Four-Way Microstrip-Based Power Combining for Microwave Outphasing Power Amplifiers

A lossless multi-way outphasing and power combining system for microwave power amplification is presented. The architecture addresses one of the primary drawbacks of Chireix outphasing; namely, the sub-optimal loading conditions for the branch power amplifiers. In the proposed system, four saturated power amplifiers interact through a lossless power combining network to produce nearly resistive load modulation over a 10:1 range of output powers. This work focuses on two microstrip-based power combiner implementations: a hybrid microstrip/discrete implementation using a combination of microstrip transmission line sections with discrete shunt elements, and an all-microstrip implementation incorporating open-circuited radial stubs. We demonstrate and compare these techniques in a 2.14 GHz power amplifier system. With the all-microstrip implementation, the system demonstrates a peak CW drain efficiency of 70% and drain efficiency of over 60% over a 6.5-dB outphasing output power range with a peak power of over 100 W. We demonstrate W-CDMA modulation with 55.6% average modulated efficiency at 14.1 W average output power for a 9.15-dB peak to average power ratio (PAPR) signal. The performance of this all-microstrip system is compared to that of the proposed hybrid microstrip/discrete version and a previously reported implementation in discrete lumped-element form.