Filters Using Op-Amps | Band Pass, Band Stop, High Pass & Low Pass Filter

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A filter is often used in electronic circuits to block (or allow) a select frequency to the circuit. For example, if the circuit in question works on DC power, we could use a low pass filter (LPF) and only allow low frequencies to pass through. The filter is designed around a cut off frequency, and only allows the low frequencies to pass through. Now, while designing we can set the cut off frequency as required. The same can be done for high pass, band pass and band stop filters. Let’s look at each of them in detail.

Low pass filter

first order active low pass filter circuit
Figure: First-order Active Low Pass Filter Circuit

A LPF is used in circuits that only allow low frequencies to pass through. It is often used to block high frequencies and AC current in a circuit. Given below is a sample LPF circuit using op-amp.Ideally, the frequency output of a LPF is like this,low pass filter outputBut this is not the case with real circuits. As there is a slight attenuation.first order active low pass filter outputThis attenuation can be further minimised by adding multiple stages as below.

second order low pass filter

 

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LPF is popular with speakers to block high pitches, some electric guitars and radio transmitters.

High pass filter

A high pass filter is used in circuits that only require high frequencies to operate. It blocks most low frequencies & DC component. Given below is a sample high pass filter circuit using op-amp.

high pass filter circuit
Figure: Circuit for High pass filter

Ideally, the frequency output of a high pass filter is like this,

high pass filter output
Figure: Frequency Output of a High Pass Filter

But this is not the case with practical circuits. As there is a slight attenuation.

high pass filter output

This attenuation can be further minimised by adding multiple stages as with LPF.

Band pass filter

A band pass filter is a combination of a high pass and a LPF. It allows only a select range of frequencies to pass through. It is designed such a way that the cut off frequency of the LPF is higher than the cut off frequency of the high pass filter, hence allowing only a select range of the frequencies to pass through. Presented here is a sample band pass filter circuit using op-amp.

band pass filter circuit
Figure: Circuit for Band Pass Filter

Ideally, frequency output of a band pass filter is like this,

Frequency Output of a Band Pass Filter
Figure: Frequency Output of a Band Pass Filter

But this is not the case with practical circuits. As there is a slight attenuation.

band pass filter output

Band pass filter is popularly with speakers to block high pitches, some electric guitars and radio transmitters.

Band stop filter

A band stop filter also known as notch filter is used in circuits that block only a select range of frequencies and allows others to pass through. It would be an inverse of the band pass filter, and can be created by using the same input at a high pass and a LPF.

band stop filter Circuit
Figure: Circuit for Band stop Filter

Note: All graphs are plotted in the frequency domain


This article was first published on 17 November 2017 and was recently updated on 22 December 2018.

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1 COMMENT

  1. I think that where you say: “This attenuation can be further minimised by adding multiple stages as with LPF.” that you mean “maximized” as in more attenuation. The signal would be minimized at a higher attenuation.

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