Design Of High-Efficiency Doherty Power Amplifier

With the development of wireless communication technology, besides those well known public wireless communication services, the personal services represented by including telephone, wireless local network and Bluetooth and so on are gradually penetrated into people's lives and becoming un-replaceable communication tools. With the rapid development of technology and increasing communications needs of the people, the wireless communications industry will continue to move forward to flourish.Power amplifiers are the one of the most important core module in the radio front-end of wireless communication system. Due to the critical influence to the overall performance of communication system, they received widespread attention and study. In modern wireless communication, the third generation mobile communication system such as WCDMA and so on requires faster and faster wireless data fast transmission. So, non-constant envelope QPSK modulation is adopted which requires high ratio of high peak to average of communication signals and higher requirements of linearity of the amplifier. In real application, a common way of power back is adopted to satisfy the requirements of linearity which will sacrifice the large scale of efficiency, however, not only the operation cost of wireless communication is increased, but also stability of the system is deteriorate due to heat. So how to improve the efficiency of power amplifier under the condition of meeting the requirements of linearity is the key point of modern power amplification. Doherty technology has become the focus of current research due to its simplified structure, easy to implement, especially maintaining the high efficiency with power back-off and being able to strike a balance between linearity and efficiency.This article is about design and simulation of power amplifier using Doherty technology. Firstly, an introduction to the basis for RF power amplifier, including performance indicators, amplifier classification and power amplifier in communication system; secondly, an analysis of working principle and advantages or disadvantages of Doherty structure based on active load-pull theory; Thirdly, design and optimization of Doherty power amplifier using ADS platform and Freescale's LDMOS power amplifier model. Not only required detail is considered in every design phase, but also the affections of efficiency and linearity of Doherty amplifier due to the main and auxiliary bias conditions, the proportion of power distribution, impedance transform are analyzed. Through continuous design optimization and simulation, an asymmetric Doherty power amplifier is finally required which is suitable for a WCDMA downlink with frequency band of 2110-2170MHz. The Doherty power amplifier has the average output power of 25W (44dBm) in the 2140MHz, and the gain is 15dB with in-band gain flatness less than ldB, power added efficiency of 43.6%. Compared with balanced AB class power amplifier with the same output power level, the efficiency has increased 19%,while it's third order intermodulation distortion of-26.2dBc in the two-tone harmonic simulation, Higher efficiency increase at smaller cost of linearity. And finally, the PCB is drawn using ADS.