Nonlinear Distortion Of RF Power Amplifiers

RF power amplifier is one of the main components in front-end of wireless transmitters. With the development of mobile communication technology, requirements of linearity on the power amplifier are increasing. The commonly used linearization technologies to significantly improve the linearity should be specific for the nonlinear characteristics of power amplifiers. Therefore, research on non-linear RF power amplifier is significant.Currently, researches on nonlinear BJT power amplifiers focus on the cause of nonlinearity of amplifiers and the establishment of the nonlinear model. Among them, researches on the cause of nonlinear are mainly about the contribution of each element in the equivalent circuit to the nonlinear distortion. Little work has referred the relationship between the nonlinear distortion and the operating point. And the way modeling of BJT power amplifiers based on the measurement data, which is used to obtain the common model (SGP, MEXTRAM, VBIC95, etc.) parameters, limit the study of causes of the nonlinear distortion. This thesis summarizes the development of Volterra series theory and its common use in circuit analysis. The analysis method based on Volterra series is applied to the nonlinear analysis of BJT power amplifier. An expression of the relationship between the power amplifier nonlinear distortion and the operating point has been obtained. The simulation results verified the potential of the change in operating points to improve the linearity of BJT power amplifiers. In the nonlinear modeling, the paper uses simulation software to achieve automatically extraction of parameters of the small-signal equivalent circuit, which is used to establish the complete SDD model based on look-up table. It is verified that this nonlinear model can be used to characterize the power amplifier. This model provides a way to study causes of the nonlinear distortion and the contribution of each component to the overall nonlinear distortion.The result of this thesis has been applied to the design of power amplifier circuit in TETRA digital trunked radio transmitter. The linearity has been improved significantly (IIP3 improved by 10dBm) by optimizing the operation point.