| 摘要 |
<p>A line circuit includes tip and ring voltage amplifiers (20, 40) each having an output resistively connected via tip and ring feed resistors (12, 13) to tip and ring leads (2, 3) in a telephone line. The tip and ring feed resistors have closely matched ohmic values, and are part of a resistance network (10) which also includes first and second voltage dividers (14, 15). The first voltage divider (14) is connected in series between the output of the tip voltage amplifier and an end of the ring feed resistor remote from the output of the ring voltage (15) amplifier, and includes a first voltage tap (6). The second voltage divider is connected in series between the output of the ring voltage amplifier and an end of the tip feed resistor remote from the output of the tip voltage amplifier, and includes a second voltage tap (7). A control circuit (60) includes a differential input connected across the first and second voltage taps, in operation the control circuit causes the tip and ring voltage amplifiers to provide energizing direct current for the telephone line. Tip and ring feedback networks (16, 17, 18, 19), connected across the respectively tip and ring feed resistors, provide feedback signals to the tip and ring voltage amplifiers such that in operation common mode rejection of longitudinally induced a.c. signals on the telephone line is enhanced.</p> |
