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Calculating photodiode current and voltage

  1. Jul 24, 2010 #1
    Hi I'm pretty new to the forums. I've been searching and can't find anything on this. Can anyone show me how to calculate the current/voltage produced by a photodiode?
  2. jcsd
  3. Jul 24, 2010 #2
    Here are the specs for a typical Hamamatsu Si PIN photodiode.


    Look up the spectral response for the wavelength you are interested in (see dotted line for 100% quantum efficiency* in plot). PIN photodiodes are essentially current sources, because the current output is nearly independent of the applied voltage. The depletion depth is much larger than the photon absorption length.

    * 100% quantum efficiency is 1 photoelectron per incident photon

    Bob S
  4. Jul 31, 2010 #3
    People tend to use the diodes in one of the following three ways:

    1. Reverse biased and treated as a current source
    2. Shunted into a low value impedance (typically 50 ohms)
    3. Operated in the "photo-voltaic" mode

    This first method is by and far preferred in that it gives you a current that's proportional to the number of photons. With the proper amplifier, this method is typically fairly sensitive and has a fair bandwidth.

    The second method is often used for wide bandwidth applications. Most applications involving a 50 ohm load are based on photo diodes going into test equipment and the sensitivity is poor. However, the linearity is still pretty good - voltage is proportional to photons for a reasonable excitation.

    The third method involves leaving the photo diode "unloaded." The following instrument or amplifier has a high input impedance and with even a little light, the diode reaches a saturation point where the output voltage is limited. Typically, you can get up to about .6v and it takes progressively more light to make a moderate increase output voltage.
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