A dual-mode microstrip ring resonator filter with active devices for loss compensation

A novel bandpass filter design is presented which uses a dual-mode microstrip ring resonator with a pair of FETs which provide negative resistance to compensate for the losses in the ring. The measured results show excellent performance, with a 32-MHz, 3-dB bandwidth centered at 1.48 GHz. The technique used is particularly attractive because of its ability to implement two transmission poles and two transmission zeros with only one ring if a suitable mode-coupling structure is designed. The advantage of the combined passive resonator and active device technique is that the poles and zeros of the filter are accurately defined by the ring dimensions, and the FETs only provide negative resistance to compensate for the ring losses. Hence, the filter is less sensitive to environmental changes than a filter based on entirely active techniques.<>