Narrow-band bandpass filter with enhanced Q and selectivity based on coupled negative resistance

This paper proposes a novel narrow-band bandpass filter with relative 3dB bandwidth of less than 1% and enhanced selectivity by utilizing negative resistance. The HEMT device is employed to realize the negative resistance with a common-source and drain inductance and capacitance feedback structure. Q-enhancement is accomplished by the negative resistance coupled to other resonators. The measured results show that the fabricated active filter has a relative 3dB bandwidth of 0.9% and insertion loss of 0.37dB. Moreover, the skirt of the filter is sharpened and the selectivity is enhanced by using the negative resistance.