NPN junction as a phototransistor amplifier.

NPN junction as a phototransistor amplifier. The semiconductor device in this circuit is an NPN phototransistor with a clear plastic window. Light hitting the device is absorbed in both the NP and PN junctions, freeing electrons and creating holes, which increases the base current of the transistor. The circuit is set up as an amplifier, so the increased base current is amplified by the factor _. The projection meter displays the amplified current on the screen. Note: The sensitivity of the phototransistor is varied by the amount of the negative bias allowed by the 25 K ohm rheostat. The sensitive area is very small (< 1mm x 1mm), so the light must be moved back and forth until you hit it squarely.