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When I see TIA schematics, I can see either the photodiode to GND or against VCC. What is the difference?

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nowox
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    Both of your circuits produce zero bias across the photodiode. If you want reverse bias in the second diagram, set the non-inverting input to GND, and deal with the fact that the output voltage will be below ground. – The Photon Jul 02 '22 at 13:08
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    Or use the first diagram, but apply a negative voltage to the PD anode. – The Photon Jul 02 '22 at 13:09

1 Answers1

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First: photodiode is a current source, which suggests its internal resistance is mighty high...often hundreds of Megohms for a small-area diode.

Zero-bias (GND):

  • minimum leakage current and temperature-induced currents added to photo-current
  • minimum noise
  • highest capacitance, slowing response time.

High-reverse-bias voltage source (Vcc):

  • less diode capacitance, for fast response time.
  • leakage current is higher, causing DC offset.

In some circuits, the photodiode is placed between op-amp "-" and "+" inputs - this is equivalent to zero-bias operation.

glen_geek
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  • Also it seems the high-reverse-bias, would need a Vref on V-, not on the other case right? – nowox Jul 02 '22 at 11:58
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    Linearity is also greatly increased by using bias. – user1850479 Jul 02 '22 at 12:46
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    Any bias supply *must* reverse-bias the diode. The resulting leakage currents are in the same direction as photo-current. So if anode attaches to bias-supply, the supply must be more negative - a cathode attached to bias-supply would require V+. – glen_geek Jul 02 '22 at 12:47
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    Reverse bias also widens the depletion zone, which improves the quantum efficiency, especially in the infrared. – John Doty Jul 02 '22 at 19:46