What is Active Filter


An active filter is an electronic circuit that uses active components such as operational amplifiers (op-amps) to achieve a desired frequency response. Active filters offer better frequency response, accuracy, and versatility compared to passive filters. Active filters are commonly used in applications where passive filters are not suitable due to their limited frequency response or low Q-factor.

Characteristics of an Active Filter:

  • Provides gain or amplification to the filtered signal.
  • Low output impedance, allowing for easier connection to other circuits.
  • The cutoff frequency and Q-factor can be adjusted by changing the values of the components or through active components such as op-amps.
  • Active filters can have a steeper slope and better accuracy than passive filters.
  • Active filters require a power supply to operate.

Construction of Active filter:

Active filters are constructed using op-amps, capacitors, and resistors, and sometimes inductors. The op-amps in an active filter are configured as a voltage follower, amplifier, or integrator, depending on the type of filter and its design. The use of op-amps in an active filter allows the filter to achieve high input impedance and low output impedance, which results in a high Q-factor and better attenuation of specific frequencies.

Working of Active filter:

Active filters work by using op-amps to amplify, attenuate, or phase shift the input signal at specific frequencies. The op-amps in an active filter provide gain or attenuation to the input signal depending on the configuration of the op-amp. The filter circuit’s output is then fed back to the input through a feedback loop, which results in the desired frequency response.

Types of Active filter:

Active filters can be broadly classified into four types based on their frequency response and topology:

  1. Low-pass filters: Allow low-frequency signals to pass through while attenuating high-frequency signals.
  2. High-pass filters: Allow high-frequency signals to pass through while attenuating low-frequency signals.
  3. Band-pass filters: Allow signals within a specific frequency range to pass through while attenuating signals outside that range.
  4. Band-stop filters (notch filters): Attenuate signals within a specific frequency range while allowing signals outside that range to pass through.


Active filters have a wide range of applications in electronic circuits. Some of the common applications of active filters are:

  • Audio signal processing in music systems and amplifiers
  • Power supply filtering to remove noise and ripple
  • RF and microwave communication systems
  • Image processing and video filtering in multimedia applications
  • Medical signal processing for heart and brain monitoring
  • Industrial automation and control systems
  • Instrumentation and measurement systems to remove noise and interference.

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