Intuitive Approach to Active Digital Filter Design. Part IV: Principle of Active Biquads

The recently developed intuitive approach to active digital filter (ADF) design represents a straightforward and efficient method for designing digital filters. This approach explores the similarity between digital integrators and operational amplifiers in analog filter design. By connecting digital integrators as signal followers, first-order ADFs are implemented. By inserting first-order ADFs into the feedback loop of another ADF, second-order or biquadratic ADFs (biquads) are created. Cascading multiple biquads allows the construction of higher-order filters. This paper, Part IV of the series, focuses on the design of active biquads as fundamental filtering stages in ADF design. A universal biquad architecture, capable of simultaneously generating low-pass, high-pass, band-pass, and band-tejection responses, is introduced and thoroughly examined. Special attention is given to the use of positive feedback as an approach to increase the Q-factor in band-pass biquads. Finally, the application of biquad notch filtering to suppress 50 Hz power-line interference in electrocardiogram signals is demonstrated.