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    Photodiodes w/ JFET VCR feedback

    Hi all, I'm working on an optical sensor and am looking to increase my SNR. I've constructed a circuit to cancel out LED emitter noise with feedback control. 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 (I believe it should balance when I move VR1). Any suggestions on what I could be missing from moving from the simulation to prototype?

    Click image for larger version. 

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    Last edited by alexan_e; 26th February 2014 at 00:42. Reason: attached images to the post

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