Improve roll-off of Sallen-Key filter

Doug Glenn

EDN

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.

а) Improve roll-off of Sallen-Key filter
b) Improve roll-off of Sallen-Key filter
Figure 1. The addition of a twin-tee network (b) considerably improves
the roll-off rate of the circuit (a).

The notch frequency,

is equal to approximately twice the desired cutoff frequency.

Improve roll-off of Sallen-Key filter
Figure 2. The improved cutoff rate of the filter results in a
quasi-elliptical response.

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.

EDN

You may have to register before you can post comments and get full access to forum.
EMS supplier