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High-order digitally programmable CFOA universal filter structures based on state variable approach

In this paper third-order and fourth-order digitally programmable universal filter structures are synthesized. The proposed electronic circuits are created based on the well-known state variable approach and techniques for designing digitally programmable active filters and oscillators. The first filter can realize third-order low-pass and high-pass electronically tunable transfer functions, while the second one can realize basic biquadratic tunable functions namely: low-pass, high-pass bandpass and band-reject. The cutoff frequencies of the proposed filters are electronically tuned via external digital words, applied to the address inputs of CMOS digital potentiometers. The digital potentiometers are connected as variable resistors in CFOA programmable integrator structures. The validity of the filters is demonstrated by simulations and experimental testing of sample electronic circuits.