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Designing of Analog Active Filters

Written By: 

A. M. Bhatt


What are the filters?­­

We all know in communication field, filter plays a vital role as it is needed in most of the applications. Its main function is to remove unwanted frequency components from information signal. It’s also used to reduce noise effect produce by different sources in information signal. Whenever any information signal is transmitted into the channel (wire or wireless), along with it lot of other unwanted frequency components are added due to channel noise and noise from other sources. So to extract the desired information signal there is a need to filter it out using such filter circuits.  

There are different types of filter for different kind of applications. So let us see different types of filters first and then we shall see the examples of such types of filters.    


Types of filters: -

Filters are broadly classified in to active filter and passive filter. Here is the difference between them.

Active filters

Passive filters

Filtering components are resistance and capacitance only. Doesn’t include inductors

It uses inductors, capacitors and resistors as filtering components

Active components like op-amp, FET, transistors etc. are used

Only passive components are used

It requires basing voltage

Doesn’t require any basing voltage

Its bandwidth is limited due to active component

It doesn’t have any bandwidth limitation

Gain is limited to the gain of active components used

No such limitation on gain

Small in size due to resistors and capacitors are used

Bulky in size because of inductor is used

Due to active components like op-amp and FET input impedance is high

Input impedance is comparatively low

Output impedance is low

Output impedance is medium or high

Basically used to suppress unwanted frequency components from information signals

Mostly used in power supplies to reduce power supply variation problems and spike noise problems.

Passive filters don’t have sharp cut off and gain adjustment such as active filters. That’s why they are not used in communication applications but they are used as a filter to produce pulsating DC output from rectified DC input in DC power supplies.


Further, we may classify filter based on the type of signal they process and the components they use.

A) Analog filters: - They process analog signals and they use analog components like resistors, capacitors, inductors, transistors, op-amps etc

B) Digital filters: - They process discrete signals and they use digital components like computer, micro-processor, micro-controller etc

The main function of filters is to filter out required frequency components from mix frequency signal. They allow range of frequency to pass that is known as pass bend and rejects (or suppresses) other frequencies known as stop bend. The cut off frequency is the parameter that separates out these two bends. So depending upon these pass band and stop band there are four types of filters.

  1. Low pass filter (LPF) : - It allows to pass all the frequencies lower than its cut off frequency and stops all other frequencies
  2. High pass filter (HPF) : - it allows to pass all the frequencies higher than its cut off frequency and stops all other frequencies
  3. Band pass filter (BPF): - it allows to pass band of frequencies between its higher cut off and lower cut off frequencies.
  4. Band reject filter (BRF): - it stops band of frequencies between its higher cut off and lower cut off frequencies.


analog filter

Fig. 1: Frequency Response of different Analog Active Filters


  1. Band pass filter can be further divided in

·         Wide band pass filter - it has wider bend and low quality factor Q (Q < 1)

·         Narrow band pass filter - it has very sharp bend and very high Q (Q > 10)


Band reject filter is also divided in

·         Wide band reject filter - it rejects wide range of frequencies

·         Narrow band reject filter - it also known as notch filter, and used to reject specific frequency


Here I have discussed the designing of analog and active filters only. The main components in active filters are op-amp, resistors and capacitors. The designing of filters is on the basis of required cut off frequency. In LPF and HPF single cut off frequency is given while in BPF and BRF there are 2 cut off frequencies. Designing is based on equation, some assumptions and some criteria that are derived from experiments and results of experiments. 


So I have given step by step procedure to design all above filters that includes such assumptions and criteria.

A.      Designing of active low pass filter

B.      Designing of active high pass filter

C.      Designing of active band pass filter

D.      Designing of active band reject filter