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Enthalpy

Photocurrent Bias

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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.

post-53915-0-86718600-1430065998.png

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

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