이동통신용 RF 전력증폭기의 효율 개선에 관한 연구

Abstract

As the wireless communication systems evolve, power amplifiers (PAs) in the advanced transmitters are required to provide high efficiency and linearity for wide instantaneous bandwidth and large peak-to-average power ratio (PAPR) signals across wide operating bandwidth. For the purpose, this dissertation presents various techniques to improve the efficiency, linearity, and bandwidth characteristics of the PA. In particular, for the high-efficiency, this dissertation considers the efficiency improvement methods for the unit cell PA and the high efficiency transmitters such as the Doherty amplifier, envelope tracking (ET) and Advanced Class-S technique. For the efficient transmitter system, the saturated PA is proposed and compared with Class-E PA at a high frequency. Switching-mode PAs such as Class-E PA have been widely investigated because they provide high efficiency performance. However, the class-E PA does not have any circuit solution above the maximum operation frequency (fmax,E). To improve the efficiency at the high frequency region, we have investigated the saturated PA which is an optimized version of the Class-E PA. The saturated amplifier has high efficiency in the high frequency region with a power matched fundamental load and the proper second and third harmonic loads. From the simulation and experiment results, it clearly show that the saturated amplifier provides excellent output power and efficiency compared to those of the Class-E PA. Secondly, a highly efficient asymmetric broadband Doherty PA (DPA) is introduced to achieve the high efficiency and broadband operation for a femto-cell base stations. The DPA is usually implemented with a hybrid configuration in the base stations. For the femto-cell base station, the PA should be small. So, a hybrid configuration DPA is not an appropriate structure for a small-cell base station because it occupies a large area. The device size selection of the carrier and peaking PAs is also limited due to device availability. Therefore, we design the DPA using monolithic microwave integrated circuit (MMIC) process to get a small size and proper cell size ratio of the carrier and peaking PAs. To enhance the bandwidth and efficiency, a new circuit topology is proposed. The efficiency for amplification of a high peak-to-average power ratio (PAPR) signal is improved by designing an asymmetric Doherty PA (DPA). The efficiency is further enhanced by inserting the second harmonic control circuit at the carrier PA. For a broadband operation, the phase compensation network is merged into the input matching circuit and the offset line is resonated-out by a dc-feed inductor. The quarter-wavelength transformer has a low-Q characteristic compared to that of a conventional one. Thirdly, a highly linear dual-band Doherty PA (DPA) is introduced for a femto-cell base stations. The digital predistortion (DPD) technique is widely used for the base station to linearize the PA. However, the DPD circuit board consumes a lot of power for a femto-cell base station. Therefore, the DPA should have high linearity in order to linearize the PA using a light PD system. A high linearity is obtained by applying a third-order inter-modulation (IM3) cancellation method for the Doherty structure. The dual-band operation is obtained by applying a switched capacirtor. Finally, an advanced Class-S transmitter based of a tri-level delta-sigma modulator (DSM) is introduced for LTE application. By converting the timevarying envelope signal to the oversampled pulse stream, the power amplifier is turned on and off while the wanted signal is transmitted with good linearity by the noise shaping characteristic of the delta-sigma modulator. With the multi-bit approach, a signal-to-noise and distortion ratio is further enhanced. However, efficiency of the PA is reduced with the multi-bit DSM. To get a high efficiency for tri-level DSM system, an average PA is proposed and demonstrated.