Electrical Revolution

26/07/2021

Band Reject Filter ( Band Elimination Filter ) ( Band Stop Filter )

It is a filter in which frequencies below lower cut off frequencies and above higher cut off frequencies are passes whereas stop band frequencies are attenuated.

Classifications of Band Reject Filter

There are two types of band reject filter

( 1 ) Wide band reject filter

( 2 ) Narrow band reject filter

Wide band reject filter

The wide band reject filter is made by using low pass filter, high pass filter and summing amplifier.
The lower cut off frequency ( f_L ) of the high pass filter must be larger than the higher cut off frequency of the low pass filter.
The passband of the low pass filter and high pass filter must be equal.

Frequency response

Narrow band reject filter ( Notch filter )

It is also called as notch filter.
The Q factor of the narrow band reject filter is always greater than 10.
As the Q factor is greater than 10 , its bandwidth is much smaller than that of wide band reject filter.

Q > 10

There are two types of notch filter

( 1 ) Passive notch filter ( Twin T notch filter )

( 2 ) Active notch filter

Twin T notch filter ( Passive notch filter )

It consists of two T shaped networks made of resistors and capacitors.
One T network is made of 2 resistors and one capacitor whereas other T network is made of two capacitors and one resistors.
The notch frequency ( attenuation frequency ) is given by

f = 1 / 2πRC

Disadvantages of Twin T notch filter

Active notch filter

The figure of merit of the notch filter can be increased by using active notch filter as shown in the figure.
It is simply voltage follower circuit.
The output voltage of the OP – Amp is fed back by R/2 and 2C circuit.

Applications of band reject filter

This type of filter is used for rejection of single frequency, 50 Hz / 60 Hz power frequency hum.
To eliminate undesirable frequencies in the communications and biomedical instruments

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