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In this article, a novel design method of multiband bandpass filters (BPFs) which have flexibly tunable center frequencies is presented by using T‐shaped stub‐loaded step‐impedance resonators (SLSIRs)’ multimode characteristics. By controlling the impedance ratios and the length ratios of the T‐shaped open stubs, the center frequencies of a multiband BPF can be decided. To verify the effectiveness of the proposed method, a 1.34/2.2/3.3/4.5 GHz quad‐band filter using split‐ring based on SLSIRs is designed, fabricated, and measured. The proposed filter is composed of a pair of tri‐section T‐shaped open stubs transmission lines, which form a folded split‐ring. By controlling the impedance ratios, the length ratios of the open stubs and the width of the gap of the folded split resonance loop, the quad‐band center frequencies are flexibly tunable. Owing to the 0° feed structure, seven transmission zeros can be realized near the pass‐band for enhancing the selectivity. The measured results show good agreement with the theory and the EM simulation.

Hybrid continuous mode can be utilized to design highly efficient broadband power amplifiers (PAs). In this paper, a broadband Doherty PA (DPA) with enhanced back-off efficiency is realized by using the hybrid continuous mode. Especially, the carrier PA operates under hybrid continuous mode at lowpower regions, enhancing the bandwidth and back-off efficiency of the DPAs. For validating the proposed theory, a broadband DPA working over 1.5-2.6 GHz is implemented. The experimental results show that the designed DPA achieves a saturation power of 43.7-45 dBm, a drain efficiency of 41.3%-55.1% at 6-dB output back-off power level, and a drain efficiency of 57%-74.6% at peaking power level.INDEX TERMS Broadband power amplifier, continuous mode, Doherty, high efficiency.

A modified triangular loop resonator with an open-circuited steppedimpedance stub, which can generate two closely spaced passbands at 5.2 and 5.8 GHz, is analysed. For verification, the triangular loop resonators along with stub-loaded resonators (SLRs) are then employed to design a compact 2.4/3.5/5.2/5.8-GHz quad-band bandpass filter (BPF) for wireless local area networks as well as worldwide interoperability for microwave access applications. Each of the four centre frequencies in the proposed quad-band BPF is able to be controlled due to the design freedom.

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