Figure 6.13: Buffer amplifier as part of active filter.
Filters often contain embedded amplifiers between passive-filter stages as shown in figure 6.13.
Figure 6.13: Buffer amplifier as part of active filter.
These filters have a limited performance since the poles are still real and hence the knees are not sharp. For example, a three stage high-pass filter with buffer amplifiers has a transfer function
and only drops 18 db/octave.
For complex poles we must use either integrators or differentiators. Consider figure 6.14.
Figure 6.14: Active filter with complex poles.
The closed-loop gain is
By exchanging the input resistor for a capacitor we can change between a low-pass and high-pass filter.