output impedance at output terminal 64 and includes a plurality n of amplifiers of which a lesser number m are in operation a control arrangement illustrated as a block 86 is coupled to switches 46 52 for controlling their operating state control arrangement 86 controls switches 46 52 so that no more than m amplifiers are coupled to junction point 62 at any one time thus in the event that an operating amplifier such as 22 should fail the configuration of switch 46 could be reset by control logic 86 to couple the common terminal of switch 46 to the lower switch terminal thereby decoupling amplifier 22 from junction 62 and connecting short circuited transmission line 68 to junction 62 instead and further by resetting the position of switch 48 to decouple the common terminal from the lower switched terminal and couple it instead to the upper switched terminal thereby decoupling short circuited transmission line 70 from junction point 62 and connecting redundant amplifier 24 to junction point 62 such control requires that control logic block 86 be coupled by paths not illustrated to amplifiers 22 28 with the illustrated arrangement and the described connections and control the composite amplifier of fig1 simultaneously provides the reliability of redundant amplifiers together with impedances at output terminal 64 which remain invariant despite failures of.