Abstract

A battery feed circuit for supplying DC power to a telephone loop and for coupling signals to and from said loop is disclosed. A differential amplifier having its inputs connected by way of a twisted resistive bridge to the telephone loop provides a voltage at its output which includes the signal from said telephone loop and has a DC component which is proportional to the DC current flowing in the loop. Signals are coupled to the telephone loop by a low-inductance three-winding transformer which has two of its windings connected in series with an external power supply and the twisted bridge in order to supply current to the telephone loop. A second differential amplifier is connected in different ways in several embodiments between the output of the first differential amplifier and the third winding of the transformer to provide an automatic flux-cancelling action in order to avoid saturation of the transformer core. Larger gain is achieved in the first differential amplifier by clamping its DC output to zero through use of an integrator circuit connected between the output and one of the inputs of the first differential amplifier. In one of the embodiments, power is supplied to the telephone loop by a constant current source.