How do you design a second-order Butterworth active filter?

A second-order Butterworth filter has maximally flat passband response with -40dB/decade rolloff. Using Sallen-Key topology: Choose cutoff frequency fc, select C1 = C2 = C for simplicity, calculate R = 1/(2*pi*fc*C). For Butterworth response, Q = 0.707, requiring gain K = 3 - 1/Q = 1.586 for unity-gain stable filter. Components Rf and Rin set the gain. The Butterworth polynomial coefficients determine the exact damping ratio for flat response. Multiple sections can be cascaded for higher order.