INPUT EXISTENCE DETECTION CIRCUIT

摘要

PROBLEM TO BE SOLVED: To make input disconnection detection time shorter and to make input disconnection release time and the input disconnection detection time well-balanced by balancing the change time of a detection signal from an input disconnection state to an input existence state and also in the reverse direction. SOLUTION: A transistor Q whose gate is applied output voltage of a voltage amplifier 10 to supplies charge current that is smaller than in the conventional manner to a capacitor C in accordance with its input voltage and also with the intervention or resistance R, makes the capacitor C charged and outputs the charge voltage of the capacitor C at a connection point of the resistance R and the capacitor C as peak voltage. The charge and discharge time of the capacitor C can be set by setting the value of the resistance R that is connected to an emitter of the transistor Q of a peak detector 11A. Input disconnection release time and input disconnection detection time of an optical input existence detection circuit 6A are respectively decided by the following. the input disconnection release time being about (VT/Iin +R).C, and the input disconnection detection time being about (C/Iin ).ΔV.