Grid-Current Flow as a Factor in the Design of Vacuum-Tube Power Amplifiers

In the design of class C amplifiers analytical methods have been proposed which give good results from the standpoint of plate-circuit operating condition. Similar methods have not been available for the determination of grid-circuit conditions. It is the purpose of this paper to describe a method by which this result can be obtained. An analytical determination of grid-circuit operating conditions requires a knowledge of grid current as a function of the grid and plate voltages. Since good approximations for the space current in a triode are available, an expression for grid current can be had if the ratio of plate-to-grid current as a function of plate and grid voltage can be determined. Theoretical considerations show that the ratio of plate current to grid current should be a function of the ratio of plate voltage to grid voltage. The nature of this functional relation is determined experimentally and a comparatively simple and accurate expression for grid current in cases in which there is only a small amount of secondary emission is thus obtained. The design procedure for class C amplifiers is based upon the allowable grid and plate dissipation. The method of determining optimum operating conditions on the basis of plate-circuit analysis alone is modified to include the analytical determination of the corresponding grid-driving power and grid loss. This procedure results in the determination of the operating conditions which will give the highest output which can be obtained without exceeding the allowable grid or plate dissipation.