Difference between revisions of "Photodiodes and Phototransistors"
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==Phototransistors== |
==Phototransistors== |
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[[image:phototransistor amplifiers.png|right |
[[image:phototransistor amplifiers.png|right]] |
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Phototransistors are transistors with the base terminal exposed. Instead of applying a voltage to the base, the photons from striking light activate the transistor. |
Phototransistors are transistors with the base terminal exposed. Instead of applying a voltage to the base, the photons from striking light activate the transistor. |
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Revision as of 14:19, 6 July 2006
Phototransistors
Phototransistors are transistors with the base terminal exposed. Instead of applying a voltage to the base, the photons from striking light activate the transistor.
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:
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.
References
- Fairchild Semiconductor, "Design Fundamentals for Phototransistor Circuits," PDF, http://www.fairchildsemi.com
