A theoretical analysis and experimental confirmation of the optimally loaded and overdriven RF power amplifier

Although the conventional Class B approach to RF amplifier design yields high output power and reasonable collector efficiency (78.5 percent at maximum output power), neither the power nor the efficiency are optimum, and both are dependent on RF drive level. This paper presents an analysis of appropriately selected collector voltage and current waveforms which determine the load impedance at the fundamental and harmonically related frequencies; these conditions define the ClassB "optimum efficiency" case with 100 percent collector efficiency and 1.27 times the conventional Class B value of output power. If the RF drive level is increased, and the collector voltage and current waveforms are appropriately selected so that the amplifier is overdriven, a different load impedance is determined; these conditions define the "optimum power" case with 1.46 times the conventional Class B value of output power and 88 percent collector efficiency. The "optimum power" case has the added advantage that the output power and collector efficiency are essentially constant over a predetermined range of drive level. Finally, the theory is verified by the construction and testing of a UHF power amplifier having a power output of 46 watts and an over-all dc to RF conversion efficiency of 65 percent with a 1 dB for 10.5- dB insensitivity of output power to RF drive.