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opa128: opa128 photodiode amplifier applications note circuit

user5166883
Prodigy 20 points
Part Number: opa128


Hi,

This question relates to application note sboa061.pdf,  "DESIGNING PHOTODIODE AMPLIFIER CIRCUITS WITH OPA128"

I'm interested to understand the bias part of the photodiode circuit, figure 4.  There is an error in the circuit diagram I think -  the bias supply for the photodiode is currently marked as ground.  Presumably both the resistor and the 0.1uF capacitor are connected to the bias supply?  It should be obvious but I can't quite see the function of the resistor and capacitor, so hesitate to guess (and wonder what value to use for the resistor). 
 Wouldn't  the circuit still function if those components were omitted, and the photodiode connected directly to a bias supply, and also have less noise?

www.ti.com/.../sboa061.pdf

Thanks, Peter

  • 0
    TI__Mastermind 23050 points

    Hi Peter,

    I think you are correct about the diagram, I don't see a reason to include this R and C either. I believe the resistor was intended to be the bias voltage source and the cap would then serve as a local bypass. The goal here is to operate the diode in the photo-conductive mode. It is much more common to do so using the circuit topology below because this allows you to bias your photodiode and also move your output for single supply operation. The current through the photodiode will also result in a positive output voltage.

  • 0 in reply to Zak Kaye
    Prodigy 20 points

    Thanks Zak, that's very helpful.  Despite having read around a bit, I had not appreciated before that that configuration would reverse bias the photodiode.

    Peter