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 R1 = R6 (see Fig. B).
If R1 = R6 = R2, the gains of both the notch and bandpass outputs are unity, regardless of the Q factor, as determined by R3, R1, R2, R4, R5, and R6. The resonant (or cutoff) frequency is given by w, -1/Ro x Co. Depending on the capacitor values and frequency w, resistance Ro might also share the same monolithic network for maximum space economy. As with the classic configuration, resonant frequency w can be electrically controlled by switching resistors Ro, or by using analog multipliers in series with the integrators.