> ______________________________________________________________________
> Post your free ad now! http://personals.yahoo.ca
> _______________________________________________
> Laser mailing list
> Laser at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/laser
>