Class-J SiGe X-Band Power Amplifier Using a Ladder Filter-Based AM-PM Distortion Reduction Technique

In this paper, a class-J power amplifier for operation in the X-band realized in SiGe bipolar technology is presented. The proposed design combines the high efficiency of class-J operation with solutions to mitigate AM-PM distortion down to a level compatible with high spectral efficiency modulation schemes like 64-QAM. The design of a transformer-based output matching network and the trade-off between efficiency and pure class-J operation related to a non-ideal second harmonic termination are discussed. The main sources of AM-PM distortion are identified in the base-emitter and collector-based bipolar junction transistor capacitance variation with power level and solutions that make the design less sensitive to these impairments are proposed. Linear MIM capacitors are used at the power amplifier (PA) core output to realize the proper class-J second harmonic termination, while an input matching network based on a Bessel ladder filter is adopted to make the signal phase at the PA core input less sensitive to base-emitter capacitance variation. A class-J PA previously designed for maximum power-added efficiency (PAE) is used as a reference throughout the paper to highlight the trade-off's entailed by AM-PM distortion reduction. The designed PA features a peak gain of 15 dB at 10 GHz and 8-to-12 GHz -3 dB bandwidth. The saturated power is 22 dBm with a 33% PAE, while the output p1dB is 19 dBm. The AM-PM distortion at p1dB is 5 degrees.