Design of non-distorting power amplifiers

The paper deals with the problem of obtaining the maximum output possible from a given amplifier tube, while keeping the distortion down to a negligible amount. A tube can be rated for this purpose in terms of the watts output obtainable, when a sine wave voltage of as great an amplitude as can be advantageously utilized, is applied to the grid. This maximum sine wave output is very much less than the rating of the same tube for oscillator purposes. Starting with a set of static characteristics for a given lube, the dynamic characteristics for any resistance load is readily plotted, and the power output and distortion can be read from the dynamic characteristic. A simple rule has been given by Mr. W. J. Brown for determining the best conditions of load resistance and grid bias for a given plate supply voltage. The best load resistance is shown to be twice the internal plate resistance of the tube. If the supply voltage exceeds a certain value, the application of the rules just mentioned would lead to excessive healing of the anode, and therefore a different procedure is followed, calling for greater grid bias and higher load resistance. There is an advantage in using low impedance tubes. The balanced or push-pull circuit, while reducing distortion, will not make up for failure to operate the tubes under proper conditions, nor will it greatly increase the permissible output per lube. The dynamic characteristic for a reactive load is not readily plotted, but for design purposes it is sufficient to determine the best operating conditions for a resistance load, and then make the impedance of the reactive load high enough to keep the plate current variations within the same limits as for the resistance load. An important application of the principles outlined here, is the design of radio telephone transmitters where serious distortion results from overworking the modulator tubes. For moderately deep modulation there should be from two to four modulating tubes for each oscillator tube. Certain details of design are discussed in the closing paragraphs.