Neltner Labs

Tutorials and Projects from Brian Neltner

Photodetector Circuit Explainer

I found that there wasn’t a good beginner compilation of the different approaches for using photodiodes that explains forward vs reverse bias, why some designs use a negative instead of a positive bias voltage, or why transimpedance amplifiers are often used. So I made a few example circuits to get the basics in one place.

Photodiode Explainer.png

The biggest reason to use a transimpedance amplifier (TIA) is that the voltage noise gain is one while the current gain is extremely large. This works well because the photodiodes act as current sources. In the TIA designs to the right of the cheat sheet, the current flows into a “virtual ground” at the negative input pin of the op-amp (so-called because it is forced by the op-amp gain to be equal to the positive input pin) and since it cannot flow into the high-impedance op-amp input pin it instead all has to flow through the RSENSE resistor that provides current to voltage conversion with extremely high gain.

In addition, by creating a virtual ground the voltage across the photodiode doesn’t change if RSENSE is large (which in the middle two designs would cause a decrease in the bias voltage). You may imagine that if 10uA flows into a 10k resistor the voltage is 100mV, causing an apparent 100mV drop in the reverse bias voltage. Finally, all of these designs have low impedance outputs from the op-amp and high-impedance inputs. If you were to simply use a large sense resistor to detect the current and use that voltage directly, current drawn from that signal will cause errors. Resistor-only design not shown because they’re a terrible idea for large gains.

The TIA is an inverting amplifier by design, meaning that current flowing in from the sensor results in a negative output voltage. This is fine for bipolar supply systems, but I like the negative bias design so that my voltages can be directly converted with an analog to digital converter.

In many circuits simply using voltage mode gain will work just fine, but here are the available options in broad strokes. There are many excellent resources for improving any of these designs once you’ve selected a starting point.

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