A flexible tri-band impedance transformer for frequency-dependent complex impedances with simple structure

This paper proposes a simple tri-band impedance transformer for frequency-dependent complex impedances. The proposed transformer is a cascade of a pi-model transformer, three series transmission lines, and a tri-band susceptance stub network, the structure is simple, and the matching sequence of the three frequencies can be changed. All parameters of the design can be obtained from the derived analytical formulas, and the process of the design is elaborated. Two design examples demonstrate the proposed design method, and a prototype is fabricated for verification. Good agreement between the simulated results and measured results is obtained. Compared with the existing works, the transformer features a simple structure, offering flexibility to match frequency-dependent complex impedances, and a simple method. Additionally, it features a very short total electrical length and wider bandwidth. Therefore, the proposed transformer is suitable for the tri-band active circuits and wireless communication systems.