| 摘要 |
The subscriber circuit has a balanced amplifier and a built-in call-current polarity changing circuit. Moreover, the output stage circuits of the direct feed-back and offset cancel circuits of the subscriber circuit have the same configuration. By switching the input voltage supplied to the direct feed-back and offset cancel circuits by a switch, the signals fed back to the inputs of the balanced amplifier are switched. Thus, the polarity of the loop current can be inverted. Furthermore, it is possible to switch the loop current without adopting an extra circuit, and the part of the circuit in which the switches are not required to withstand pressure can be constructed with CMOS thereby allowing minimization of the circuit size.
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