An important property of a bandpass filter is its shape factor, which is the ratio of its bandwidth at high attenuation to that at low attenuation. The smaller the shape factor, the better the filter.

Another important parameter is the group delay time, which determines how fast pulse shaped and sinusoidal signals can traverse the filter. Within the passband, the delay time should be constant to ensure the faithful transfer of the signal.

These parameters often do not show up very well in the usual active filter with local feedback for each second-order section ( multiple feedback filter ).

The magic word in electronics is ‘feedback’. The graphs in Fig. 1 and 2, pertaining to the circuit in Fig. 3, show the importance of feedback. Figure 1 gives the frequency vs gain response if R14 and R16 are omitted (no feedback). The bandwidth at -3 dB is 50 Hz and that at -40 dB is 450 Hz. That gives a shape factor of 9.

With the extra feedback provided by R14 and R16, the filter has a bandwidth of 200 Hz at -3 dB and of 660 Hz at -40 dB. That gives a shape factor of 3.3.

Moreover, the group delay in the passband is constant within acceptable limits.

This shows that with the extra feedback the filter gives a better performance than in its traditional set-up.