TRANSMITTING CIRCUIT

PURPOSE:To make a transmitting circuit high-impedance at a transmission stop time when viewed from a transmission line by connecting a diode which has a polarity opposite to that of a current induced at the transmission stop time in a closed circuit formed with a primary winding of an output transformer. CONSTITUTION:At a transmission time, an AC output is generated from an oscillating circuit 23 by a transmission signal S from a transmission/reception signal processing part 4, and the AC output drives alternately transistors Trs 9 and 10 constituting a class ''B'' push-pull amplifying circuit through a transformer 8. The signal current amplified by Trs 9 and 10 passes a DC power source 21, a capacitor 22, and a diode D27 and is flowed from a center tap 11 of a primary winding 14 of an output transformer 16 to Trs 9 and 10 alternately, and the AC output is sent to a communication line 5 through a secondary winding 15 of the transformer 16. At a transmission stop time, the signal S from the processing part 4 is stopped, and Trs 9 and 10 are cut off, and the primary side of the transformer 16 is in the open state by the directivity of the D27, and the circuit has a high impedance when viewed from the communication line. A reception signal passes a transformer 24 and is demodulated by a receiving circuit 3 and is processed in the processing part 4.