Balanced to Unbalanced: An Overview of Multifunctional Wideband Balanced-to-Unbalanced Four- and Five-Port Filtering Power Dividers

Fifth-generation (5G) communication techniques are growing in popularity, and 5G will become the commercial operation standard for mainland China in 2020 [1]. As the next-generation mobile communication network, 5G will not only greatly improve communication efficiency; it will also change the way people live and work. The 5G communication system contains the millimeter-wave and sub-6-GHz bands. For the sub-6-GHz band, circuits are becoming ever more compact and multifunction; thus, electromagnetic interference and multicoupling problems will greatly affect the entire system's performance [2]-[4]. In response, balanced circuits with higher immunity to environmental noises [5]-[8], lower electromagnetic interference [9]-[11], and better dynamic range [12]-[15] have attracted increasing attention in the past few years, and a large number of balanced circuits are being conceived [16], [17]. Currently, all feature differential-mode input and output terminals, so they cannot be connected directly to single-ended circuits. Balanced filters, antennas, and power dividers that can be cointegrated with balanced active circuits are required for the development of fully balanced transmitter and receiver RF chains, such as the balanced power dividers that can be connected to a Doherty power amplifier in a balanced system. In this case, balancing is only necessary at the input and output stages. It is expected that the transistor stage is still a single-ended structure, where only two amplifier devices are needed [18].