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.
Abstract-A lossless power combining network comprising cascaded transmission line segments in a tree structure is introduced for a multi-way outphasing architecture. This architecture addresses the suboptimal loading conditions in Chireix outphasing transmitters while offering a compact and microwavefriendly implementation compared to previous techniques. In the proposed system, four saturated power amplifiers interact through an all-transmission-line power combining network to produce nearly ideal resistive load modulation of the branch power amplifiers over a 10:1 range of output powers. This work focuses on the operation of the combining network, deriving analytical expressions for input-port admittance characteristics and an outphasing control strategy to modulate output power while minimizing reactive loading of the saturated branch amplifiers. A methodology for combiner design is given, along with a combiner design example for compact layout. An experimental four-way outphasing amplifier system operating at 2.14 GHz demonstrates the technique with greater than 60% drain efficiency for an output power range of 6.2 dB. The system demonstrates a W-CDMA modulated signal with a 9.15-dB peak to average power ratio (PAPR) with 54.5% average modulated efficiency at 41.1 dBm average output power.Index Terms-base stations, outphasing, power amplifier (PA), wideband code division multiple access (W-CDMA), Chireix, LINC, load modulation.
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