Compact Power Amplifier with Improved Back-off Efficiency

Abstract

The widespread use of wireless communication promotes the development of technology in modern society. Near Field Communication and Radio Frequency Identification are two main applications which require RF power amplifiers to provide higher power for long distance transmission. Some techniques operate at 433MHz, like Active Tag Reader. Power amplifiers transfer DC power to AC power, the ability of energy saving depends on the efficiency of the power amplifier. It is known that power amplifiers suffer from efficiency degradation at reduced drive levels. The efficiency is also low for low input power and output power. The Doherty power amplifier is a good choice for back-off efficiency improvement for its easy implementation. The conventional Doherty power amplifier is large in size due to the adoption of quarter wavelength impedance transforming network. This project includes three different designs for reducing the size and enhancing the efficiency of the conventional Doherty power amplifier operating at 433MHz and make comparison of them. Firstly, microstrip lines are used for realizing branch-line coupler and output coupler for the Doherty power amplifier. Secondly, microstrip lines in the first design are replaced by lumped components for size reduction, power-added-efficiency (PAE) is not as high as expected due to high loss of lumped components in reality, however the size has been reduced efficiently. Finally, to enhance power-added-efficiency (PAE) and reduce size, the third design combines the input matching network and branch-line coupler for size reduction, less lumped components are used, so the circuit has higher PAE with smaller size. All three designs are simulated and measured, results are consistent with each other. The goal of this project is achieved successfully.