Contents I. Introduction
POWER amplifiers (PAs), which are used in mobile or wireless transmitter systems, have to satisfy tight requirements on power performance. In particular, the highest output power and efficiency levels achievable from a device, together with its linearity, represent the major concern of PA designers. The design results, therefore, in a tradeoff between such conflicting re-quirements, e.g., linearity versus efficiency or high output power versus low distortion. Potential design approaches are related to the operating frequency and bandwidth requirements, as well as to the available device technology. For narrowband applications, with up to 10% fractional bandwidth, commonly adopted design solutions are based on harmonic tuning (HT) strategies, e.g., class F [1], [2] or related ones [3]–[5], to improve PA per-formance, both in terms of output power and efficiency (i.e., drain or power-added, ). In these approaches, a suitable combination of device input and output harmonic terminations, practically limited up to the third one, is adopted. Output voltage waveform is shaped to fulfill device physical constraints while assuring a higher fundamental component as compared to a class-A amplifier. As a consequence, improvements on output power, power gain, and efficiency are attained [6]. The control of second-harmonic terminations, both at the input and output device ports (hereinafter, 2nd HT PA), demonstrated major performance improvements [7]. However, 2nd HT PA design re- quires additional design efforts related to the control of the input harmonic terminations to properly generate the output harmonic components with appropriate phase relationships [6], [7]. More-over, the resulting output voltage waveform exhibits a peaking behavior toward device breakdown.
