Advantages of Phototransistor | disadvantages of Phototransistor

This page covers advantages and disadvantages of Phototransistor. It mentions Phototransistor advantages or benefits and Phototransistor disadvantages or drawbacks.

What is Phototransistor?

• It is a transistor controlled by exposure to light. It is similar to photodiode controlling an ordinary bipolar transistor.
• It can be either bipolar transistor or field effect transistor (FET).
• Its body is encased in resin or plastic or encased in a metal shell with window in it.


• Phototransistor has two leads which connect internally with its collector and emitter (or source and drain in FET). The base of transistor (or gate in FET) responds to light and controls the flow of current between the leads.
• In the absence of light, bipolar phototransistor permits leakage between collector and emitter of 100 nA or less. When exposed to light, it conducts upto 50mA. This differentiates it from photodiode which can not pass much current.
• The figure-1 depicts most common variant of phototransistor i.e. Bipolar NPN.
• Examples: Optek/TT Electronics OP506A, Vishay TEKT5400S, Vishay BPW17N etc.

Benefits or advantages of Phototransistor

Following are the benefits or advantages of Phototransistor:
➨There are numerous applications of phototransistor such as light measurement, light sensitive switch, opto-coupler etc.
➨The dark current of a phototransistor is much higher compare to photodiode.
➨It has ability to sink current of about 20 mA to 50 mA. Hence it can be connected to component having low impedance. For example, phototransistor can drive piezoelectric audio transducer or LED indicator directly. This means it can directly interface to small loads.
➨Phototransistor is more responsive to light than photoresistor.
➨Phototransistors are cheaper.
➨Phototransistors can produce a voltage which can be used in conjunction with microcontroller housing ADC.
➨Output current of phototransistor is easily obtainable.

Drawbacks or disadvantages of Phototransistor

Following are the drawbacks or disadvantages of Phototransistor:
➨It is insensitive to incident light from other directions than particular narrow window unlike photoresistor which is sensitive to incident light from anywhere in front of it.
➨Silicon phototransistors do not handle higher voltages above 1000V.
➨They are more susceptible to electricity surges/spikes and EM energy from radiations.
➨It has nonlinear characteristic and it is temperature sensitive.
➨Large dispersion between individual units.

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