| 摘要 |
Subscriber line interface circuit for connecting a subscriber's telephone line to, for example, an automatic telephone exchange comprising a pair of amplifiers whose outputs are connected to the wires of the telephone line and which are fed by a d.c. voltage supply source. This supply source provides the telephone line with direct current via the amplifiers. In practice, the telephone line must be terminated by a symmetrical impedance of a prescribed value. To this end the interface circuit comprises means for determining the weighted sum of the current flowing through one and through the other wire of the telephone line such that the sum current is independent of the longitudinal currents in the telephone line. By means of an impedance, two antiphase feedback voltages are generated at the inputs of the amplifiers, these amplifiers then having a symmetrical impedance at their outputs.
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