Codesigned Wideband Filtering Amplifier With High Reverse Isolation Using Unilateral Frequency-Selective Gain Stage

This letter presents the RF design and experimental validation of a codesigned wideband filtering amplifier that simultaneously achieves flat in-band gain, high reverse isolation, and low noise figure (NF). The proposed architecture incorporates a wideband unilateral frequency-selective gain stage (UFGS) featuring resistive transmission-line (TL) feedback to suppress its reverse transmission while maintaining high gain and low NF. The UFGS is codesigned with coupled-line sections and open-circuited stubs to enable wideband filtering characteristics and multiple transmission zeros (TZs) for enhanced selectivity and out-of-band rejection. A step-by-step practical design procedure is also developed following conventional coupled-resonator design principles. For experimental validation purposes, a filtering amplifier prototype was manufactured and tested at 2.13 GHz, demonstrating a 3-dB fractional bandwidth (FBW) of 72% and a flat-gain bandwidth (FGBW) of 66%, along with a wide upper stopband spanning 3.1–5.5 GHz. Additionally, it achieves 15-dB peak gain, 55-dB directivity, 1.9-dB NF, 4.2-dBm output 1-dB compression point, and 16-dBm output third-order intercept point at the center frequency.