The sensitivity of photo diode can be increased to larger value about 100 times by adding a junction resulting in a NPN transistor. The photo transistor is usually connected in a common emitter configuration with base open. The illumination (Radiation) is concentrated on the region near the emitter base junction. A direct voltage V is applied between the emitter and collector such that the collector junction is reverse biased and emitter applied voltage appears across the reversed collector junction.
Initially that there is no radiant excitation (illumination) this condition, the minority carriers are generated thermally and the electrons crossing from base to the collector as well holes crossing From collector to base constitute the reverse saturation current Ico. With Ib=0 the collector is given by IC=(1+BETA) ICO.
- Photo Detector
- Optical coupler
- Used in electron optical control
- Light beam reception
- Light operated switch ( Common Emitter and Collector Amplifier circuit)
HOW TO TEST WITH DIGITAL MULTIMETER?
- Connect DMM positive Red test lead to BASE
- Negative black lead to collector= .618V (COLLECTOR)
- Negative black lead to emitter = .648V (EMITTER)
Display reading shows good.
- Connect DMM Negative Black test lead to BASE
- positive red lead to COLLECTOR
- positive red lead to EMITTER
Display reading shows OL or OL GOOD.
Verification: If the DMM above reading shows the condition is GOOD
Verification: If you get reading in forward bias as 0000 or OL or 1, and in reverse bias as 0000 (or) low values the photo transistor can be FAULTY and needs replacement.