In such cases, phototransistors provide a lower cost alternative. Signal of a photodiode can be amplified through external circuitry but it is not considered to be cost effective. As many of the applications demand for high output which is not possible for a photodiode to work with, so the phototransistors are used for the same. The gain for a phototransistor has a generalized range from 100 to over 1500.
This combination had a particularly specific task to overcome a major limitation of photodiode which was unity gain. The phototransistor were realized as a photodiode whose output current is fed to the base of a transistor. Phototransistors are considered as the photodiode attached to an amplifiers that are integrated on a single silicon chip. The other features to consider for the phototransistors were its relatively less cost, simple and small structure. One of the other points for choosing the phototransistor over others was its compatibility with most visible or near infrared light sources including IREDs, neon bulbs, fluorescent bulbs, incandescent bulbs, and other, Also, it gave faster results with almost instantaneous output. Current parameter not being the only reason, but also the voltage generation which was not like the photoresistors, which became a point for the selection of phototransistors over others. As the phototransistors produce higher current than the photodiodes, they are preferred for many applications.
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