Abstract: An electronic dipole circuit operating at constant current, designed to be used for looping a telephone line from a private branch exchange is disclosed. The circuit comprises a first transistor with the current which is constant in the emitter circuit owing to the presence of a second transistor. A device connected between the collector and the base of the first transistor delivers a constant current. Such a device may comprise, for example, a phototransistor or a field-effect transistor placed in series with a phototransistor.

Abstract: An electronic feeding bridge for a telephone line uses a two-stage circuit to provide high a.c. impedance between the load and the line and between the line and the load. The first stage includes one NPN type transistor and one PNP type transistor with their bases coupled together by a capacitor. The second stage includes one PNP type transistor and one NPN type transistor with their bases coupled together by a capacitor. The emitters of the transistors of the first stage are connected to the emitters of the transistors of the second through of a pair of resistors. The collectors of the second stage are connected to the load through of a pair of diodes, and the collectors of the transistors of the first stage are connected directly to the line.