Analog predistortion for improvement of RF power amplifier efficiency and linearity
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The objective of this research is to implement analog systems that enable highly efficient and linear operations of the RF PAs used in wireless communication systems. Particularly, analog predistortion (APD) techniques have been developed while providing higher levels of performance required by 4G cellular systems in this work. Analog circuits such as a variable gain amplifier, a phase shifter, a logarithmic amplifier, a phase detector and an operational amplifier for use in this APD system were identified, and the required specifications of these analog circuits for this APD system were defined. Based on the specifications, analog circuits were designed and fabricated using a SiGe BiCMOS 0.18-µm process. Then, their performances were experimentally verified. Last but not least, an APD system was implemented in board levels by utilizing the fabricated circuits on printed circuit boards. Digital control capabilities such as digital-to-analog converters and digital potentiometers were added to this system. They maximized the performance of the APD system. Measured results show that a 5-dB improvement in the adjacent channel power ratio (ACPR) was achieved at 28 dBm output power for an LTE BW 10 MHz signal. Total power consumption of this test board was 437 mW, which is lower than the power consumption of a commercially available digital predistortion. The limitations of ACPR correction are because of a noise generated by gain blocks and group delay in correction loops.