Line circuit using miniature line transformer

摘要

A line circuit is disclosed employing a transformer having tip and ring windings and a third balancing winding designed to be serially connected in circuit with the tip and ring windings and poled so as to balance out their DC magnetization. The AC components of longitudinal current flowing through the tip and ring windings are prevented from flowing through the balancing winding by a capacitor bypass which offers a low impedance path compared to that of the balancing winding. This capacitor is prevented from short circuiting the AC voice frequency voltages induced by transformer action in the balancing winding by driving the balancing winding from the collector terminal of a transistor biased to remain out of saturation and which, therefore, exhibits a high impedance to changes in collector voltage. The resistance in the emitter circuit of this transistor determines the level of DC current delivered through the balancing winding to the tip and ring windings and the telephone line. The transistor base is supplied with current drive by a Darlington pair which avoids longitudinal unbalance in the tip and ring conductors. By effectively preventing the effects of DC magnetization a smaller and lighter weight transformer core is obtained.