Large-signal theory of UHF power triodes

Using a parallel plane model, an analysis is made of the UHF electronics of triodes operating under Class B conditions. The theory, in its most general form, includes the effects of space charge in both the grid-cathode region and the plate-grid region. However, most of the solutions contained in this paper were obtained by neglecting space-charge effects in the plate-grid region. Over the wide range of UHF operating conditions investigated, it is found that the reduction of a triode´s power output due to transit times is accounted for almost entirely by transit-time effects in the plate-grid region only. The principle effect of transit times in the grid-cathode region is to introduce a phase shift. This phase shift is quite important, however, for it contributes to a mechanism of regenerative feedback from output to input. The reduction of transit-time effects for a fixed operating frequency is limited by four factors: temperature-limited saturation of the cathode emission current, reliable mechanical spacing considerations, electrode dissipation capabilities, and amplifier bandwidth requirements. It is shown that there exists an optimum plate-grid spacing which leads to the highest possible RF output power consistent with the required bandwidth of the amplifier.