Jump to content

Photocurrent Bias

Recommended Posts

Hello everybody!

Some sensors need a huge input impedance: if highly resistive, if capacitive, at low frequencies - reasons vary. Commercial resistors exist up to 22Mohm, uncommonly 100Gohm, and high values integrate badly on a chip; instead, I propose to polarize the amplifier by photocurrents.


The loop can provide a feedback at frequencies lower than the signal, or include the signal frequencies, or set a zero when not sensing the signal - or other uses and their combinations.

Over a resistor, photodiodes have the advantages of a huge impedance up to some 0.2V, especially if used in the photovoltaic mode (=without external bias) which has zero residual current. This reduces the noise, especially where capacitances and frequencies are small.

Over a zero switch, photodiodes advantageously inject no switching charge nor leakage current.

Discrete photodiodes exist with a capacitance <<1pF. Within a chip, photodiodes can be made even smaller, adding very little capacitance to a Mos input, and protecting against static charges. In both cases, an optical attenuator can match their sensitivity to the range of light sources. If >0.2V are needed, the diodes can be in series, more can be connected - but two diodes with a bigger bandgap and small leakage would be preferable.

I didn't try that one, but it can only work.
Marc Schaefer, aka Enthalpy

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.