A typical photodiode model contains the following key elements: a diode in parallel with a current source, and the current source is proportional to the light intensity. Parasitic elements CD and RD will affect device performance.

In photovoltaic mode, the photocurrent flows in the loop shown in Figure 2 and provides forward bias to the diode. Since the voltage-current relationship of the diode is logarithmic, the open-circuit output voltage is approximately logarithmically related to the photocurrent and is corrected by a small current through RD. Therefore, the relationship between output voltage and light intensity is highly nonlinear. Some applications will benefit from the logarithmic relationship because changes in light intensity over a large range (the eye is a perfect logarithm) will cause similar changes in voltage. Since the diode voltage-current characteristic is temperature-dependent, the absolute relationship between voltage and light intensity is poor.

In photovoltaic mode, diode capacitance limits frequency response. Rapid changes in light intensity will charge and discharge CD. This is not a mode for fast response.

The output can introduce buffering or can also be amplified in phase. In order to achieve low input bias current, CMOS or JFET operational amplifiers can be used so that the op amp does not become a load on the photodiode at low light intensities.

In photovoltaic mode, when a load is introduced at the output end, there will be a significant drop in voltage. In order to output the highest power, the load value adopted is determined by the light intensity.

Photoconductive mode - The diode voltage is constant, as shown in Figure 3, usually 0V. A transimpedance amplifier is usually used to convert photocurrent into voltage. The capacitance of it can be reduced by applying reverse bias to it, but this will cause leakage of dark current. When there is no forward voltage across the diode, the response is linear with light intensity. In addition, since the voltage across the capacitor does not change with changes in light intensity, frequency response is greatly improved. Since capacitance forms a pole in a negative feedback loop, it is necessary to reduce its value. In order to achieve stability, a feedback capacitor CF is usually introduced.

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By simply loading a photodiode with a resistance value of about 50 ohms or so, you can get many benefits from photoconductive mode. If the diode voltage does not exceed 20 mV, there is no need to forward bias it and the response will still be reasonable and fast. However, sensitivity will be low.

Avalanche photodiodes are special modes that require reverse bias voltages close to breakdown voltages. This allows output currents to be amplified at low light intensities.

There are many trade-offs when choosing photodiodes, including size of photodiodes, capacitance, noise, dark current and package type. Generally speaking, it’s best to choose smaller photodiodes with reflectors or lenses that can focus light sources. Texas Instruments does not produce individual photodiodes; however for many basic applications OPT101 which integrates photodiodes and transimpedance amplifiers on one chip provides a complete solution.