A Second Differential Bandpass Filter with Tuning Center Frequency and Constant Q

Abstract

A second order differential filter with Q-enhancement and tunable active inductor is presented. The design technique for a tunable Q-enhancement SiGe HBT active inductor operating in the wide RF-band for the filter is described. Multi-regulated Cascode circuit is employed to enhance the quality factor Q by increasing the output impedance. Tunable active resistor is introduced to boost the tuning ability of the active inductor. Employing the proposed active inductor, the center frequency of the filter is tuned in the frequency of 1.05~2.45GHz by adjusting the bias current, Q reducing with different bias current can be compensated via tuning the feedback effective resistance of the active inductor, and Q has almost constant value of 209~225 at the frequency of 2.15GHz.