What is a Filter? Describe the classification of Filters.

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.

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

o    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, ƒ_c 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, ƒ_c 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 cannot be achieved in actual practice.

Ideal Low Pass Filter

A filter that provides a constant output from d.c. up to a cutoff frequency fc and then passes no signal above that frequency is called an 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 ƒ_c .

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

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

Therefore, the bandwidth is ƒ_c.

Ideal High Pass Filter

A filter that passes signals above a cutoff frequency ƒ_c   is a 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 ƒ_c  , called the cutoff frequency, and above this frequency, the gain is constant.

Hence, signal of any frequency beyond ƒ_c   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.

The 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 f_c2 and f_c1, where f_c2> f_c1 and two stop bands: 0<f< f_c1 and f> f_c2.

The bandwidth of the band pass filter is therefore, equal to f_c2 - f_c1, where f_c1 and f_c2 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.

It has a band stop between two cut off frequencies f_c2 and f_c1 and two pass bands: 0<f<f_c1 and f>f_c2.

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

This is also called as band elimination or notch filter.