Design of an RF low-noise bandpass filter using active capacitance circuit

A novel RF active bandpass filter (BPF) is proposed and its noise performance is optimized by noise analysis. In the proposed design, a resonator consists of an active capacitance circuit together with a conventional inductor. The active capacitor is made of a field-effect transistor that exhibits negative resistance as well as capacitance. It can, therefore, compensate the loss of an inductor. Whereas the conventional active filters using negative resistance circuits usually have a high noise figure, the proposed active filter shows good noise property achieved by adopting a novel active capacitance circuit, which is proven by noise analysis and the experimental result. The measured second-order active BPF shows bandwidth of 95 MHz, 0.1-dB insertion loss, 0.3-dB ripple, and a noise figure of 2.4 dB at the 1.9-GHz band, which agrees well with the simulated results.