Phototransistor

A phototransistor is a light sensing transistor, which has a special clear window for light to reach the base junction. This component has a collector and emitter junctions, just as a bipolar junction transistor (BJT), however, the base junction is polarised by light instead of an electrical signal. The transistor detects light in the form of photons, which are small particles of light with energy. When they fall on the base junction of the transistor, they generate electrons. These electrons produce a very small current, which controls the collector current flowing through the transistor, just as in an ordinary (BJT).

Since the amount of current flowing through the collector depends upon the amplification parameter (hFE) of the transistor, and the current at the base generated by the light, this type of transistor is still an amplifier. Notice that the base junction of this device is much larger than in conventional BJT to allow a greater surface area onto which the light may fall.

Applications

These were widely used in celluloid film projectors as optical pickups for the audio track. They are also currently widely used in opto-isolators.

Circuit Diagram

Here is a typical circuit diagram of a common-emitter configuration. As you can see, the collector terminal provides the output voltage. There is also a load resistor RL between the collector and the supply voltage.

Modes of Operation

A phototransistor typically operates in either switch mode, or active mode. In active mode, the output varies proportionally to the light level falling on the base junction. The range of light variation it detects depends upon the value of the load resistor.

In switch mode, it operates within saturated and cut-off regions just as a BJT, to simply provide an ON or OFF signal in response to light falling on the base junction. A load resistor of value 5 kΩ typically operates the phototransistor in switch mode. A simple ON or OFF signal is very useful when fed to another transistor to drive a relay or emergency light.

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