Photodiodes and Phototransistors

Phototransistors

Phototransistors are transistors with the base terminal exposed. Instead of applying a voltage to the base, the photons from striking light activate the transistor. Other than that, the phototransistor behaves just like a normal transistor. Two common configurations are shown on the right.

• Common-Emitter Amplifier - goes from "high" to "low" with light.

• Commond-Collector Amplifier - goes from "low" to "high" with light.

The phototransistor can be used in two different modes: 1) active & 2) switch. These modes are controlled by changing the value of the resistor. The equations are:

Fairchild recommends a 5kohm resistor or greater to use as a switch

• Switch Mode - when operating as a switch, the transistor can be switched between the cut-off ("off") and saturated ("on") states. This means that when light strikes the phototransistor, it will conduct. Otherwise, it will insulate.

• Active Mode - In active mode, the output of the transistor is proportional to the intensity of the light.

Comparison

• Frequency Response

  Photodiodes are much faster than phototransistors

• Gain

  Phototransistors have a higher gain

Applications

• Optocoupler

  Optocouplers are used in electronics-sensitive applications. For example, you may use this in a mobile robot application to separate the microcontroller circuitry (low voltage/power) from the motor driver circuitry (high voltage/power).

References

• Fairchild Semiconductor, "Design Fundamentals for Phototransistor Circuits," PDF,