PHOTOELECTRIC CURRENT DETECTION CIRCUIT

摘要

PROBLEM TO BE SOLVED: To attain high speed response by connecting bases of 1st and 2nd transistor(TR) with a bias resistor and supplying a bias current to the 1st TR via a resistor from the 2nd TR. SOLUTION: A photo diode 38 converts an optical signal into an electric signal. A voltage detection circuit 36 detects a terminal voltage of a load resistor 34 based on photoelectric current to provide an output of a photoelectric current detection signal. Since a base of a current detection TR 30 and a base of an auxiliary TR 46 are connected by a bias resistor 44, a bias current is always supplied to the TR 46. That is, the TR 46 supplies a bias current to a base of the TR 30 via the resistor 44 so that a base-emitter voltage is kept to be a voltage slightly lower than the ON-voltage when no optical input is received. The photoelectric current detection speed is increased by about a multiple of ten by this circuit and the circuit is realized at a low cost.