| 摘要 |
PURPOSE:To make small the size of power feeding circuit and two/four wire conversion circuit incorporatingly and to achieve high circuit integration, by cancelling the reception signal and outputting only the transmission signal, in a differential amplifier. CONSTITUTION:An inverted reception signal is inputted to an addition circuit of a power feeding circuit SP, for example, and it is added with an output voltage of a voltage division circuit VD and inputted to a current conversion circuit VI. Thus, the reception signal is transmitted to the line constituting the 2 wire type line through superimposition on the call current and transmitted to a telephone set Tel. On the other hand, the transmission signal delivered from the telephone set Tel to a line constituting the two wire line, is transmitted from the circuit SP to the line. The output is inputted to one input terminal of differential amplifiers DA1, DA2 together with the reception signal transmitted in an opposite direction, and the reception signal is inputted to another input terminal of the amplifier DA1. The reception signal is cancelled at the amplifiers DA1, DA2 and only the transmission signal is outputted to a transmission signal transmitter SEN. |
