Abstract: A switch for transmitting a signal with gain from one of a plurality of input transmission lines (10A,10B,10N) to a common output line (14) is described which achieves high isolation by omitting the transmission line between the output matching section and the junction of the input and the output. In one embodiment the output of each amplifier, a field effect transistor (38,40), is coupled directly to a common output (34) and the output matching circuitry is also omitted from the input lines, there being provided a common output matching and bias network located in the output line. In another embodiment the output matching network (26a,26b;64a,64b) is coupled directly to the common junction (16;56) and the transmission line length between the output matching network and the junction is omitted for each input line.

Abstract: A transmission line switch allows a single output line (3) to be switched to one of two or more input lines (1,2) to which it is permanently connected at a common junction (4). Each input line (1,2) has an associated amplifier stage (10) which can be biased in a normal high gain (`on`) state, or in an isolation (`off`) state. Suitable biasing in the `off` state ensures that the amplifier stage output presents a low impedance to its own input line, the length (L) of which is chosen to reflect a high impedance at the junction (4) with the other lines. Correct design enables the return loss and insertion loss of the `on` path to be kept to low values while simultaneously offering a high insertion loss to the `off` path signals.