What is a phototransistor?
The phototransistor is the type of diode junction photo-sensitive device. In addition to silicon and germanium photodiodes, that utilises the light-sensitive qualities of the p-n material. The phototransistor possesses the ability to detect not only the presence of a light signal, but it can amplify its signal strength at the same time.
How the Phototransistor Works?
The principle of the NPN transistor is applied to the phototransistor. In general, the phototransistor is an NPN transistor based.
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We let light fall onto the base-emitter junction. What with each photon being converted into an electron and the electron allowed to flow into the emitter region, this constitutes a withdrawal of electrons from the base just as in the case of the microphone circuit.
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As a result, a much larger current, a current that is proportional to the light’s intensity falling onto the phototransistor, flows in the collector-emitter circuit.
It is possible to obtain current gains that result in many hundreds of electrons being controlled by just one photoelectron. As a result of this electron control multiplication ability, the phototransistor is capable of very weak light detection.
Applications of Phototransistors:
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Used for the purpose of electrically isolating portions of electronic or electrical circuits from other portions. The optoisolator may be considered a contact-less high-speed switch having its input and output separated by an infinitely high impedance.
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Phototransistors find extensive use in those applications where modulated light is transported over long distances requiring light signal relaying (i.e., repeating) or requiring final demodulation at the end of a lengthy transmitted light path.