Efficiency is often the difference between a power amplifier (PA) being selected or rejected for a particular application. Because higher-power PAs can require large amounts of bias energy to achieve a target output-power level, a difference in efficiency of just a few percent can mean a difference in the size and cost of a power supply for a particular PA. A basic overview of PA efficiency can also help to better understand how that efficiency can impact the overall performance of a system, as well as the performance of other PA parameters (notably linearity).
An amplifier with high efficiency uses power-supply energy more effectively than an amplifier with lower efficiency. At lower efficiency levels, wasted power-supply energy is typically converted into heat at the amplifier’s active devices, which are increasingly gallium-nitride (GaN) transistors for RF/microwave PAs. GaN high-electron-mobility-transistor (HEMT) devices are noteworthy for a number of features that enable high-efficiency PAs at microwave frequencies, including high breakdown voltages, high current densities, high transition frequencies, low on-state resistances, and low parasitic capacitances. To protect the active devices and the surrounding electronic circuits, the heat must be removed. This results in the need for additional hardware, such as fans and heatsinks, which can add to the size, weight, and cost of a power amplifier.