Design, Fabrication, and Comparative Study of Electronically Tunable Active Filters

The Kerwin-Huelsman Newcomb (KHN) biquad is the natural choice for use as a ‘universal’ filter building block out of the several existing 3-amplifier biquadratic filter sections based on the two integrator loop or state variable realization of 2nd order systems. A modified version of the KHNbiquad, which is electronically tunable almost linearly over a decade range of center frequency, ispresented. For electronic tunability, instead of the direct use of a FET as a voltage variable resistanceto control the time constants of the integrators, a novel approach of using voltage-controlled variable-gainamplifiers at the inputs of both the integrators and one immediately after the input signal has beensuggested. The circuit is suitable for use as a tunable universal filtering module and can be easilydigitally programmed