Evaluation of knee voltage effect and soft turn-on characteristic on the load modulated continuous class-B/J power amplifier

Abstract

The impact of knee voltage effect and soft turn-on characteristic of the practical transistor on the design space of load modulated (LM) continuous Class-B/J power amplifiers (PAs) is analyzed and improved design space is proposed in this paper. For LM continuous Class-B/J PAs with a practical transistor, the knee voltage effect has a noticeable effect on the dynamic load-lines when the dynamic load modulation at the current generator plane (CGP) is proceeding. The chosen load impedance of LM continuous Class-B/J PAs with optimum output power and efficiency at the 0 dB output power back-off (OPBO) can be affected by the knee voltage effect. The soft turn-on characteristic can lead to a non-null drain quiescent current at Class-B/J mode, which limits the dynamic OPBO range with high efficiency. Based on the Pedro load-line theory, the improved design space of LM continuous Class-B/J PAs can offer a more accurate prediction on the efficiency contours and provide more effective theory guidance in designing high-efficiency LM PAs.