Monolithically integratable telephone circuit for supplying ringing

Abstract

A telephone circuit, which may be monolithically integrated, for supplying ringing signals to a subscriber's line and for detecting an off the hook condition during ringing includes a circuit for supplying ringing signals coupled to exchange control components for generating a constant frequency sinusoidal signal which gives rise to the ringing signals and an enabling circuit, controlled by the exchange components, for enabling the supplying of the ringing signals to the line and for activating a circuit for detecting a direct current on the line. The circuit for detecting a direct current on the line supplies a signal to a circuit for controlling the detection of an off the hook condition, and which supplies an inhibiting signal for the enabling circuit and check, only after a predetermined time, whether the circuit for detecting a direct current on the line is still supplying a signal and for generating a signal informing the exchange components that the set has been off the hook. The circuit also includes a circuit which amplifies the ringing signals and simultaneously supplies the telephone line. A circuit for generating timing signals synchronizes the operation of the enabling circuit and the circuit which controls the detection of the off the hook condition, and also the supplying of ringing signals to the line, during the times in which the sinusoidal signal generated by the exchange control components has a zero amplitude.