Most radio frequency and microwave high power solid state amplifiers are design to operate into a matched load. This frequently takes a value of 50 ohms, although it may be 75 ohms, a waveguide output or otherwise. Although the load may well be other than a matched load, it is desirable that its impedance should differ by as little as possible. By this means, a system designer does not need to consider the electrical length of an intermediate transmission cable, as this will not alter the impedance presented to the amplifier.
In reality, it is frequently not possible to present a matched load to an amplifier, particularly where wide frequency ranges are concerned, so a degree of mismatch is encountered. In severe cases, the amplifier may be presented with a complete reflection. In addition, the amplifier is required to perform predictably under fault conditions, where the load may be disconnected or short-circuited. The purpose of this application note is to explain the implications of operating an amplifier into an output mismatch.