2B - Photo-transistor Construction and Characteristics In...

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Unformatted text preview: Photo-transistor Construction and Characteristics In the NPN transistor the collector is biased positively with respect to the emitter so that the base/collector junction is reverse biased. therefore, with no light on the junction normal leakage or dark current flows which is very small. When light falls on the base more electron/hole pairs are formed in this region and the current produced by this action is amplified by the transistor. The sensitivity of a phototransistor is a function of the DC current gain of the transistor. Therefore, the overall sensitivity is a function of collector current and can be controlled by connecting a resistance between the base and the emitter but for very high sensitivity optocoupler type applications, Darlington phototransistors are generally used. Photodarlington transistors use a second bipolar NPN transistor to provide additional amplification or when higher sensitivity of a photodetector are required, but its response is slower than that of an ordinary NPN phototransistor. It sensitivity of a photodetector are required, but its response is slower than that of an ordinary NPN phototransistor....
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2B - Photo-transistor Construction and Characteristics In...

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