Battery feed circuit for telephone subscriber line

Claim 1

A battery feed circuit for supplying DC current to a two-wire subscriber line, comprising first and second amplifiers respectively connected to first and second ones of the two wires of said subscriber line, for providing said first and second wires with respective DC voltages, said first amplifier having a power source terminal for supplying a power source voltage thereto, first and second output resistors respectively connected between outputs of said first and second amplifiers and said first and second wires, for providing each said amplifier with a respective output impedance for the respective wire, a DC voltage feedback loop having as an input a connection to at least said first wire, and having an output for feeding back a DC voltage proportional to the DC voltage of said first wire to said first amplifier, and a switching power supply for comparing said power source voltage supplied to said first amplifier with a voltage corresponding to the output of said DC voltage feedback loop, and for controlling said power source voltage to be lower than the voltage of said output of said DC voltage feedback loop, wherein said switching power supply includes: a zener diode connected between said power source terminal and said output of said DC voltage feedback loop, for providing a DC reference voltage as said voltage corresponding to the output of said DC voltage feedback loop; a reference signal generator for generating a reference wave signal; a first comparator means for comparing said power source voltage with said DC reference voltage, and for outputting a respective high or low voltage depending on whether said power source voltage is higher or lower than said DC reference voltage; a second comparator means for comparing said high or low voltage with said reference wave signal, and for outputtting at an output a rectangular wave having a broad or narrow width in respective corresponding to said high or low voltage output from said first comparator means; and a switching transistor circuit means, including a transistor, a capacitor and first and second resistors, said first transistor being connected to said output of said second comparator means, said power source voltage being supplied by switching the collector current of said transistor in accordance with said rectangular wave supplied to said transistor via said first resistor, said capacitor being supplied with a discharge path through said second resistor, and said capacitor being charged by a charging path having a respective time constant and which includes said first and second amplifiers.