Study Of Microwave Power Amplifier Linearization Techniques

With the rapid development of wireless communication, the demand to the frequency resources and the efficiency of power amplifier has become more and more important. The power amplifier is the necessary component in radio frequency system and also a main part of nonlinear products. The linearity of PA has been the most crucial factor to restrict the efficiency of PA, so it has the important actual meanings and the topic of challenge to linearize the power amplifier.Most of the wireless communication systems are required to achieve least adjacent channel emissions and high bandwidth efficiency, that is to say signals must be transmitted in a limited band. However, distortion caused by nonlinear devices certainly will disturb other channels. In order to take good advantage of the limited band and contain more channels, modern communication systems often use linear modulation and wideband transmission techniques. Typical modulation techniques, such as QAM (Quadrature Amplitude Modulation) and QPSK, which have varying envelopes. Devices working near saturation will produce inter-modulation distortion while those kind of signal flowing through nonlinear power amplifier, specifically the distortion brought by the AM-AM and AM-PM character of the amplifier will disturb other users. In modern mobile communication systems, the adjacent channel interference requirements range from -40dB to -60 dB relative to the in-band carrier level.The linear modulation signal which has the advantage of high band efficiency is sensitive to the nonlinear distortion of the system because of its nonconstant envelope. In order to obtain high output power, amplifier has to work at saturation situation, while the nonlinear distortion becomes severer as the power is increasing.Linearization which can improve spectrum efficiency and signal distortion is a good method to solve this problemThis paper first introduces the research purpose and its significance and the development of the situation at home and abroad. ChapterⅡintroduces the non-linear amplifier characteristics and its model. ChapterⅢintroduces commonly used amplifier linearization methods such as feedback, pre-distortion and feed-forward. ChapterⅣfocuses on the pre-distortion linearization technique and its characteristics, as well as the commonly used pre-distortion methods and their advantages and disadvantages are compared. A predistorter has been designed which can reduce IM3 and IM5 of the amplifier. The principle of this linearizer is represented and analyzed in detail, and the results of the amplifier with and without linearizer are given. The results show that distortion reduction could be achieved.Chapter V analyzes the basic principles of feed-forward amplifier and presents a hybrid linearization technique that integrates RF predistortion and adaptive technique into the structure of the basic analog feedforward linearizer to overcome many drawbacks of the basic feedforward power amplifier system. This hybrid linearization technique insert a RF predistortion unit in front of the main amplifier and select rational adaptive technique according to its working principle, so the linearity and power efficiency can be enhanced and increase the performance and practicability of whole amplifier are improved. System simulation results show that the improvement of basic feedforward power amplifier system is effective.