Page 1018 - Basic Electrical Engineering
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Figure 14.55 A phototransistor trigging an SCR on low illumination level
Figure 14.56 Photo-darlington transistor
14.13.8 Photo-darlington
A phototransistor can be connected with another transistor to produce higher
output. The arrangement as shown in Fig. 14.56 is called a photo-darlington.
Photo-darlington is a phototransistor connected in darlington with another
bipolar junction transistor. With this arrangement, the output current obtained
will be higher than what could be obtained in a phototransistor or a
photodiode. The sensitivity of the device to illumination is high but the
switching time is somewhat more here as two transistors are getting involved.
