What is Active Low Pass Filter


An active low pass filter is an electronic circuit that passes low-frequency signals with minimal attenuation and attenuates high-frequency signals. 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 low pass filter finds application in various electronic circuits such as audio signal processing, power supply filtering, and instrumentation.

Characteristics of Active Low Pass Filter:

  • Allows low-frequency signals to pass through while attenuating high-frequency signals.
  • The cutoff frequency 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 at low frequencies and a roll-off response at high frequencies.
  • Used in audio amplifiers, equalizers, and communication systems.


The active low pass filter circuit consists of an op-amp, a feedback resistor, and a capacitor. The op-amp is configured in a voltage follower or amplifier mode, and the feedback resistor and capacitor are connected in series between the op-amp output and the input. The input signal is applied to the non-inverting input of the op-amp, and the output signal is taken from the op-amp output.


In an active low pass filter, the capacitor acts as a frequency-dependent impedance. At low frequencies, the capacitor’s impedance is high, which results in the signal passing through with minimal attenuation. At high frequencies, the capacitor’s impedance decreases, which results in the signal being attenuated. The feedback resistor and capacitor form a low pass filter that determines the cut-off frequency of the filter. The op-amp provides gain to the input signal and compensates for the signal loss due to the filter.


Some of the applications of active low pass filters are:

  • Audio signal processing in music systems and amplifiers
  • Power supply filtering to remove noise and ripple
  • Signal conditioning in sensor applications
  • Instrumentation and measurement systems
  • Active crossover networks in speaker systems

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