Phototransistors BJT

Phototransistors are solid-state light sensors that possess internal gain. Phototransistors can be viewed as photodiodes whose output current is fed to the base of a conventional small signal transistor. Phototransistor gain is of the order of 100–1500 as compared to unity gain of photodiodes. However, the response time of phototransistors is of the order of few to hundreds of microseconds, which is much larger than that of photodiodes.

Figure below shows the circuit symbol of a phototransistor.

Circuit symbol for phototransistors