A Miniaturized Differential CMOS BPF With High Selectivity and Improved In-band Flatness Based on Transformer-Type Resonators

A miniaturized 4th order on-chip bandpass filter (BPF) with high selectivity and improved in-band flatness is proposed in this paper. It consists of two pairs of transformer-type resonators (TFRs) which are constructed by a transformer loaded with parallel capacitors. With the transformer-type structure, one resonator is formed inside another resonator, thus fully utilizing the chip area and resulting in a miniaturized structure. Meanwhile, the two pairs of transformer-type resonators (TFRs) are placed side-by-side and naturally form a quadruplet structure, which introduces two transmission zeros (TZs) at each side of the passband thus significantly improving the selectivity and stopband rejection of the BPF. The proposed on-chip BPF with 5.85 GHz center frequency (CF) and 31% 3-dB fractional bandwidth is manufactured by a commercial 55nm bulk CMOS technology. The measurement results show that the BPF has two TZs located at 4.6GHz and 8.3GHz and the shape factor of the BPF is 1.5.