TC 9-62
3-56. Another special optoelectronic device in common use today is the PHOTODIODE.
Unlike the LED, which produces light, the photodiode uses light to accomplish special
circuit functions. Basically, the photodiode is a light-controlled variable resistor. In total
darkness, it has a relatively high resistance and therefore conducts little current. However,
when the PN junction is exposed to an external light source, internal resistance decreases
and current flow increases. The photodiode is operated with reverse bias and conducts
current in direct proportion to the intensity of the light source.
3-57. Figure 3-31 shows a photodiode with its schematic symbol. The arrows pointing
toward the symbol indicate that light is required for operation of the device. A light source
is aimed at the photodiode through a transparent "window" placed over the semiconductor
chip. Switching the light source on or off changes the conduction level of the photodiode.
Varying the light intensity, controls the amount of conduction. Since photodiodes respond
quickly to changes in light intensity, they are extremely useful in digital applications such
as computer card readers, paper tape readers, and photographic light meters. They are also
used in some types of optical scanning equipment.
Figure 3-31. Photodiode
3-58. A second optoelectronic device that conducts current when exposed to light is the
PHOTOTRANSISTOR. A phototransistor is much more sensitive to light and produces
more output current for a given light intensity that does a photodiode. Figure 3-32 shows
one type of phototransistor that is made by placing a photodiode in the base circuit of an
NPN transistor. Light falling on the photodiode changes the base current of the transistor,
causing the collector current to be amplified. Phototransistors may also be of the PNP type,
with the photodiode placed in the base-collector circuit.
Figure 3-32. Phototransistor
3-20
TC 9-62
23 June 2005
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