High-Selectivity Single-Ended/Balanced DC-Block Filtering Impedance Transformer and Its Application on Power Amplifier

In this article, a single-ended filtering impedance transformer with good frequency selectivity and high transforming ratio is proposed at first, comprising of the two cascaded coupled-line sections with a stub. Also, the balanced structure is expanded based on the differential- and common-mode analysis. The closed-form analytical equations and complete design procedures are provided in detailed. The working principle of the proposed single-ended impedance transformer is applied on the design of the filtering power amplifier (PA). Finally, a 3.3-3.6 GHz bandpass filtering PA with the measured drain efficiency of 62.3%-68.9% is designed, simulated, and fabricated, which displays the good filtering response and power amplification. Good agreement between the simulated and measured results is observed, thus demonstrating the validity of the impedance transformer and PA.