Auto-balance circuit for battery feed in a telephone circuit

摘要

An active current limited in only the ring side of a telephone circuit has the control branch of a current mirror resistively connected across a supply voltage for establishing the magnitude of loop current in its other branch. A first resistor is also connected between adjacent one sides of the two branches for causing the control voltage and loop current to vary as a function of loop length. Extraneous metallic voltages on the A and B leads are AC coupled to associated inputs of a differential amplifier having its output connected through a second resistor to the one side of the control branch for modulating the loop current provided by the other branch. Opposite sides of the second resistor are fed back to associated input terminals of the amplifier. The amplitude modulated loop current is also fed back to an input of the amplifier for minimizing the affects of induced metallic voltages on the tip and ring lines.