Linear Power Amplifier Design

The modern wireless communication systems have made significant progress toward using non-constant envelope modulation technology for high-data-rate and spectral efficiency considerations. Such as W-CDMA basestation and repeater applications, the power amplifiers require a high linearity to amplify the high peak-to-average ratio (PAR) signal source without distortion for avoiding interference between different channels. To satisfy linearity requirements, the power amplifiers are usually biased at class A or AB mode and must operate at a large amount of back-off from the peak output power. However, the class A or AB mode power amplifiers have a low efficiency due to the back-off operation which increasing the difficulty and complexity of thermal management and cooling system. The contradiction between the linearity and the effiency make the design technique of the power amplifiers with high efficiency and linearity across a wide instantaneous bandwidth become a hot issue.In this paper, based on the adequate analysis on nonlinear characteristic of power amplifier, a type of RF predistortion approach to linearize power amplifier is presented which utilizes the intermodulation signal generated by an intermodulation signal generator to cancel the intermodulation component of power amplifier itself. A high performance linearized power amplifier for W-CDMA repeater application has been designed and implemented.The linearity of a 20 W high power Doherty amplifier is designed using the bias circuit optimization, uneven power driving and individually optimized matching and combining circuits. To reduce the memory effects, the load impedances of the bias circuit should be reduced to short the envelope frequency voltage component in order to avoid limitation of the cancellation performance of RF predistortion techniques.The measurement results show that the IM3 distortion rate of the power amplifier reaches -70dBc and the improvement in IM3 cancellation is more than 25dB for two-tone signal with 1MHz signal spacing in the frequency range from 2.11GHz to 2.17GHz at a 42dBm output power. For a forward-link two-carrier W-CDMA signal at a -14dBm input power level, the predistortion power amplifier delivers an adjacent channel power ratio(ACPR) of -50dBc at a 42dBm average output power with a cancellation of 9dB.The research demonstrates that the RF predistortion linearizated power amplifier can improve the linearity distinctly with simple structure, low cost and stable performance, which is essential to modern wireless communication system if combining with the low memory effect and high efficieny Doherty power amplifier.