[3] the standard phototransistor symbol
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[1] a standard phototransistor
Phototransistor
Phototransistor
[2] A phototransistor is a light-sensitive transistors that works like a standard BJT, but IO using base current, it uses incident light to control current flow between its collector and emitter terminals.
The magnitude of collector-emitter current it passes depends on the light intensity its base receives. Thus it can be used as solid-state light sensor with built-in current gain, Beta (β) producing a much larger output current (milliamps) than a photodiode (microamps) for the same amount of light intensity.
etm: "photo-" (Gr.) = light
[1] Similarly, a photodiode is a semiconductor converting light into electricity. It absorbs incoming light particles (photons), creating free electrons and holes, creating electric current. The current's magnitude matches the light brightness.
Operation
Operation
[1] Like a regular BJT, a phototransistor consists of 2 PN-junctions and terminals; the emitter, base, and collector.
But the phototransistor can also be a photodiode whose output photo-current is fed directly to the base of a regular small signal transistor, making phototransistors much more sensitive to variations in light intensity than photodiodes, but respond slower to such changes."
While a photodiode's output signal can always be amplified via an op-amp or other circuits, it's often more practical and cost effective to use 1 phototransistors for light detection.
Also, phototransistor can pass emitter currents of up to a few milliamps compared to the microamps of a photodiode, making them easier to interface to than photodiodes.
[1]
[1] Phototransistors can be used as optical switches in optocoupler devices to electrically isolate the output phototransistor switch from the input single being generated by the LED. The transmissive sensor detects an object or rotating slotted or coded disk that blocks the light passing through the gap between the LED and phototransistor of the optical switch.
Types
Types
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