[Laser] simplified KA7OEI receiver

[Stuart]

Hello Steve,
I have also made a simplified  receiver, but I did not go as far as you have gone. My circuit is well down the page on the projects/basic receivers tabs of this site, it omits the second op-amp and has the diode  RC coupled since we use it to work in the full daylight also. I use a LED instead of the photodiode so I can use it on transmit also.
When measuring the performance of the receiver I found it essential to get the drain voltage of the FET to half the power supply voltage, or the sensitivity goes down. Several others have found this also, that's   why the complex bias circuit is there to run the FET at its optimum point.
Over here in the UK, some of us came to the conclusion that the performance of any front end really depends on the diode and first stage device, since several designs have given the same results.
If you look on our Yahoo Group "UKNanowaves", in the files area (which I think is public), there are some other circuits by Roger Green M0RJG, that use only two devices, are quite simple, and give the same results as the "full" KA7OEI circuit. 


Good luck,


Stuart, G8CYW




Message: 1
Date: Tue, 13 Dec 2011 06:30:30 -0800 (PST)
From: steve kavanagh <skavanagh73 at yahoo.ca>
Subject: [Laser] Preamp design and other related stuff
To: laser at mailman.qth.net
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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