CHANNEL CIRCUIT

PURPOSE:To obtain an optimum talking current in the talking circuit of an exchange, by increasing the constant current of a constant-current circuit when a voltage exceeding the reference voltage of a voltage comparator is inputted in the constant-current circuit. CONSTITUTION:A voltage control circuit ICO, transistor (TR) Q0, and resistor R0 constitute a DC constant-current circuit CC and a resistor R2 and diode D for constant voltage constitute a voltage comparator VC. A capacitor C', comparator IC1, TRQ1, and resistors R1 and R3 constitute a means CM which changes an electric current. When the voltage across channels A' and B' reaches a voltage which conducts the diode D, the TRQ1 is turned on and the electric current of the circuit CC bypasses the resistor R1 and, as a result, the emitter voltage of the TRQ0 drops. Therefore, the output of the circuit IC0 controls the base of the TRQ0 to restore the emitter voltage to the original state and the circuit CC is stabilized to a constant current determined by the parallel resistance value of the resistors R0 and R1. Namely, the electric current is increased.