Velocity tapering in microwave amplifiers

The low-to-moderate RF conversion efficiencies characteristic of kinetic energy conversion devices may be enhanced by appropriately tapering the RF circuit phase velocity. The nonlinear interaction equations have been solved in closed form for an HKB (hard-kernel-bunch) model to obtain an optimum taper form. Several velocity taper forms have been investigated in experimental amplifiers and the power output, gain, efficiency, and phase shift characteristics of these amplifiers are discussed in detail. Conversion efficiencies up to 50 percent and near constant output power for a 20-dB variation in RF drive power have been realized in near optimally tapered circuit amplifiers. Further improvements realized include operation at a single voltage for maximum gain and power output and a nearly linear phase shift characteristic for a ±10 percent voltage modulation.