... what should be the optimum values for R1,C1 and range of R3
Please explain more. What do you want? As far as i can get you want to measure the current when there is no light and no infrared source on it?
And what is your application?
I aim at designing a circuit that would generate an output current of order pA say some 10pA.This is to test an amplifier circuit that would amplify this pA current to a measurable voltage of range 0-5V
I think these values aren't critical at all. I'd assign R1 a voltage drop of 1V at the highest expected dark current (typ. 10pA, max. 0.5nA @ 25°C) at your highest expected temperature, say R1 = 1V/10nA = 100MΩ. With a ceramic cap of C1 = 1nF or 10nF (close between the photodiode's cathode and GND) you have a ≈1Hz low pass filter against noise and op. voltage changes.
R3 isn't necessary at all, but anything between 1..10 MΩ doesn't hurt if you insist.
Don't you think there are other way to generate such low current and test your amplifier. What so special about this circuit?
What could be an alternate method for generating such small current?
I used a voltage source in the mV range along with 1 Giga Ohm input resistance so that I can get some 1 to 10 pA current input to circuit.
Apart from the question, if other circuits may be better suited as low current generator, why don't you use standard low leakage diodes instead of photo diodes?
Don't you think there are other way to generate such low current and test your amplifier. What so special about this circuit?
Are you goin to test it in space, vacume but cosmic rays may affect there also. Ig ohm may be close to air resistance.
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