Filter definition | Common types of filter | Important Use


  • In this article, we will discuss about few basic concept related to filter and try to answer few questions as follows :
  • What is a filter ? filter definition
  • How does a filter work?
  • What are the types available ?
  • What are the Active Filters? Explain with definition
  • What are the Passive Filters? Explain with definition
  • What are the points of conflict between active and passive types.
  • What is the filter symbol?
  • Examples
  • What are the Applications of filter in optics and electronics industry ?
  • What is the Response curves for most common types?
  • What is Time response of a filter ?
  • What is Frequency response of a filter?
  • What is Order of a filter?
  • How first-order output differs with A second-order filter ?
  • What is Corner frequency, Cut off frequency, break frequency
  • What is Bandwidth (BW) ?
  • What is Resonant Frequency?
  • What is Resonant Filters
  • What is ideal and real filters ?

What is a Filter?

A filter is a frequency selective network consisting of four terminals with reactive elements to transmit a specified range frequency.

  • The band of frequency transmitted through it is called Pass-band.
  • The band of frequency, which gets attenuated by it, is called attenuated on Stop-band.

Filters are of two types- analogue digital. Now, based on the components used, They are of two types – Active types and Passive Types.

The below image represents a diagram of active types(one of the very popular and important types).

Filter Circuit : active filter
Filter Circuit : active type

Know about Active High Pass Types. Click Here!

What are the main characteristics of an active filter?

As the titles suggest, these types are made using active components. Some of the active transistors are – transistors (BJT, FETs), any other electronics devices which are capable of amplifying a signal or can produce powers.

If there is a need to increase the characteristics, various stages are joined in a certain or specific ways.

How to design an active low pass filter?

To design an active types, we may use IC741, an Op-amp, configured with 8 pins. The op-amp is to be supplied with DC power along with resistors and capacitors of different values.

Passive Filters

Passive types are designed using passive devices.

Know about Applications of Active High Pass Types. Click Here!

Comparison of Active and passive types

An active type may have an advantage, increasing the signal power available in comparison with the input. Whereas a passive one dissipates energy from the signal. For various ranges of frequencies, such as at sound frequencies and under, an active type may realize a specified transfer function with no use inductors, that are comparatively big and costly components in contrast to resistors and capacitors, and that are more costly to create with the essential high quality and precise values.

Numerous stages may be cascaded when wanted to enhance attributes. By comparison, the layout of multiple-stage passive blockers needs to take into consideration every phase’s frequency-dependent loading of the previous stage. Since inductors aren’t utilized, they can be reached in really compact dimensions.

  • Passive type suffers from attenuation of signals. There are various ways; one popular method of control or restoration is by using amplification through the Active type applications. The major point of conflict between the active and passive types is the ‘amplification’.
  • Compared to a passive one, active type are composed of active components in operational amplifiers, transistors, or FET’s within their circuit design, as described in earlier sections. These components draw power from the external power source, use it for amplification of output. That’s an added advantage compared to a passive one.

Know about Active Low Pass Types. Click Here!

Why is an active filter needed at low frequencies?

  • An active type is needed at lower frequencies because it helps achieve low output impedance while providing high input impedance. It also stabilizes different frequency ranges as multiple stages can be cascaded with it.

Difference between Active & Passive types

Know about Applications of Active Low Pass Types. Click Here!

These can be categorized and sub-categorized from several points of view. The most common divisions and sub-divisions are- active or passive type; high-pass type low-pass type, bandpass type, band-reject/notch type or all-pass type; digital or analog type discrete-time or continuous-time type; linear or non-linear type; infinite impulse response (IIR) or finite impulse response (FIR) and so on.


Active types and Passive types are designed to modify certain band of frequency in a desired way. They have different types according to their needs. The categories are given below.

  • Low pass types (LPF)
  • High pass types (HPF)
  • A bandpass types (BPF)
  • Band reject/stop types (BSF)


They are nowadays used in many purposes of the electronic circuit, and its applications are immense. Moreover, it’s possible to improve the circuit gain by using different filters in different ways, either active or passive types, especially in active types. Active types use amplifiers, and we know that it helps increase gain. This article will discuss two type, such as Low pass type, High pass type with appropriate diagrams and simulated wave shapes for both active and passive condition in the following sections with the importance of using higher-order in the HPF and LPF.

In electronics, some applications are as follows:

  • In Radio communication System for Radio tuning to a specific frequency: They are used to enable radio receivers to only “see” or “detect” the desired signal and reject all the other signals by assuming their different signal frequency. So noise-free signals can be received. The high-frequency bandpass types are used for channel selection in central telephone offices.
  • Power Supply Design: They are used to remove noise or high frequencies usually present on AC input lines. These are also applied to decrease the ripple.
  • Analog-to-digital conversion (ADC): They are utilized in most of the ADC input to minimize aliasing.
  • Modify digital images: It can be used to modify digital images also.
  • Data analysis: They are also very helpful to remove specific frequencies in data analysis.

Frequency Response & Time Response:

Time-domain refers to the change of signal’s amplitude with respect to time. In contrast, in the frequency domain, Frequency refers to the occurrence of an event in a given period.

A Frequency Response of different type
Image credit: SpinningSpark real life identity: SHA-1 commitment ba62ca25da3fee2f8f36c101994f571c151abee7, Bandform templateCC BY-SA 3.0

What is Bandwidth (BW)?

For filters, bandwidth is the difference between the upper and low -3dB points.

For example, if a bandpass filter has -3dB cut-off points and set to 200Hz and 600Hz, then the filter bandwidth will be = (BW) = 600-200 = 400Hz.

What do you mean by the Q Factor?

Q factor is given by the ratio of resonant frequency to the BW.

Q = 2 * π * (Maximum Amount of Energy Stored) / (Energy Dissipated per Cycle)

A greater Q value represents the filter is more selective as Q factor is a parameter which judges the selectivity.

Resonant Frequency

Resonant frequency is simply given by the frequency of the given resonant circuitry. A Resonant circuit is also popularly known as the tank circuit or the LC circuit. A resonant circuit is designed using parallelly placed inductors and capacitors and resistors.

A parallel Resonant Circuit

 Oscillation of a system is given by the following equation –


f= frequency in Hertz

L= Inductance in Henry

C= Capacitance in Farads

Orders of Filters

Higher-order filters provided more excellent roll-off rates between passband and stopband. Higher-order filters are also required to achieve required levels of attenuation or sharpness of cut-offs.

Active type and Passive type also have variation in different types of orders, such as:

  • First Order Low pass active types, First Order High pass active types, First-order Bandpass active types, First-order Band stops active types.
  • Second order Low pass active types, Second order High pass active types, Second-order Bandpass active types, Second-order Band stops active types.

The frequency response of second-order are shown below –

Frequency response of Second-order

Ideal type & Real type:

Sometimes, for the reason of simplification, we often use the active filters to approximate ways. Later they are modified and termed as ‘ideal filter’. Filters, which operate in reality considering all the possible factors, are real ones.

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About Soumali Bhattacharya

I am currently invested in the field of Electronics and communication.
My articles are focused towards the major areas of core electronics in a very simple yet informative approach.
I am a vivid learner and try to keep myself updated with all latest technologies in the field of Electronics domains.

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