The classical "state-variable" (two-integrator) filter (see the figure, 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 classical 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.1 It was found by employing that configuration, a fourth notch-filter output can be obtained if R11 = R6 (see the figure, 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, R
1Bernie Hutchins, "Filter's Gain Is Independent Of Q," EDN, Nov. 24, 1982.