A photodiode can be regarded as a high-impedance non-ohmic photosensitive device whose current is almost independent of applied voltage. The incident light falls on a reverse-biased semiconductor junction, and the separation of electrons from holes will allow the junction to conduct despite the reverse-bias.

Photodiodes are constructed like any other diodes, using silicon, but without the opaque coating that is normally used on signal and rectifier diodes. The junction area may be quite large, so the photodiode may have more capacitance between electrodes than a conventional signal diode.

This can be compensated by using a feedback capacitor in the circuit, illustrated in Figure 5.6, which shows a typical circuit for using a photodiode along with an operational amplifier for a voltage output.

The feedback resistor R will determine the output voltage, which will be RI, where I is the diode current.
• Some LEDs can be used as photodiodes with peak sensitivity values in the infra-red or in the visible spectrum, and in some circuits it can be convenient to use the same device as both a receiver and an indicator.

Characteristics for photodiodes specify the output current into a short circuit, and the current will be much lower into a resistance of appreciable value. The sensitivity can be quoted in terms of incident light measurements, but Table 5.2, shows, more usefully, the output of some types when the incident light is provided by various typical sources.

Figure 5.8 shows typical circuits using a photodiode using an operational amplifier as a load. The circuit in (a) is used for high sensitivity and operation down to DC levels. The circuit in (b) is preferred when speed of response is preferred to operation at very low frequencies.

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