Design of a compact multifunction biquad filter with five filtering responses and its modification as a sinusoidal oscillator

Abstract This paper presents a systematic methodology for designing a voltage-mode multifunction biquad filter employing the commercially available active element, LT1228. The proposed topology adopts a three-input single-output (TISO) configuration. The circuit comprises a single LT1228, two capacitors, and three resistors, offering a highly compact design. The TISO biquad filter is capable of providing five fundamental filtering responses: low-pass (LP), band-pass (BP), high-pass (HP), band-reject (BR), and all-pass (AP) responses, within the same circuit architecture. Notably, these responses are achieved without the need for additional inverting amplifiers or reconfigurations of passive components. The parameters of the proposed filter, encompassing the natural ( ω 0 ) and quality factor ( Q ), are determined by electronic adjustment. Furthermore, we can adapt the proposed biquad filter to function as a current-controlled sinusoidal oscillator. The oscillation frequency is adjustable independently of the oscillation condition. We verified the performance and validity of the proposed circuits through PSpice simulations and experimental implementations in a laboratory setting. The proposed circuits hold immense significance for applications requiring precise signal generation and filtering, such as communication systems, test instrumentation, audio processing, automation and control systems.