SIGNAL SWITCHING CIRCUIT AND SIGNAL CHANGE-OVER CIRCUIT USING THE SAME

A transmission line is connected in series to the emitter of an output transistor of an ECL circuit which outputs a logical signal, and a pair of series-connected switching diodes is connected to the output end of the transmission line. A parallel switching diode is provided which has its anode connected to the common connection point of the pair of series-connected switching diodes and has its cathode connected to a common potential point. A current pull-in circuit responsive to a control signal and a bias supply circuit are connected to the common connection point. The current pull-in circuit is turned ON to pull in current from the pair of series-connected switching diodes, thereby turning them ON. At this time, the parallel switching diode is turned OFF and the logical signal from the ECL circuit passes through the pair of series-connected switching diodes and is then output. Conversely, when the current pull-in circuit is turned OFF, the pair of series-connected switching diodes is reversely biased, and consequently, no current flows through the emitter of the output transistor of the ECL circuit. As a result of this, the logical signal is not provided on the transmission line.