| With the fast development of wireless communication techniques, the mobile communication systems occupy frequency bands with the carrier frequencies spanning over multi-octave bandwidth and the signals are featured by high peak-to-average power ratio(PAPR) and wide instantaneous bandwidth. To linearly amplify the modulated signals with high PAPR, the power amplifiers(PAs) have to operate at the back-off power with low power efficiency, leading to energy waste. In addition, several PA modules in parallel connection are usually required to fulfill the requirement of broad operating band and large instantaneous bandwidth, which will increase deployment cost as well as maintenance difficulty of the mobile communication systems.Therefore, the PAs in the base stations should have broad operating band, large instantaneous bandwidth, high efficiency and good linearity to save energy, reduce emission and cut the cost. Unfortunately, the bandwidth, efficiency and linearity of the PA are contradictory to each other, which make the power amplifier design more challenging.We focuses on the efficiency enhancement and bandwidth extension of the RF PAs in this thesis, the main contributions of which are summarized as follows.1. The impact of the carrier transistor’s load impedances on the Doherty PA’s(DPA’s) efficiency is analyzed, and a strategy for the optimum selection of the carrier transistor’s load impedances is proposed to improve the back-off efficiency of the DPA. A 2.35 GHz asymmetrical Doherty power amplifier is designed and fabricated to validate the proposed approach. The efficiency of higher than 55% is achieved at 8d B back-off and an average efficiency of greater than 50% with a linearized ACLR of lower than-47 d Bc is measured when tested with 100 MHz LTE-advanced signals.2. An optimization based method for the matching network design is proposed to mitigate the factors that limit bandwidth of the DPA. In addition, a LC resonator-type bias network is proposed to minimize the memory effect of the DPA under wideband signals. The fabricated wideband DPA shows a 6d B back-off efficiency of over 43% in 21% fractional bandwidth. When tested with 100 MHz instantaneous signals, the ACLR asymmetry of lower than 1.5d B is measured, together with a linearized ACLR of better than-50 d Bc.3. A harmonic-tuned power amplifier(HT-PA) with multi-octave bandwidth based on resistive harmonic load impedances is proposed. The principles of the resistive harmonic load impedances are described, followed by the impact of which on the efficiency, output power and bandwidth of the HT-PA. A multi-octave HT-PA operating from 0.4 to 3.0 GHz is implemented, and a high efficiency of 58~78% is obtained in 153% fractional bandwidth. Driven with 20 MHz instantaneous signals, the average efficiency of the proposed HT-PA is higher than 35% when the ACLR of which is lower than-30 d Bc.4. Broadband reconfigurable matching network is proposed for the bandwidth enlargement of the HT-PA. The employment of resistive harmonic load impedances usually decreases the efficiency of the HT-PA. To solve this problem, a broadband reconfigurable matching network realized using the PIN diodes are proposed to extend the bandwidth of the HT-PA without degrading the efficiency. The realized reconfigurable HT-PA is capable of operating from 0.7 to 2.5 GHz, with a measured efficiency of 70~84% in 113% fractional bandwidth. Driven with 20 MHz instantaneous signals, the average efficiency of the proposed HT-PA is higher than 40% when the ACLR of which is lower than-30 d Bc.
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