Filter Has Four Outputs

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 U_D rather than U_A 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, R4, R₅, and R₆ are preferably of a monolithic resistor network. The resonant (or cut-off) frequency is given by ω= 1/R_o × CZ_o. Depending on the capacitor values and the frequency ω, the resistors R_o may also share the same monolithic network for maximum space economy. As with the classical configuration, the resonant frequency ω can be electronically controlled by switching resistors R_o, or by employing analog multipliers in series with the integrators.

¹Bernie Hutchins, "Filter's Gain Is Independent Of Q," EDN, Nov. 24, 1982.