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Photodiode photometer

Download for Macintosh or for PC.

The purpose of this circuit is to generate an output voltage that is proportional to the current flowing through a photodiode (which is in turn proportional to the light intensity falling on the photodiode).

Adjust the slider to change the light intensity on the photodiode. The resulting photocurrent flows almost entirely onto the feedback resistor Rf. The feedback resistor sets the gain of the circuit. Vout = -IinRf. Note that the current between the - and + inputs of the op amp is very small compared to the resistor current, even if Rf is larger than the differential input resistance of the op amp, because of the very small voltage between the inputs. (The fraction of the input photocurrent that flows into the op amp is displayed below).

You can also change the differential input resistance (typically 1 to 100 Megohm) and and the open-loop gain (typically 50000 to 200000).

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This page is maintained by Prof. T. C. O'Haver , Department of Chemistry and Biochemistry, The University of Maryland at College Park. Comments, suggestions and questions should be directed to Prof. O'Haver at
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