Integrable line circuit for use with miniature line transformer

摘要

A line transformer circuit in which the DC component of magnetization produced in the tip and ring windings by the battery supplying loop current to the telephone set microphone may be neutralized by an oppositely-poled, serially-connected balancing winding is disclosed. One end of the balancing winding is connected to the supply battery and is, therefore, at AC ground. The other end is kept at a point of high AC impedance to ground by the collector of a transistor so that audio frequency voltages induced in the balancing winding cannot load the transformer. The AC currents in the tip and ring windings are provided with a virtual ground return path by means of a differential amplifier controlled Darlington pair in circuit between the tip and ring windings and the balancing winding. The AC components of the Darlington pair collector current are bypassed to ground by a shunt capacitor at the junction of the Darlington pair collector terminal and the third winding. The differential amplifier senses voltage changes at the virtual ground so that the Darlington pair is driven to suppress these changes. Advantageously, the differential amplifier may be powered from the voltage drop appearing across the Darlington pair thereby facilitating circuit integration.