| 摘要 |
<p>A line interface circuit for a two-wire telephone line comprises two feed resistors (40) connected in series with two primary windings of a transformer (38), and differential amplifiers (50) capacitively coupled to a secondary winding of the transformer and coupled to a current sensing network (44) for providing a balanced output dependent upon current flowing through the feed resistors. Each amplifier has d.c. and a.c. feedback paths (52 to 58) arranged to compensate for resistance of the transformer secondary winding. A further amplifier (62) is arranged in a path from the outputs to the inputs of the differential amplifiers to reduce common mode signals at the differential amplifiers. An output of the further amplifier and a potential divider (66) across the line are coupled to a summing node (68) to provide a signal representing common mode current on the line.</p> |
