Doug Glenn EDN The welldocumented SallenKey active filter is a staple of analog design. This Design Idea shows a way to obtain better rolloff by adding just a few common passive components. Figure 1a shows a typical implementation of a threepole, lowpass version. In operation, you adjust the ratio of capacitors C_{1} and C_{2} to give a peaked response for the two poles within the feedback loop. The peaked response compensates for the initial rolloff in the third pole formed by the R_{3}C_{3} section at the input. In Figure 1b, a twintee notch filter replaces the R_{3}C_{3} section at the input.
The notch frequency, is equal to approximately twice the desired cutoff frequency.
Select a value for R_{4} that's approximately onethird to onefourth the value of R_{1}, and then adjust R_{4} 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 quasielliptic 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 rolloff, just scale the frequency and impedance to your application. The highpass dual of this circuit works as well as the lowpass version. 

