What is Active Band Stop Filter


An active band stop filter, also known as a notch filter, is an electronic circuit that attenuates a specific range of frequencies while allowing frequencies outside the range to pass through. The filter achieves this by using an active component such as an operational amplifier (op-amp) in combination with passive components such as resistors and capacitors. The active band stop filter finds application in various electronic circuits such as audio signal processing, radio communication systems, and instrumentation.

Characteristics of Active Band Stop Filter

  • Attenuates a specific band of frequencies while allowing frequencies outside that band to pass through.
  • The center frequency and bandwidth can be adjusted by changing the values of the components or through active components such as op-amps.
  • Provides gain or amplification to the filtered signal.
  • Has a low output impedance.
  • Has a flat response outside the stopband and a roll-off response within the stopband.
  • Used in audio and communication systems to filter out unwanted frequencies.


The active band stop filter circuit consists of an op-amp, two feedback resistors, and two capacitors. The op-amp is configured in a non-inverting or inverting amplifier mode, and the feedback resistors and capacitors are connected in a notch filter configuration between the op-amp input and output. The input signal is applied to the inverting or non-inverting input of the op-amp, and the output signal is taken from the op-amp output.


In an active band stop filter, the two capacitors act as frequency-dependent impedances that form a notch filter. The two feedback resistors set the gain and Q-factor of the filter. The op-amp provides gain to the input signal and compensates for the signal loss due to the filter. The frequency range that the filter attenuates is determined by the values of the two capacitors and the two feedback resistors.


Some of the applications of active band stop filters are:

  • Audio signal processing in music systems and amplifiers
  • Radio communication systems for filtering out specific frequencies
  • Instrumentation and measurement systems
  • Seismic data acquisition and filtering
  • Medical signal processing and filtering

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