Doherty Power Amplifier Design

Nowadays, the envelope-varied modulation has been widely used. The OFDM(Orthogonal Frequency Division Multi-plexity) modulation, such as WCDMA(Wide-band Code Division Multiple Access), LTE(Long Term Evaluation), WIMAX(Worldwide Interoperability for Microwave Access) have played more and more important roles. The envelope-varied communication system transmits the high peak-to-average ratio(PAR) signal, which also has more bandwidth. The power amplifier is one of the important parts in the communication system, which has been required to be more efficient.This paper aims to find the way to solve this problem. Firstly, it focuses on the Doherty power amplifier and analyzes the principle of it, such as the symmetric Doherty amplifier, asymmetric Doherty amplifier, N series Doherty amplifier and the Saturated Doherty amplifier.Secondly, the Freescale's LDMOS transistor MRF7S21080H is used for designing the Doherty amplifier, which has 40W average output power. The simulation methods of the loadpull and the Doherty combining have been introduced. Then, the test of the whole amplifier system has been done. The test results and the method of the Doherty combining tuning have been introduced. From the test results, the driver amplifier and the Doherty amplifier have met the requirement of the system.Finally, the 3rd Generation Semiconductor GaN has been analyzed, for its advantages in the power device. The simulation with the transistor has been done, which is NPT25100, from the Nitronex Corporation. During this work, the design experience has been accumulated, which is useful for the further research.