A Class-J Power Amplifier with Varactor Based Dynamic Load Modulation

Abstract

In order to reach increasingly higher data rates and energy requirements in mobile networks, energy efficiency and broadband operation in power amplifiers have been driving parameter in the research of wireless transmitters. This thesis presents the theory for a broadband design of a dynamic load modulated class-J power amplifier. Calculations show that a drain efficiency higher than 70% down to an output power back off of 7.7 dB can be maintained for a fractional bandwidth of 36% by tuning the transistor load reactance during the appropriate operating conditions. The modulation of the transistor load reactance can, for example, be achieved with varactors. The concept is demonstrated in a gallium nitride high electron mobility transistor power amplifier with silicon carbide varactors. The power amplifier achieved a power adde iency over 50% for 1.70 to 1.80 GHz down to 5 dB output power back off, with the maximum output power of 40.4dBm for continuous wave measurements. For modulated signals, the power amplifier showed excellent linearity and high efficiency. For a 3.84MHz 6:6 dB peak to average power ratio W-CDMA signal at 1.75 GHz, the power amplifier achieved an adjacent channel leakage ratio of -48 dBc, an average power added efficiency of 44.9% and an average power of 33.1 dBm. The correlation between theory, simulated results and measured results is discussed to show the potential of the broadband, dynamic load modulated class-J power amplifier concept.