Photodiode -- photoconductive (reverse bias) vs photovoltaic (zero-bias) for tachometer

Question

I'm building a tachometer for speeds 0 to 3600 rpm (various speed motors turning things down to whirligigs blown by the kids.). I've got a photo diode running in photovoltaic mode generating about 50 mv pulses. (Of course, this is a function of distance and illumination; my hand held sensor is 3 or 4 inches away from the rotating object.)

Somewhere I got the notion that running in photoconductive (reverse bias) mode would get a better signal from the diode, but my cursory research suggest that generating a current mode signal wouldn't give any different results from a voltage mode signal, as far as the quality and strength of the signal feeding into the amplifier.

In other words, photo-conductive mode wouldn't have any amplification effect relative to photo-voltaic mode.

Am I missing something? Can you think of any reason to prefer one mode over the other?

Answer 1

It depends on how you are sensing the current. If you are just using a resistor to ground, then the photodiode will become forward biased and you get a maximum voltage of about 0.5V. (If you use a TIA opamp circuit that won't happen.) If you reverse bias it then you can get output voltages up to the bias supply. With a TIA opamp circuit the only advantage (as HH plasma said) is a higher frequency response.