Integrated Transformer-Coupled Balun Bandpass Filters With an Optimal Common-Mode Rejection Ratio

This paper describes a compact transformer-coupled balun bandpass filter design based on integrated circuit technology. Using a planar transformer with high-density fully symmetrical wiring not only greatly reduces the component size, but also provides a superior balance at the differential output ports. In addition, applying the tapped feed-line structure to the balun bandpass filter is accomplished by carefully studying the impedance-matching method and transmission-zero mechanism. Interestingly, the proposed balun bandpass filter can guarantee an optimal common-mode rejection ratio level in a specific operating frequency range by precisely controlling the fractional bandwidth of the filter passband.