Research On S-band Broadband Doherty Power Amplifer

As an important component of the transmitter, the design theory and method of broadband and high efficiency power amplifiers(PAs) have been researched by scholars both at home and abroad. The next-generation wireless communication system needs to operate on different communication standards/frequency bands for different applications,thus the design of broadband PA has become the most challenging problem. In this thesis, the research of the broadband and high efficiency PA design is aimed at constant envelope signals and varying envelope signals. The research contents are as follows:Firstly, according to the theory of the Class-J power amplifier, the harmonic controlled unit is added in the output matching network to reduce the effect of harmonic impedance. The simplified large signal model of the transistor is used to calculating the optimum output impedance over the target band. Moreover, the pi section matching network is used which contains lump and distribution elements as the input matching network. A Class-J PA is designed in the 2.2~2.8GHz band exhibiting a drain efficiency of more than 61% and output power higher than 40 d Bm. A good linearity performance is measured with adjacent channel power ratio(ACPR) lower than-30 d Bc.Secondly, in the view of varying envelope signals, the Doherty PA is researched.With the limitations of bandwidth, back-off efficiency and linearity in the Doherty PA, a parallel structure which has low impedance transformation ratio is used. Furthermore, to avoid the impedance modulation mismatch, an offset line is added behind the matching network. The implemented Doherty PA achieves an operation bandwidth from 2.15 to2.6GHz, and it obtains a performance of more than 40% efficiency at 7-8d B back-off and 43 d Bm output power. Linearized modulated measurements using a WCDMA signal show an ACPR less than-31 d Bc.Thirdly, by considering the limited factors in the broadband Doherty PA design, the simplified real frequency technology is applied to solve the broadband matching problem. Besides, the impedance inverter is replaced by a broadband network.Measurement results show that an optimal Doherty PA is realized from 1.7 to 2.6GHz with a drain efficiency higher than 40% at 5-6d B power back-off.The design method of broadband Class-J PA and Doherty PA is proposed in this article, which provides the basis for the design of broadband PA. Besides, two other kinds of broadband structure are introduced to discuss the broadband characteristic ofthe Doherty PA. These methods provide the preview of the design of broadband and high efficiency PA.