# _Ku_-Band CMOS Power Amplifier with Three-Stack Power Stage to Enhance Output Power and Efficiency
## 1 Introduction  
With the recent rapid development of wireless communication technology, commercialization of 5 th generation mobile communication technology in millimeter-wave bands is becoming visible. Thanks to the development of such 5 th generation mobile communication technology, research into beamforming transceivers is also being actively conducted [1; 2; 3].  
In particular, a _Ku_-band is used as the frequency band for satellite communications with 10.7-12.75 GHz for downlink and 13.75-14.6 GHz for uplink [4; 5; 6]. Considering the heavy free-space loss in satellite communications, it is possible to reduce the output power requirement for a single power amplifier by spatially combining power using a large-scale phased-array transmitter such as a beamforming system [6; 7]. The beamforming technology provides various technical advantages in wireless communication and radar systems. However, unlike the previous system, a large number of high-frequency circuit units are required to be used, and, accordingly, various tasks to be solved are generated. In particular, heat generated by a large number of high-frequency circuit units degrades the high-frequency characteristics of circuits, which is a major cause of deteriorating the performance of the entire system. Therefore, for practical implementation and commercialization of a beamforming system, it is essential to efficiently dissipate heat generated at the front end of the transceiver; a more fundamental way is to suppress heat generation itself. In order to efficiently suppress heat generation in the beamforming system, it is important to improve the efficiency of a power amplifier that uses the most power at the front end of the transceiver [8; 9; 10].  
Accordingly, studies to improve the efficiency of the power amplifier are being conducted very actively to date. However, since the output power of the power amplifier and the efficiency are in a trade-off relationship, it is required to ensure high output power and minimize efficiency degradation. Most research related to improving the efficiency of thepower amplifier is focused on improving efficiency in the power stage, which consumes the most dc power. However, dc power consumption in the driver stage for driving the power stage cannot be ignored. Therefore, to improve the efficiency of the entire power amplifier, it is reasonable to reduce the dc power consumption not only in the power stage but also in the driver stage. As a result, it is required to consider the entire structure of the power amplifier, including the driver stage as well as the power stage, in order to ensure high output power and minimize efficiency degradation.  
In this study, we investigated the power and driver stages that comprise a power amplifier to improve output power while suppressing degradation of the efficiency. Based on this, we proposed the structure of the driver stage and power stage to improve the efficiency of the CMOS-based _Ku_-band power amplifier for beamforming systems of satellite communications and verified its feasibility through verification design. In Section 2, we investigated the output power, impedance, and efficiency characteristics of the common-source and stack structures. Section 3 presents the proposed power amplifier structure. The simulation results of the designed power amplifier are shown in Section 4. Finally, we provide the measurement results of the designed power amplifier in Section 5.