4th-order Sallen-Key low-pass
11 parts
Two Sallen-Key sections in a row make a fourth-order filter, but only if their Q values are staggered — 0.54 for the first, 1.31 for the second. That pairing is what makes the combined response maximally flat; give both stages the same Q and you get a sagging response with the corner in the wrong place. Same 1 kHz corner as the single-stage `Sallen-Key low-pass` example, twice the slope past it. The op-amps are ideal-gain macromodels with no bandwidth limit, so this skirt keeps falling as far as you sweep, where a real device's would flatten into its own feedthrough floor.
- V1 1V·1kHz
- R1 1kΩ
- R2 1kΩ
- C1 172nF
- OP1
- C2 147nF
- R3 1kΩ
- R4 1kΩ
- C3 416nF
- OP2
- C4 61nF
