I got a circuit for a photodiode, but unfortunately without datasheet and explanation. Now I am a bit confused what the purpose of the different parts is:
simulate this circuit – Schematic created using CircuitLab
As far as I understood, the resistor R1 and the diode D1 are providing a voltage drop, while the capacitor C1 provides a voltage buffer. R2 is doing ?, and C2 is coupling out DC current. Is that correct? And what is the purpose of R2?
The photodiode has some parasitic capacitance. Its time constant is hence C times the load resistance. R2 plays the role of this load here; a sufficiently small value such that the response time will be acceptable for whatever application this circuit is intended. It also converts the output current into a voltage, thus giving the circuit a definite gain (200 V/A) independent of the load (within reason).
C2 also needs not to be fully charged if it is to work as a coupling capacitor, and R2 allows for that to happen by providing a d.c. path to ground.
