Question

(C) It is predicted that the sun emits solar flares of various types that flash for 10 ps (flare pulse) to 1 ms (monster flare). In order to know whether your detector can see the flares, you look up FDS010 and found junction capacitance is CJ 6pF, shunt resistance R_SH1 MOhm and series resistor 500 Ohm. See effective diagram below. Estimate the range of the solar flash duration your photodector can record. W-oAnode Rs PH o Cathodee

Answer 1

The concept of this problem lies in finding out the maximum and minimum time constants (= R_eqC) for which the photodiode can work. The time taken to detect the flash is 5 times the time constant = 5R_eqC which is the time for the R-C circuit to attain steady state value.

For the minimium and maximum flash detection, the resistors R_S and R_SH will come into play as time constant and 5 times the value gives the flash duration.

Hence minimum flash duration that can be sensed = 5 x 500 x 6 = 150 ps

Maximum flash duration that can be sensed = 5 x (1 x 10⁶) x (6 x 10^-12) = 30 x 10^-6 s = 30 _$\mu$ s.

This gives the range of flash detection.