Active Band Stop Filter | Notch Filter | 5 Important use


  • Band stop filter definition
  • Frequency response of a band pass filter
  • Mathematical expression
  • Characteristics of BSF
  • Applications of band stop filter
  • Notch Filter
  • Difference between Band Pass & Band Reject Filter
  • Advantages & disadvantages
  • Frequently asked questions

What is an Active Band Reject/Stop Filter?

Definition of a Band Stop Filter?

Band reject filter is combined of low pass and high pass filter which eliminates frequencies or stop a particular band of frequencies.”

Active Band Stop Filter
Circuit diagram of Active Band Stop Filter



Band rejection is obtained by the parallel connection of a high pass section with a low pass section. Now, the general rule is that, the cutoff frequency should be higher than the cut-off frequency of the low-pass area.

There is another way to create it. If a multiple feedback system is incorporated with an adder, then that functions like the desired operation. It is called as notch.

The Frequency response of a band stop filter is calculated by considering the frequency and gain.

 The frequency response of a band-stop filter 


The bandwidth is chosen through the lesser and greater cut-off frequency. Notch filter is used to remove the single frequency. From this frequency response, we can also obtain Passband ripple and stopband ripple.

                                 Pass Band Ripple= -20log10(1-∂p) dB

                                 Stop Band Ripple= -20log1o(s) dB

Where ∂p= magnitude response of the passband filter

             ∂s= magnitude response of the stopband filter

Mathematical Expression for a Band Stop Filter:

Band Reject Filter can also be obtained by using the multiple -feedback bandpass filter with an adder. A notch filter is created using a circuit which eliminates the output of a bandpass filter from the unmodified signal.

     Band rejects active filter using multiple feedback BSF with an adder.


Characteristics of a Band Reject Filter:

  • A band-stop filter works a frequency remover which is not within a specific range, reason it is called a rejection filter.
  • A band-stop filter passes frequencies of a particular bandwidth with maximum attenuation.
  • Different types of band-stop filters produce a maximum rate of roll-off rate for a given order and flat frequency response in the passband.

Applications of a Band Stop Filter:

  • An active Band Pass filter is used in the public addressing system and speakers for enhancing the quality.
  • A bandstop filter is also used in telecommunication technology as a noise reducer from different channels.
  • BSF is used in radio signals to remove static on the radio devices for better and clear communications.
  • Besides radios and amplification, this filter is also used in many other electronic devices to decrease a specific range of frequencies, known as ‘noise.’
  • In the medical field, BSF is used in making many useful devices like ECG machines, etc.
  • It also plays a vital role in image processing.

What is a Notch filter?

Notch filters find applications when there is a need to attenuate the undesirable frequencies while passing the necessary frequencies.

Difference between Band Pass Filter & Band Stop Filter:

Advantages & Disadvantages of a band stop filter:

A band stop filter attenuates the frequencies that are below the cut off range, so key advantage of using this filter is, it eliminates the external and unwanted noise or signals as well as gives us a stable output.

On the other hand, due to some certain limitations a band stop filter does not function properly under sustainable conditions. The parallel arrangement between the high pass and low pass filter my vary about the change of frequencies.

Frequently Asked Questions (FAQs):

What is the Q factor or ‘Quality Factor’?

Q is given by the ratio between the resonant frequency to the bandwidth. It is an important parameter and it helps us to calculate the selectivity.

The higher the value Q, the more selective is the filter, i.e., narrower is the bandwidth.

How do a Band stop filter work?

A band stop or band reject filter always cuts or rejects frequencies that are not within a certain range, as the name implies. Besides this, it also gives easy passage to the frequencies to pass which are not in the range. These types of filters are often termed as ‘Band Elimination Filters’.

How to design a Band Reject Filter?

To make a Band Stop/reject filter we always need a Low Pass Filter(LPF) & a High Pass Filter(HPF). Therefore we combine them and  make a ‘parallel’ connection with both the filters to create a band reject filter.

What does a Notch Filter do?

Notch Filter is also band reject filter. They can be used to fix frequency noise sources which are from the line frequency within a certain limit. Notch filter is also used to remove resonances from a system. Like a Low pass filter, notch filter creates less phase lag in a control loop.

Find out the differences between between a band reject filter & a notch filter?

A band reject filter or band stop filter is a filter that carries or passes the frequencies without altering and attenuates them in a specific range to low level. This is the opposite of a band pass filter.

On other hand, a notch filter is a band stop filter which has a narrow stop band and has good high ‘Quality factor’(Q-factor).

What is Ideal Filter & Real Filter?

Sometimes, for the reason of simplification, we often use the active filters to approximate ways. We upgrade them into an ideal and theoretical model, which is called ‘Ideal Filter.’

The use of these standards is insufficient, leading to errors; then, the filter should be treated based on accurate real behaviour, For example, the Real filters.

The characteristics of an ideal filter are:

  • The response transits between zones in a sudden way.
  • It does not create any distortion when the signal passes through the transit zone.
  • The pass of the signal causes no loss.

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About Soumali Bhattacharya

I am currently invested in the field of Electronics and communication.
My articles are focused towards the major areas of core electronics in a very simple yet informative approach.
I am a vivid learner and try to keep myself updated with all latest technologies in the field of Electronics domains.

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