Op Amps for Everyone, 5th Edition

16.3. Low-Pass Filter Design

Eq. (16.1) represents a cascade of second-order low-pass filters. The transfer function of a single stage is

Ai(s)=A0(1+ais+bis2)

(16.2)

For a first-order filter, the coefficient b is always zero (b₁ = 0), thus yielding:

A(s)=A01+a1s

(16.3)

The first-order and second-order filter stages are the building blocks for higher-order filters.

Often the filters operate at unity gain (A₀ = 1) to lessen the stringent demands on the op amp's open-loop gain.

Fig. 16.11 shows the cascading of filter stages up to the sixth order. A filter ...