The well-documented Sallen-Key active filter is a staple of analog design. This Design Idea shows a way to obtain better roll-off by adding just a few common passive components. Figure 1a shows a typical implementation of a three-pole, lowpass version. In operation, you adjust the ratio of capacitors C₁ and C₂ to give a peaked response for the two poles within the feedback loop. The peaked response compensates for the initial roll-off in the third pole formed by the R₃-C₃ section at the input. In Figure 1b, a twin-tee notch filter replaces the R₃-C₃ section at the input. The notch frequency, F=1/(2πR₄C₄), is equal to approximately twice the desired cutoff frequency.

Select a value for R₄ that's approximately one-third to one-fourth the value of R₁, and then adjust R₄ as needed to allow use of standard capacitor values. The graph in Figure 2 shows the improvement in the cutoff rate of the filter; the result is a quasi-elliptic response. A breadboard of the circuit in Figure 1b uses 5% parts. The measured results show good agreement with the Spice simulation. To take advantage of the faster roll-off, just scale the frequency and impedance to your application. The highpass dual of this circuit works as well as the lowpass version.

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