Difference between revisions of "Photodiodes and Phototransistors"

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* '''Frequency Response'''
* '''Frequency Response'''
*; Photodiodes are much faster than phototransistors
*:: Photodiodes are much faster than phototransistors


* '''Gain'''
* '''Gain'''
*; Phototransistors have a higher gain
*:: Phototransistors have a higher gain



==Applications==
==Applications==

Revision as of 15:42, 6 July 2006

Phototransistors

Phototransistor amplifiers.png

Phototransistors are transistors with the base terminal exposed. Instead of applying a voltage to the base, the photons from striking light activate the transistor. Other than that, the phototransistor behaves just like a normal transistor. Two common configurations are shown on the right.

  • Common-Emitter Amplifier - goes from "high" to "low" with light.
  • Commond-Collector Amplifier - goes from "low" to "high" with light.

The phototransistor can be used in two different modes: 1) active & 2) switch. These modes are controlled by changing the value of the resistor. The equations are:

Phototransistor modes.png

Fairchild recommends a 5kohm resistor or greater to use as a switch

  • Switch Mode - when operating as a switch, the transistor can be switched between the cut-off ("off") and saturated ("on") states. This means that when light strikes the phototransistor, it will conduct. Otherwise, it will insulate.
  • Active Mode - In active mode, the output of the transistor is proportional to the intensity of the light.


Photodiodes

Comparison

  • Frequency Response
    Photodiodes are much faster than phototransistors
  • Gain
    Phototransistors have a higher gain

Applications

References