A High-Efficiency Grid-Modulated Amplifier

A method of applying impedance inversion to the grid-modulated amplifier is described. Two tubes are employed in the modulated stage with their plate circuits connected through the electrical equivalent of a quarter-wave transmission line, and the modulating voltage is applied to the grids in series with the grid bias. By this means, high efficiency and large power output per tube are obtained. Efficiences of 65 to 80 per cent are readily secured during both modulated and unmodulated intervals as compared with 60 to 65 per cent for the high-efficiency linear amplifier. The output per tube is also greater than that of the high-efficiency linear amplifier. Only a small amount of modulating power is required, and this system is particularly adaptable to negative feedback. The effect of the impedance-inverting network on modulated waves is discussed. A simplified method of adjusting the tuned circuits of the impedance inverter is described. Formulas are derived for output and efficiency for both the high-efficiency grid modulator and the high-efficiency linear amplifier.