Photodiodes w/ JFET VCR feedback

Discussion in 'The Projects Forum' started by RitchRock, Feb 25, 2014.

  1. RitchRock

    Thread Starter New Member

    Apr 10, 2013
    8
    0
    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?

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  2. burger2227

    Member

    Feb 3, 2014
    190
    24
    LED's don't have emitters, just anodes and cathodes.

    Never had one that made noise either...
     
  3. RitchRock

    Thread Starter New Member

    Apr 10, 2013
    8
    0
    Hi burger,
    An LED is an emitter - it emits light.

    C
     
  4. RitchRock

    Thread Starter New Member

    Apr 10, 2013
    8
    0
    Also, this is for a fiber optic sensor. Noise often seems to be an issue in these sort of systems.
     
  5. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    631
    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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