Photodiodes w/ JFET VCR feedback

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RitchRock

Joined Apr 10, 2013
8
I've constructed a circuit to cancel out LED emitter noise with a feedback control circuit. PDref monitors just the emitter, while PDsig carries the sensor signal + emitter noise. Feedback consists of integrator -> JFET VCR -> current divider. Please see attached circuit. LT Spice simulation works well, but in real-world implementation I don't see the JFET resistance changing. Any suggestions on what I could be missing from moving from the simulation to prototype?

 

DickCappels

Joined Aug 21, 2008
10,140
With the source of the JFET, you would need to be able to drive its gate negative with respect to the source (and ground) by as much as -6 volts (that's the maximum VGS(off). You need to increase your negative power supply voltage, find N JFET with a lower maximum VGS(off), or try different 2N4341 JEFTs until you find one that works. Also, you need to alow another 2 volts or for the opamp because its output can only get to within about 2 volts of the negative supply.
 
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