Modulation in Vacuum Tubes Used As Amplifiers

Recent developments in amplifier design tending toward more rigorous quality requirements have shown that the solutions of Van der Bijl and Carson are inadequate for certain purposes since they are based upon a convenient assumption which is not satisfied in fact. In particular, a detailed investigation of carrier current repeaters used for the simultaneous transmission of several channels, and upon which in consequence the modulation or crosstalk requirements are particularly severe, showed the modulation currents measured to be quite different from those specified by the theory, as was the law of variation of these currents with the circuit constants. The cause of the discrepancy was found to reside in the neglect of the variation of the amplification factor (n) with both plate and grid potentials. When the actual state of affairs was taken into account in the analysis by the application of a general method involving no assumptions, theory and experiment were found to be in good accord. The new expressions have been developed in terms of the amplification factor (ji), the internal output resistance of the tube (K u ), and their differential parameters, which are involved in the representation of the characteristic tube equation by a double power series. Expressions for the current components are developed in terms of the coefficients of the series, and modifications of Miller's method for greater convenience and precision in determinations of tube characteristics are described from which the series coefficients may be evaluated.