Broadband radio frequency power amplifiers for 5G wireless communications

Power amplifier (PA) is one of the key components in radio frequency (RF) front-end transceiver and its performance has a critical impact on wireless systems. In the coming fifth-generation (5G) wireless telecommunication system, due to the growing data traffic, there are increasing demands for higher data rate, wider bandwidth and higher spectrum efficiency. Moreover, modulated signals with high peak-to-average power ratio (PAPR) are widely adopted in 5G systems, requiring PAs to provide high power dynamic range. In this thesis, PA design methodologies aiming at performance enhancement with respect to bandwidth, efficiency, linearity and power dynamic range are comprehensively discussed under 5G sub-6 GHz applications. In each topic, both theoretical analysis and circuit verification are provided. Firstly, the broadband solution for maintaining PA's high efficiency is discussed. The broadband continuous mode operation is analyzed. Based on it, a practical design methodology to construct output matching networks for broadband continuous mode PA is proposed. This solution is designed for Monolithic Microwave Integrated Circuit (MMIC) process which enables compact circuit design with high power density. A prototype PA with 0.25 um Gallium Nitride (GaN) MMIC process is implemented. Secondly, in addition to broadband continuous mode PA design, its linearity performance is analyzed. The simulation reveals that the linearity behaviour varies with frequency inside the extended impedance space provided by continuous mode. To unify the linearity in broadband, a circuit level linearity compensation method adopting clipping contours is presented. A design example implemented with 0.25 um GaN HEMT is demonstrated to validate the method. Thirdly, the bandwidth and efficiency performance at output power back-off is investigated. A design methodology for the broadband Doherty power amplifier (DPA) is presented. To extend the bandwidth and output power back-off range simultaneously, a modified load modulation network is proposed. Based on the analysis and case studies, a series of generalized formulas for estimating design parameters are then introduced, offering broadband solution for arbitrary current ratio and power back-off. A prototype DPA is demonstrated and implemented with packaged GaN devices using proposed formulas. The last part proposes another design methodology for enhancing efficiency at output power back-off. A new way of load modulation is realized with a three-stage load modulated power amplifier architecture. This new load modulation mechanism enables flexible output power back-off and efficiency enhancement within large power dynamic range. Under the proposed architecture, the power back-off can be reconfigured without redesigning the circuit.