Four-Output Filter Circuit

The classic state-variable (two-integrator) filter (see Fig. A) is famous for its insensitivity to device parameter tolerances, as well as its ability to provide three simultaneous separate outputs: high pass, bandpass, and low pass. These advantages often offset the fact that a quad operational amplifier is needed to implement the circuit. A modification of the classic scheme that applies the input voltage via amplifier UD, rather than UA provides a bandpass output with a fixed peak gain that doesn`t depend on the Q of the filter. It was found by using that configuration, a fourth notch-filter output can be obtained if - R6 (see Fig. B).
Four-Output Filter Circuit - schematic

If R, -R^ = R2, the gains of both the notch and bandpass outputs are unity, regardless of the Q factor, as determined by R3, Rl, R2, R4, R5, and R6. The resonant (or cutoff) frequency is given by, - 1/R0 x CQ. Depending on the capacitor values and frequency, resistance R0 might also share the same monolithic network for maximum space economy. As with the classic configuration, resonant frequency can be electrically controlled by switcliing resistors R0, or by using analog multipliers in series with the integrators.

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