Band Reject Filter OR Band Elimination Filter

 

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.

Range of frequencies
f < fL	fL < f < fH	f > fH
Allowed to pass	Not allowed to pass	Allowed to pass

 

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

• The frequency response of the wide band reject filter is shown in the Figure.

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

• Low figure of merit

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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