= Sallen-Key equal-component stability bound
{c}
{title2=$K<3$}
For equal frequency-setting <resistors> $R$ and <capacitors> $C$, write $\tau=RC$. The denominator is $1+(3-K)s\tau+s^2\tau^2$. With positive $\tau$, <asymptotic stability> requires $K<3$; at $K=3$ the ideal <poles> are imaginary, and larger gain gives right-half-plane <poles>. In particular $K=5$ gives $(1-s\tau)^2$, a repeated unstable <pole>. A finite formal gain curve must not be confused with stable filter operation.
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