Load Modulation of Harmonically Tuned Amplifiers and Application to Outphasing Systems

Abstract

Modulation of load impedance is an effective way to maintain efficient power amplifier (PA) operation over high dynamic range modulated signals. For high efficiency, a load modulation approach can be applied to inherently efficient classes of PAs such as those with harmonic tuning: class J, class F, and inverse class F. This paper presents an analysis of harmonically tuned amplifiers operating under load modulation conditions, deriving the optimal loading trajectories for these multiple classes of operation. Because these load trajectories are complex, it is then shown-through a series of analysis, simulations and measurements-that harmonically tuned amplifiers are better suited for outphasing systems, than conventional amplifiers such as class B. A design methodology is proposed and validated through design and measurement of a 900 MHz outphasing system, comprising of two Gallium Nitride (GaN) class J branch PAs, delivering 44.6 dBm with 75% PAE at saturation, while mantaining PAE above 60% over a 7 dB output power back-off.

Analysis of Optimal Outphasing Load Trajectories for GaN PAs

“…The complete outphasing load trajectories are determined by approximating the PA branches as voltage sources with the two-port combiner found from (1)- (3). The outphasing trajectories and performance are extracted for an outphasing system designed for: OPBO = 6 and 10 dB.…”

“…1 (a) shows the potential for efficiency enhancement of a load modulated architecture over 12 dB P OUT from Z f0 and Z 2f0 load-pull of a 900 MHz GaN PA under constant input power. The dependency of back-off efficiency of GaN PAs on Z 2f0 was analysed in [3] with simulations and experiments considering PAs operating at fixed compression (1 dB). In order to achieve the largest possible back-off efficiency load modulated architectures, such as outphasing, operate the constituent branch PAs at much larger compression levels, pushing the device to exhibit switch-like behaviour.…”

Analysis and Design of Outphasing Transmitter Using Class-E Power Amplifiers With Shunt Capacitances and Shunt Filters

“…Therefore, the high efficiency at back-off power level can be easily achieved by appropriate output combiner design, as will be discussed further in this paper. Combined with nonconstant envelope input signal techniques and drain supply modulation, the outphasing PA can demonstrate high drain efficiency and power added efficiency (PAE) for wide range of output power levels [8]- [11]. Several techniques have been proposed in order to achieve broadband operation of outphasing PA [12]- [14].…”

“…Since the two branch signals after the SCS have constant envelope, switch-mode PAs are widely used as branch amplifiers for outphasing transmitters, due to their high power efficiencies. Outphasing PAs based on class-D [27], [29], class-E [17], [18], [20]- [23], [30]- [32], class-F [8], [33]- [35] have been presented in the literature.…”

Investigation of load modulated inverse Class-F power amplifier with extended conduction angle

“…Most researchers have made great effect to develop various efficiency enhancement techniques for solving the PAs’ issue of transmitting high PAPR signal with poor efficiency, such as Doherty, envelope tracking, Chireix out‐phasing, pulse width modulation, and pulsed load modulation . As an alternative to above techniques, dynamic load modulation (DLM) technique was first introduced into Class‐E PAs by Rabb, and has been developed for the past few years .…”