Photodiode receiver - HELP

Discussion in 'The Projects Forum' started by shahidmk, Nov 20, 2011.

  1. shahidmk

    Thread Starter New Member

    Nov 16, 2011
    1
    0
    Greetings,

    I hope I am posting in the correct forum, if not please forgive the newbie mistake.
    Attached is a circuit used to detect a pulsed red laser 635 - 650nm. Could some one please explain the reasoning behind the use of the inductor. and what effects would it have if I changed the inductor to a higher value. The output signal is fed into an lm324 for appropriate signal amplification.

    Currently the ambient light is filtered using an optical filter, would it be possible to implement an electronic filter for the said wavelengths without getting too complex.

    The circuit works as it is, I am just trying to learn some of the basics. any possible suggestions to improvement of the circuit are extremely welcomed.

    cheers
     
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    The inductor provides a simple means of biasing the DC output level of the photodiode at 0V while providing reactance that develops a voltage for rapid changes in light or transients. This prevents circuit saturation regardless of the level of ambient lighting--this variable is otherwise difficult to deal with.

    I, myself, have accomplished this via a constant current load that is actively controlled, thus providing the same function as the inductor. The inductor is a really neat idea--wished I had thought of it or known about it years ago.

    I can suggest little to improve it--since its output is 0VDC, the coupling capacitor may not be needed--this depends upon the dc gain of any amplifier connected to the output. The shunt resistor effectively controls the gain. The shunt capacitor and resistor tailor the high frequency roll-off. The inductor may need to be shielded or be a self-shielding variety to prevent inductive coupling with AC flux from power wiring, transformers, lighting ballasts etc.

    Increasing the inductance may have little effect upon the output, but there is a resonant point that you may wish to consider--resonant circuit consists of both the shunt inductor and shunt capacitor--easy to calculate--then you can consider the Q factor that is a function of the inductor series resistance and shunt resistor. This resonance may increase the sensitivity substantially, especially if there is a carrier frequency present--may mess with the signal, if there is a variable data rate...
     
    Last edited: Nov 21, 2011
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