What is a Filter ? Describe The Classification of Filters

What Is A Filter

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An electrical filter is a circuit which can be designed to modify, reshape or reject all the undesired frequencies of an electrical signal and  pass only the desired signals.

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In other words we can say that an electrical filter is usually a frequency selective network that passes a specified band of frequencies and blocks signals of frequencies outside this band.

Classification of Filters

  • Depending on the type of element used in their construction, filters  are  classified into two types, such as:
  1. Passive Filters : A passive filter is built with passive components such as resistors, capacitors and inductors.
  2. Active Filters : An active filter makes use of active elements such as transistors, op-amps in addition to resistor and capacitors.
  • According to the operating frequency range, the filters may be classified as audio frequency (AF) or radio frequency (RF) filters.
  • Filters may also be classified as :
  1. Low Pass Filter : The low pass filter only allows low frequency signals from 0 Hz to its cut-off frequency, ƒc point to pass while blocking  any higher frequency signals.
  2. High Pass Filter : The high pass filter only allows high frequency signals from its cut-off frequency, ƒc point and higher to infinity to pass through while blocking those any lower.
  3. Band Pass Filter : The band pass filter allows signals falling within a certain frequency band set up between two points to pass through while blocking both the lower and higher frequencies either side of this frequency band.
  4. Band Stop Filter : The band stop filter blocks signals falling within a certain frequency band set up between two points while allowing both the lower and higher frequencies either side of this frequency band.

Fig. below shows the frequency responses of the four  types of filters mentioned above. These are ideal responses and can not be achieved in actual practice.

(a) Ideal Low Pass Filter

A filter that provides a constant output from d.c. upto a cutoff frequency fc and then passes no signal above that frequency is called an ideal low pass filter.

Frequency Response of Ideal low pass filter

The ideal response of a low pass filter is shown in fig. above.

The voltage gain i.e. the ratio of output voltage to input voltage is constant over a frequency range from zero to cutoff frequency fc.

Hence,the output will be available faithfully from 0 to fc with constant gain.

The frequencies between 0 and fc , are called passband frequencies, while the frequencies above fc are called as stopband frequencies.

Therefore the bandwidth is fc.

Ideal High Pass Filter

A filter that passes signals above a cutoff frequency fc is a high pass filter.

Frequency Response of ideal high pass filter

The frequency response of an ideal high pass filter is shown in fig. above.

The high pass filter has a zero gain starting from zero to a frequency fc, called the cutoff frequency, and above this frequency, the gain is constant.

Hence, signal of any frequency beyond fc is faithfully reproduced with a constant gain, and frequencies from 0 to fc will be blocked.

Ideal Band Pass Filter

When the filter circuit passes signals that are above one cutoff frequency and below a second cutoff frequency, it is called a band pass filter.

Frequency Response of ideal band pass filterThe frequency response of an ideal band pass filter is shown in fig. above.

As we can see from the above fig.,  the band pass filter has a pass band between two cutoff frequencies fc2 and fc1, where fc2 > fc1 and two stop bands : 0<f<fc1 and f>fc2.

The bandwidth of the band pass filter is therefore, equal to fc2-fc1, where fc1 and fc2 are lower and higher cutoff frequencies respectively.

Ideal Band Stop Filter

The band stop or band reject filter performs exactly opposite to the band pass filter.

Frequency Response of ideal band stop filter

It has a band stop between two cut off frequencies fc2 and fc1 and two pass bands : 0<f<fc1 and f>fc2.

The frequency response of an ideal band stop filter is shown in fig. above.

This is also called as band elimination or notch filter.

In our next articles we will study about each type of passive and active filters in detail.