Band-Pass Filters

There are applications where a particular band or spread or frequencies need to be filtered from a wider range of mixed signals. Filter circuits can be designed to accomplish this task by combining the properties of low-pass and high-pass into a single filter. The result is called a band-pass filter. Creating a bandpass filter from a low-pass and high pass filter can be illustrated using block diagrams.

FIGURE

What emerges from the series combination of these two filter circuits is a circuit that will only allow passage of those frequencies that are neither too high nor too low. Using real components, the following figure shows a typical schematic representation of a band-pass filter circuit.

FIGURE

The response of the band-pass filter is shown in the following figure.

FIGURE

Band-pass filters can also be constructed using inductors, but the reactive "purity" of capacitors gives them a design advantage. If we were to design a band-pass filter using inductors, it might look something like as shown in the figure below.

FIGURE

The fact that the high-pass section comes "first" in this design instead of the low-pass section makes no difference in its overall operation. It will still filter out all frequencies too high or too low.

While the general idea of combining low-pass and high-pass filters together to make a band-pass filter is sound, it is not without certain limitations. Because this type of band-pass filter L works by relying on either section to block unwanted frequencies, it can be difficult to design such a filter to allow unhindered passage within the desired frequency range. Both the low-pass and high-pass sections will always be blocking signals to some extent, and their combined effort makes for an attenuated (reduced amplitude) signal at best, even at the peak of the "pass-band" frequency range. This signal attenuation becomes more pronounced if the filter is designed to be more selective (steeper curve, narrower band of passable frequencies).