| 摘要 |
<p>A circuit arrangement for an electronic subscriber feed has loop current supervision, ringing connection and hybrid for a transition from two to four-wire operation with mutually symmetrically connected current generators for communication facilities such as branch telephone exchanges. The current generators are operational amplifiers having transistors associated therewith, the operational amplifiers being operative as pure voltage followers and having inverted inputs through which modulation of the loop current is controllable in a manner that dynamic self-balancing of said current generators is increased by amplification of the modulation voltage. A differential amplifier is associated with the operational amplifier and having an output and an input. A transistor is connected to the input of the differential amplifier and means are also provided for applying, with the loop open, a loop signal through the transistor connected to the input of the differential amplifier for controlling the differential amplifier so as to obtain a large hybrid attenuation in a manner that no a-c voltage reaches the output of the differential amplifier. The hybrid includes a complex line balancing network in combination with an impedance selected from a group consisting of a real-input resistance and a complex input impedance</p> |
