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Phototransistor basics | Phototransistor types, Phototransistor applications

This article covers Phototransistor basics and Phototransistor types including its applications. It describes different types of Phototransistors Viz. BJT phototransistor,FET phototransistor etc.

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

• The figure-1 depicts most common variant of phototransistor i.e. Bipolar NPN.
• Examples: Optek/TT Electronics OP506A, Vishay TEKT5400S, Vishay BPW17N etc.

Phototransistor applications

Following are the applications of Phototransistor:
• It may be used for light measurement or as light sensitive switch.
• Its output is interfaced with microcontroller containing ADC.
• To achieve clear ON/OFF signal, it is interfaced to input of the logic gate containing a schmitt trigger. It can also be processed by comparator for the same.
• Optocoupler contains phototransistor which is activated by internal LED device. Its purpose is to switch current while isolating one part of the circuit from the other part.

Types of Phototransistors

Following are the types of Phototransistors viz. BJT phototransistor and FET phototransistor based on their construction and principles of operation.

Phototransistor types-BJT,FET

• BJT Phototransistor: 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.
• FET Phototransistor: 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.

Other Phototransistor types or variants

Due to advancement in surface mount technologies, these variants are widely available. Although through-hole packages are also available very commonly. Phototransistors are encapsulated like LED and are also available with flat surface. Flat surface variants are sensitive to light from any direction in front of it.

➨Some phototransistor variants provide base connection or gate connection. This allows application of bias current in order to prevent low amplitude light levels from triggering the phototransistor.
➨Photodarlington: It is similar to darlington transistor. It is made of pair of bipolar transistors. The first transistor is sensitive to light where as the second transistor acts as amplifier.
• Advantages: The photodarlington is more sensitive to light compare to regular phototransistor.
• Disadvantages: Its response is slower. It is less linear.
➨Photo-FET: It is not used as standalone component. It is used in optocoupler. It is symmetrical for either of the polarity and hence used for AC signals. It works as controllable resistance when applied voltage is low in contrast ro bipolar transistors.

Also refer advantages and disadvantages of Phototransistor >> for more information.



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