The Nonlinear Drain–Source Capacitance Effect on Continuous-Mode Class-B/J Power Amplifiers

The effects of the nonlinear drain-to-source capacitance (<inline-formula> <tex-math notation="LaTeX">$C_{\mathrm {DS}}$ </tex-math></inline-formula>), or the varactor effect, on the operation of continuous-mode class-B/J power amplifiers (PAs) are described in terms of the mathematical analysis as well as experimental validation. The nonlinear <inline-formula> <tex-math notation="LaTeX">$C_{\mathrm {DS}}$ </tex-math></inline-formula> generates harmonic current when the class-B/J drain voltage is applied across it, modifying the theoretical load trajectories on the Smith chart. This is shown to significantly deviate from the continuous-mode assertion of constant power and efficiency. A novel measurement approach called voltage–pull is used to investigate the effects of the second harmonic generation by the varactor in a class-B/J PA mode. Theoretical and experimental results showed the nonlinear <inline-formula> <tex-math notation="LaTeX">$C_{\mathrm {DS}}$ </tex-math></inline-formula> degrades the drain efficiency compared to class-B on one half of the class-B/J design space while improving the other half due to the energy upconversion and downconversion, respectively. This paper provides an approach for die nonlinear characteristic optimizations for transistor manufacturers.