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Bandpass filter with extra feedback

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 ).

Bandpass filter with extra feedback Schematic diagram

Bandpass filter with extra feedback Schematic diagram

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.
Bandpass filter with extra feedback Schematic diagram

A bandpass filter allows signals within a selected range of frequencies to be heard or decoded while preventing signals at unwanted frequencies from getting through. A bandpass filter also optimizes the signal-to-noise ratio (sensitivity) of a receiver.
A bandpass filter circuit/device is used to allow only a pre-defined set of frequencies to pass through it. It will filter all the frequency that is below the set value and above the set value. It is a combination of a high-pass filter and a low-pass filter.
Generally, the dielectric band-pass filters can be used over the frequency range from 300 MHz to 100 GHz. For high-frequency applications, NRD waveguide filters (Figure 7.38) gain interest because of the extremely low-loss and low dielectric constant materials that can be used in the design.

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