| 摘要 |
In a subscriber line interface circuit of a telephone switching system, a differential amplifier (1) is coupled through the line terminals (T, R) of a two-wire subscriber line for detecting a voltage difference developed thereacross. First and second resistances (8, 9) of equal values are coupled at their first ends to the line terminals, respectively. Tip and ring drivers (2, 3) are connected to the other end of first and second resistances for driving the load impedance of the subscriber line. First, second and third feedback loops (f1, f2, f3) are provided, the first feedback loop being coupled from an output of differential amplifier (1) to the input of the drivers (2, 3) vary the impedance characteristics of the first and second resistances (8, 9) so that a high impedance is synthesized across each of the first and second resistances. The second and third feedback loops are also coupled from the output of differential amplifier (1) to the drivers (2, 3), each of the second and third feedback loops having a different passband frequency characteristic and having the effect of reducing the synthesized impedances to a predetermined value. |
