# What is a Band Stop Filter ? Draw and explain the frequency response of a band stop filter

## Band Stop Filter

The band stop filter blocks signals falling within a certain frequency band set up between two points while allowing both the lower and higher frequencies either side of this frequency band.

It is formed by the combination of low pass and high pass filters with a parallel connection instead of cascading connection.

Since it eliminates frequencies, it is also called as band elimination filter or band reject filter or notch filter.

Unlike high pass and low pass filters, band pass and band stop filters have two cut-off frequencies. It will pass above and below a particular range of frequencies whose cut off frequencies are predetermined depending upon the value of the components used in the circuit design. Any frequencies in between these two cut-off frequencies are attenuated. It has two pass bands and one stop band. The ideal characteristics of the Band pass filter are as shown below.

Where :

fL indicates the cut off frequency of the low pass filter.

fis the cut off frequency of the high pass filter.

The centre frequency  fc = √( fL x fH)

When the input signal is given, the low frequencies are passed through the low pass filter in the band stop circuit and the high frequencies are passed through the high pass filter in the circuit. This is shown in below block diagram.

The R-C band stop filter is similar to a band pass filter in which the shunt arm is replaced by the series arm  and the series arm is replaced by the shunt one.

### R-C Band Stop Filter

The circuit of a R-C band stop filter is shown below.

Such a filter attenuates the electric signal over a specified frequency range from fto fand below fand above fhas all pass band region.

The R-C band stop filter frequency response curve is shown in fig. below.

The stop band is represented by the group of frequencies that lies between fand fwhere response is below -60 dB.

The cutoff frequencies are given as :