What is Passive Band Stop Filter


A passive bandstop filter, also known as a notch filter, is an electronic circuit that attenuates a specific range of frequencies while allowing all other frequencies to pass through. The filter achieves this by using only passive components such as resistors, capacitors, and inductors. The passive bandstop filter is widely used in various electronic circuits such as audio signal processing, power supplies, and communication systems.

Characteristics of Passive Band Stop Filter

  • Attenuates a specific band of frequencies while allowing frequencies outside that band to pass through.
  • The center frequency and bandwidth are determined by the values of the capacitors and inductors.
  • Does not provide gain or amplification to the filtered signal.
  • Has a high output impedance.
  • Has a deep notch within the stopband and flat response outside the stopband.
  • Used in audio and communication systems to filter out unwanted frequencies.


The passive bandstop filter circuit consists of two passive filter sections connected in parallel. The first section is a high-pass filter, and the second section is a low-pass filter. The input signal is applied across the parallel combination of the two filter sections, and the output signal is taken across the resistor or inductor in the middle of the circuit. The resistor, capacitor, and inductor values determine the frequency range that the filter attenuates.


In a passive bandstop filter, the high-pass filter section and the low-pass filter section work together to create a “notch” in the filter response at a specific frequency. This notch causes the input signal to be attenuated significantly at the specific frequency, while allowing all other frequencies to pass through with minimal attenuation. The notch frequency is determined by the product of the resistor and capacitor values in the high-pass and low-pass filter sections.


Some of the applications of passive bandstop filters are:

  • Audio signal processing in music systems and amplifiers
  • Power supplies to filter out a specific frequency noise
  • Communication systems to remove a specific frequency interference
  • Medical equipment to filter out specific frequencies of electrical noise
  • Signal conditioning in sensor circuits
  • Tone control circuits in guitar amplifiers
  • Filtering of unwanted noise in data acquisition systems.

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