Construction of a GaN class AB power Amplifier with digital pre distortion
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The number of wireless devices in the world is growing rapidly. One of the most crucial parts of a wireless communication system is the power amplifier. This creates a huge demand, and a big interest for power amplifiers, and the technology is constantly requiring better performance from the PA. 2 of the most essential measures of an amplifiers performance is linearity, and efficiency. Modern modulation techniques as OFDM are very sophisticated, and require very linear behavior from the amplifier. Another popular trend is to make energy efficient equipment. Power amplifiers are one of the most energy consuming components in wireless devices today. An increase in efficiency could therefore increase the battery lifetime in battery powered hand held devices significantly. The problem in the design of an amplifier is that there is a general contradiction between linearity and efficiency. It is therefore a difficult task to design a PA with both good linearity, and high degree of efficiency. This problem will be the main scope of this thesis. The work conducted in this thesis will study the basic theory of RFPA design, and pre distortion. And use this to construct a class AB PA with harmonic tuning to boost the efficiency. The PA will be optimized for maximum output efficiency, which leads to a very nonlinear behavior. It will then be implemented a simple pre distortion algorithm to compensate for the nonlinear behavior. The performance of the system will be evaluated upon power added efficiency, adjacent channel power ratio, error vector magnitude, and there will also be introduced a new measure of linearity called Nonlinear to linear power ratio(NLPR). NLPR is a new metric developed by Dragan Gecan, Morten Olavsbråten and Karl M. Gjertsen that gives a metric of the total in band and out of band distortion in an amplifier. There has been conducted small signal, and 1 tone harmonic balance simulations, and measurements of the realized amplifier. The simulations gave good results with a small signal gain of 15.8dB, 41.8dBm output power, and a peak efficiency of 63\%. The measurements gave the same results except the efficiency, which was a bit lower than expected at 45.3\% due to inaccuracies in the production of the PA. \newpage The performance of the PA with and without pre distortion is simulated using a 16-QAM test sequence. Complex power series have been chosen as the base of the pre distorter. The simulations shows a significant improvement of linearity after pre distortion. The best results were obtained with a PA polynomial model order of 11, and a pre distorter polynomial order of 3. Giving a 6.49dB improvement in terms of ACPR, 2.92\% improvement in terms of EVM and 8.15dB improvement in terms of NLPR.