Opto photodiode varicap?

[Rich Grise]

When a photodiode is reverse-biased, it has some capacitance,
which has to be accounted for, right?

If you reverse-biased the PHD in an optoisolator, would it
become an isolated varicap?

Conversely, just for fun, let's hear everybody's favorite
easiest way to use optos for isolated gain control feedback.

Thanks!
Rich

 

[Tom Bruhns]

When a photodiode is reverse-biased, it has some capacitance,
which has to be accounted for, right?

If you reverse-biased the PHD in an optoisolator, would it
become an isolated varicap?

Conversely, just for fun, let's hear everybody's favorite
easiest way to use optos for isolated gain control feedback.

Thanks!
Rich

Well, I suppose it would be isolated, and it is a varicap, in that its
capacitance changes with applied reverse voltage. But what would vary
in the diode, as you varied the LED current? Hint: not the
capacitance, at least not by much. Consider that the width of the
depletion region is set mainly by the reverse voltage applied to the
diode. What happens when photons come zipping into that depletion
region?

Cheers,
Tom