An alternative approach to improving the efficiency of high power radio frequency amplifiers

The concept of using fourth-harmonic rather than third-harmonic resonant circuits in the tuned plate circuits of high-power radio-frequency amplifiers to improve operating efficiency is proposed. The advantages are circuit simplicity, greater stability, and simpler adjustment than with third-harmonic-enriched amplifiers because the grid excitation voltage can be sinusoidal. Improvements in plate efficiency, envelope distortion, and carrier shift characteristics are comparable to those achieved with third-harmonic designs: however, implementation is simpler and the amplifier more stable because the requirement to maintain a constant grid current under conditions of plate modulation becomes less critical