Non-Linearity Analysis of Drain-Source Capacitance in Extended Continuous Class-F Power Amplifier

This work presents a new design space of Extended continuous class-F (ECCF) power amplifiers with resistive second harmonic impedance after including the effect of non-linear drain-to-source capacitance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{\mathbf {ds}}~_{\mathbf {}}$</tex-math> </inline-formula> of a transistor. The expression of current waveforms is modified to include the impact of nonlinear drain to source capacitance in this mode. The analysis shows that this non-linear capacitance generates harmonic current, which alters the load trajectory on the Smith chart by achieving active and modified passive second harmonic loads. These active harmonic loads at the current reference plane of the transistor represent an active harmonic injection that increases the drain efficiency. Consequently, a design methodology is proposed to obtain passive loads for matching that ensures this active harmonic injection at the current reference plane of the transistor provides an efficiency improvement in the ECCF mode while maintaining the operation more than the octave frequency range. The proposed theory is validated by designing a power amplifier operating from 0.8-3.0 GHz (115.7% fractional bandwidth) with a measured drain efficiency of 57.24-70.3%, and an output power of 41.7-44.63 dBm. The power amplifier is also tested with modulated signals and linearized using digital predistortion to qualify spectral mask with a measured (Error Vector Magnitude) EVM of 1.54%.