Harmonically Tuned Continuous Class-C Operation Mode for Power Amplifier Applications

This paper presents the broadband class-C mode of operation, termed the “continuous class-C”, which is an extension of a standard class-C design but with the optimum fundamental and second harmonic terminations now forming a continuous arrangement of load impedances on the Smith chart. With the generic drain current waveform depending on the amplifier conduction angle, and by using the well-established continuous class-B (class-J) voltage waveforms, a set of closed-form equations for the fundamental and second harmonic impedances are derived. These provide a wide impedance design space to realize broadband class-C power amplifiers (PAs) with the constant output power, gain and efficiency over a significant bandwidth. The continuous class-C mode is subdivided into low- and high-power modes for which load impedance, output power and drain efficiency ratios are found and compared to those of the continuous class-B. New design spaces are graphically shown for both class-C modes. The proposed continuous class-C concepts are validated experimentally with on-wafer load pull measurements of an integrated 8×50 μm, ~ 1 W, 20 V GaN MMIC transistor. The proposed theory is also applied in simulations of power amplifiers operating in continuous class-B and continuous class-C modes.