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The Photonics Spotlight

Using a Current Amplifier for Optical Power Measurements and Recording with a Photodiode

Dr. Rüdiger Paschotta

Ref.: encyclopedia article on photodiodes

Honestly, it looked to me like a complete overkill when many years ago a colleague of mine suggested to buy a commercial current amplifier (also called transimpedance amplifier) for various kinds of measurements with a photodiode in our laboratory. After all, couldn't I just use a very simple home-made circuit, consisting of nothing more than a small battery and a resistor? In this way I could apply some reverse voltage to the photodiode and convert the photocurrent into a voltage, which I could then easily measure with a multimeter or monitor with an oscilloscope. The cost for that would be a tiny fraction of that of a commercial current amplifier.

Well, in principle this works, but I soon learned to appreciate the advantages of the current amplifier:

Having used this kind of equipment, never again would you bother to save some money by using a simple resistor/battery circuit instead of a current amplifier, when you need flexibility in the laboratory! You may, however, consider to build such a device yourself, or let your electronics support build one. If you don't require top performance, but need several of those devices in your lab, this can be a reasonable solution.

For high-speed applications or in cases where you need ultimate sensitivity, a universal current amplifier may no longer be suitable. Here, you have to get the amplifier as close as possible to your photodiode – ideally, built into the photodiode housing. For such purposes, there are reasonably prized current amplifier modules, if you don't want to get into the electronics details.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.

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