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The phototransistor is similar to the conventional transistor, except that it uses light energy, instead of electricity, as its input signal. The advantage over the photodiode is the current amplification provided by the transistor.

Phototransistors may be of the two-terminal type, in which the light intensity on the phototransistor alone determines the amount of conduction. They may also be of the three-terminal type, which have an added base lead that allows an electrical bias to be applied to the base. The bias allows an optimum transistor conduction level, and thus compensates for ambient (normal room) light intensity. The figure below illustrates the schematic symbols for the various type of phototransistors.

2-terminal and 3-terminal phototransistors.

The figure below shows the basic phototransistor circuit. This is seen to be common-emitter amplifier circuit. When light strikes upon the phototransistor, current carriers are released and setup a base current. This corresponds to the input current of the common-emitter circuit. This current is amplified in accordance with the amplification factor, beta, of the transistor. An amplified collector current I accordingly flows through the load RL. Before the application of light, a small dark current flows in the collector circuit.

Basic phototransistor circuit
Basic phototransistor circuit.

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