SIGNAL DETECTING CIRCUIT

摘要

PURPOSE:To accurately detect a fine input signal by interposing a bidirectional switch element between the output terminal of a DC cutting capacitor and a reference voltage source and turning on the switch element before a specific detection period. CONSTITUTION:A DC cutting circuit 30 which is arranged between a photodetecting amplifier 20 and a comparator 21 has the CMOS type analog switch Q composed of an NMOS transistor(TR) Q1 and a PMOS TR Q2 composed between the output terminal of the capacitor C2 and a reference voltage Vref. When a gate signal phi goes up to 'H' level, the NMOS TR Q1 and PMOS TR Q2 turn on and the voltage Vc at the output terminal of the capacitor C2 is set to a reference voltage Vref. The gate signal phi is held at 'L' for a specific period and then falls to the 'L' level later. Thus, the fine input signal is accurately detected.