Abstract: A constant current circuit, including a constant current source which draws a current and a constant current source which sends a current, is connected to a two-wire subscriber line. A ringing signal made up of constant stream of positive and negative current signals is transmitted to the telephone line. The OFF-hook condition of a telephone set is detected by comparing the output voltage of a constant current circuit with a reference voltage for ring trip.

Abstract: A subscriber line interface circuit receives and twice inverts currents from a 2-wire subscriber line and returns the currents to the subscriber line so longitudinal input impedance is low. Any differential input current in the subscriber lines is bypassed by a capacitor, so differential input impedance is very high. The circuit also sends a signal current from one system having one ground to another system having a different ground through one current path, while sending another current, having a phase opposite and amplitude equal to the signal current, through another path. This effectively isolates the two systems having different grounds.

Abstract: A battery feed circuit for a two-wire telephone subscriber line is miniaturized by an electronic integrated circuit without a transformer. The electronic battery feed circuit is composed of first and second operational amplifiers, output resistors, a DC voltage feedback loop, an AC signal feedback loop and a switching power supply. The switching power supply means is to control the power source voltage of the first operational amplifier, corresponding to the length of the subscriber line loop and the status of a telephone set in the loop. Low power consumption of the circuit is attained as follows. Most of power loss of the circuit can be separated as between the amplifiers and the output resistors. The loss of the output resistors are reduced by using small resistances, and the loss of the second amplifier is reduced by using a small DC bias. The loss of the first amplifier is reduced by setting its power source voltage to several volts lower than output voltage of the switching power supply.