A doherty power amplifier for 5G applications

Abstract

The ever-increasing need for high data rates and low latency steered modern communication systems toward massive MiMo architectures. In this architecture, each antenna element requires its amplifier unit. Smaller cell sizes and more user service have forced a significant increase in antenna elements, so the Power Amplifier units. Each amplifier is a substantial contributor to the power consumption budget. Therefore, efficiency is a primary concern in Power Amplifiers of cellular infrastructure systems. This study focuses on an architectural solution for Power Amplifier efficiency by demonstrating a Doherty PA (DPA) design in a widely used 5G sub-6GHz band of n78. This thesis focuses on the efficient RF output power generation of 5G base stations from an architectural perspective. A Doherty Power Amplifier is designed using for n78 5G band that is prioritized for the upcoming deployment. The design is optimized for an operation between the 3.6 - 3.8 GHz frequency band. The output power of 43 dBm (20 W) was obtained by using two of the 10W GaN HEMT transistors and a drain efficiency of 73% was obtained. The Doherty region starts at 38 dBm output power which allows an efficient operation with 6 dB power back off. A drain efficiency of up to 53% was obtained in the Doherty region and a gain of 10 dB is obtained over the entire band. The requirement of unconditional stability at all frequencies under small and large signal conditions demands a thorough analysis at different phases of design. For multi – stage stability analysis, the Ohtomo approach was also utilized in this study, for its convenience of being based on S – parameters and not requiring access to the transistor's internal components. Nyquist method is used in Ohtomo approach. Nyquist method was applied for frequencies between 10 MHz and 10 GHz. As a result, the proposed DPA is stable because there is no loop gain surrounding the 1+ j∗0 point. The prototype of the proposed DPA was fabricated and real-time small and large signal tests were performed. During the tests, it was found that the prototype DPA did not work only in the desired frequency range of 3.6 - 3.8 GHz during the design period, and after the tuning process, Doherty drain efficiency characteristics were obtained in the frequency range of 3.2 - 4 GHz (n77 5G band). Accordingly, The output power of 41.6 dBm (approximately 15 W) was obtained by using two of the 10W GaN HEMT transistors and a drain efficiency of 67% was obtained. The Doherty region starts at 32 - 38.2 dBm output power, allowing an efficient operation with 0.5 - 7.8 dB power back off. A drain efficiency of up to 67 % was obtained in the Doherty region, and a gain of 3.5 - 7.9 dB was obtained over the entire band.