Analysis of the Impact of Finite OFF-State Impedance of Peaking Branch on the Efficiency of Doherty Amplifiers

The Doherty power amplifier (PA) is widely recognized as one of the most promising power amplification techniques in wireless transmitters for both mobile terminals and base stations. In this paper, the efficiency enhancement that is achieved by the Doherty technique is briefly presented. Then, the impact of the finite OFF-state impedance of the peaking branch on the efficiency response of the Doherty PA is theoretically analyzed. For the practical validation, this problem is studied for laterally diffused metal oxide semiconductor (LDMOS) field effect transistors (FETs) as well as gallium nitride (GaN) high electron mobility transistors (HEMTs). It is demonstrated that GaN HEMTs processes superior material properties that make them very suitable for the design of high efficiency Doherty PAs. Lastly, an overview of recent implementations of GaN monolithic microwave integrated circuit (MMIC) Doherty PAs is presented.