Electronic switching circuit

1,142,079. Transistor switching circuits. STANDARD TELEPHONES & CABLES Ltd. 16 Nov., 1967, No. 52145/67. Heading H3T. In a switching circuit comprising a T network of three transistors, which have their emitters joined together, the series arm transistors 7 and 8 are biased from the same biasing circuit so that when they are conducting the emitter electrodes are at zero potential relative to earth and the shunt arm transistor 9 is biased from a separate biasing circuit so that either the series arms or shunt arm are conducting. The switching circuit is arranged between a source of D.C., A.C. or composite signal 1 and a load circuit 2, such as resistor R. To switch on transistors 7 and 8, the terminals 14, 15 are connected to -V, +V which causes these transistors to be saturated. If the signal current exceeds the biasing current of these transistors one of them will come out of saturation and switch off. To disconnect the signal source 1 from the load 2 the voltage applied to the terminal 15 is removed and a +V voltage is applied to the terminal 18 so as to turn on transistor 9 in the shunt arm. A transistor network 21 is connected across the signal source so as to be correctly terminated. For an A.C. signal source two transistors 23, 24 are required, but for a unipolar source, only one of these would be required. The terminating network 21 is biased from the same circuit as that biasing the shunt transistor 9. In a change over or selector circuit (Fig. 2, not shown), two or more switching circuits may be connected to a common load 2 so as to selectively switch different inputs (1, 11) to the load.