[Laser] Re : Preamp design and other related stuff

[Garnier Yves]

Hi Steve.
In your schematic the capacitor following the photodiode is not useful and can produce very slow recoveries biasing phenomenons after a light pulse or after power on.
Permute the photodiode leads to be in reverse mode with the current leakage from the FET gate
R drain 1K must be adjusted to give half PSU voltage on the drain in low light conditions because your diagram will work near the IDSS FET current and this value is very changing and dispersed a FET to an other even for the exactly the same item type and provider !
73 

Yves F1AVY
http://f1avyopto.wifeo.com


________________________________
 De : steve kavanagh <skavanagh73 at yahoo.ca>
À : laser at mailman.qth.net 
Envoyé le : Mardi 13 Décembre 2011 15h30
Objet : [Laser] Preamp design and other related stuff
 
I decided to try something like KA7OEI's version 3 front end, but simplified to the bare bones (so I would actually get it built !).  No bias on the photodiode, a simple AC-coupled JFET/bipolar cascode amp, and no bias on the JFET (well, self biased through the gate leakage current, with an AC coupled photodiode, so the drain current is just a bit more than Idss).  

Schematic is at

http://www.qsl.net/ve3sma/OpticalRx2011PreampSchematic.jpg

It seems to basically work, but there are so many other changes in my new receiver compared to the old one it is difficult to say how well, so far.

Any comments ?

73,
Steve VE3SMA
______________________________________________________________
Laser mailing list
Home: http://mailman.qth.net/mailman/listinfo/laser
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Laser at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html