Miniature Multi-Band Absorptive Bandstop Filter Designs Using Bridged-T Coils

In this paper, miniature absorptive bandstop filters (ABSFs) with one to three stopbands are demonstrated. First, the proposed single-band ABSF is realized by the introduction of one lossy resonator to a conventional stub bandstop filter, while the circuit size is largely reduced by employing bridged-T coils to replace the transmission line sections. Next, by employing the dual-band design of bridged-T coil, a miniature dual-band ABSF can be achieved. The proposed tri-band ABSF is then implemented through the cascade of the proposed single- and dual-band ABSFs. As a demonstration, three proposed single-, dual-, and triple-band ABSFs are implemented in a commercial GaAs pHEMT process. Much smaller circuit size than the conventional multi-band ABSFs and good measured performance are achieved.