Reseach On Envelope Tracking Power Amplifier Technology

Envelope tracking(ET) technique is a promising method to implementing high efficiency power amplifer(PA). The essence is biasing PA with dynamic drain or collector supply varying according to the power level of output signal, thus keeping PA working in high efficiency zone constantly. Although the concept of ET has been understood for very long time, satisfactory work has not yet been implemented in practice. Analysis, simulations and experiments about the key aspects while implementing ET PA have been carried out in this thesis.Starting with equation of efficiency in PA, block diagram of implementing ET PA is presented in this thesis along with some key aspestcs such as envelope shaping technique, envelope modulator, consideration of dynamic supply for PA design and digital predistortion.Envelope shaping can be divided into three catagories and they are maximum efficiency, fixed compression and fixed gain. Maximum efficiency method delivers best performance in terms of PA efficiency while fixed gain method shows best performance in terms of linearity. The linearity of PA can not be too bad to be corrected using digital predistortion(DPD), causing this kind of PA has no place in practice.Envelope modulator is the most important part while designing ET PA. Bandwidth, efficiency, noise and load capability should be carefully considered. DC-DC converter, linear amplifier, hybrid of DC-DC converter and linear amplifier are three mostly used approaches to realize envelope modulator. An envelope modulator implemented with multi-level converter controlled with multistage noise shaping(MASH) technique is presented in this thesis. Measurement results show that 80% of efficiency was achieved and the tracking capability up to 300 KHz.PA is nonlinear inherently. When biased dynamically, the linearity performance may be worse and unpredictable. Analysis is made in this thesis about the effects of dynamic supply to the optimum matching point in the design.Linearity is as important as efficiency for PAs in modern communication system. In this thesis, a digital predistorter based on neural network(NN) is proposed to correct the nonlinearity of ET PA. Experimental results illustrate that more than 26 dB improvement of intermodulation(IM) can be obtained for 8 carriers GSM signal and 12 dB improvement of adjacent channel power ration(ACPR) for 5MHz WCDMA signal.