In this paper, we propose a universal biquad filter that can realize all types of 2nd-order functions, such as Low-pass Filters (LPF), High-Pass Filters (HPF), Band-Pass Filters (BPF), Band-Elimination Filters (BEF), and All-Pass Filters (APF). Also, the filter types can be programmable digitally with built-in switches. The proposed circuit can be realized by using a CMOS technology that is suitable for a mixed digital-analog LSI. In addition, the circuit can operate in high frequencies with a low power supply voltage because it is based on a current-mode circuit. Finally, the proposed circuit is simulated by PSpice to confirm its characteristics.

A CMOS current-mode continuous-time band-pass filter using the positive feedback Q (quality factor) - enhancement technique is presented in this paper. Q of the proposed filter can be mainly determined by the ratio of the two MOSFETs' transconductances in a Q-setting part, not by the ratio of two capacitance values similar to most of conventional band-pass filters. This filter can realize high Q value in spite of small chip area. Therefore, when higher value of Q is needed, the proposed filter does not need large capacitor which occupies large area on an IC chip. The proposed filter spends smaller chip area than the conventional one under the condition of Q>2. The proposed circuit is simulated by Spectre to confirm its characteristics.