Hybrid circuit in a telephone subscriber interface circuit

Abstract

A hybrid circuit composed of semiconductor current amplifiers without an inductance and large capacitance has been found. The DC speech current supplied by the DC power source (33) and the speech signal from the modem (28) are adjusted by the current amplifiers (17a, 17b) so that the subscriber line (2) is terminated with the predetermined impedance (600 ohms), and the speech signal from the subscriber line (2) is transferred to the modem (28) through the AC monitor circuits (24a, 24b). The common mode noise induced on the subscriber line (2) is cancelled by adjusting the input current of said current amplifiers (17a, 17b) by the common mode noise detector (22) and the common mode noise monitor circuits (23a, 23b). The speech signal from the modem (28) to the subscriber line (2) does not return to the modem (28) because of the cancellation operation by the return signal detector circuit (32). The present hybrid circuit is suitable to compose a semiconductor integrated circuit (IC) because no inductance is used and no large capacitor is used.