Effects of matching on RF power amplifier efficiency and output power

To date, one of the most important design methods for RF power amplifiers (PA) still consists of matching the large-signal input and load impedances of an active device. These large-signal impedances are supplied by the manufacturer of the active device or are measured directly by the user. In both cases, the output power level and efficiency are associated with those large-signal impedances. Nevertheless, in practice, the output power, efficiency and even the operation mode depend not only on the large-signal impedances but also on the matching networks used to provide the recommended loads for the transistor. This phenomenon is caused by the high frequency behavior of the matching networks. The usual practice of using a short-circuit lambda/4 line located at the collector or drain of the transistor to supply DC power causes similar frequency-limiting effects due to the behavior of lambda/N lines at harmonics of the fundamental frequency.