Linear GaN MMIC Combined Power Amplifiers for 7-GHz Microwave Backhaul

This paper presents the design of two combined linear power amplifiers for 7-GHz microwave backhaul, realized in 0.25- μm GaN on SiC monolithic technology. Both modules are based on a combined class-AB structure conceived for maximum back-off efficiency and reduced phase distortion, which are important requirements in backhaul systems. Different second harmonic loads are exploited in the two power amplifiers, leading to different performance in terms of output power, bandwidth and efficiency. The two stages exhibit a saturated output power in excess of 35 and 36 dBm on 16% and 26% of fractional bandwidth, respectively; moreover, the measured average efficiency in the presence of modulated signals with 7.4-dB peak-to-average power ratio is 18% and 25%. Simulations and experimental results demonstrate that the second-harmonic load has little influence on the linearity of the proposed amplifiers. Compliance with the spectrum emission mask defined for the targeted application has been achieved through low-order polynomial digital predistortion, thus demonstrating the high linearity of the stages. A comparison with a Doherty amplifier realized in the same technology and for the same application shows that the two proposed stages need a simpler predistorter to achieve the linearity required by standard specifications.