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What is filter?

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Filter

 Introduction 

 A Filter is an electrical circuit that is designed to pass a specified band of frequencies while attenuating all the signals outside that band. It is a frequency selective circuit. The filters are basically classified as active filters & passive filters. They are used in circuits which require the separation of signals according to their frequencies. They are widely use in communication & signal processing.

 ADVANTAGES OF ACTIVE FILTERS OVER PASSIVE FILTERS

All the elements along with op-amp can be used in the integrated form. Hence there is reduction in size & weight.• The op-amp gain can be easily controlled in the closed loop fashion hence active filter I/P signals is not attenuated.• The op-amp has high I/P impedance & low O/P impedance, hence the active filters using op-amp do not cause loading of the source or load.

 LIMITATIONS OF ACTIVE FILTERS

The finite bandwidth of the active devices places a limit on the highest frequency of operation.• A reasonably good filter performance can be achieved approximately up-to 500 kHz, as against this passive filters can be used up to 500 MHz• The active elements are more sensitive to the temperature & environmental changes than the passive elements.• The requirement of d.c power supply is another disadvantage of the active filters.

 COMMONLY USED FILTERS

 The most commonly used filters are:-• Low Pass Filter• High Pass Filter• Band Pass Filter• Band Reject Filter• All Pass Filter &• Universal Filters.

FREQUENCY RESPONSE OF LOW 

                A Low Pass Filter has a PASS FILTER constant gain from 0 Hz to a IDEAL & PRACTICAL high cut-off frequency, Fh. RESPONSE • Practically, the gain decreases as the frequency increases & at f=Fh, the gain is down by 3 db & after Fh, it decreases at a higher rate. • After the end of a transition band, the gain becomes zero.

 FREQUENCY RESPONSE OF HIGH PASS FILTER 

For a High pass Filter, Fl is the low cut-off frequency. • The range of frequency 0<f<Fl is the stop band, while f>Fl is the pass band. • The transition band is not shown in the characteristics as it is very small.


 FREQUENCY RESPONSE OF BAND PASS FILTER 

The band pass filter has two stop bands. • The range of frequency 0<f<Fl & range of frequency Fh<f<infinity are two stop bands while the range Fl<f<Fh is the pass band. • The bandwidth is thus Fh- Fl.

 FREQUENCY RESPONSE OF  BAND STOP FILTER 

The band elimination filter is also called band rejection filter or band stop filter. • Its characteristics is exactly opposite to that of band pass filter. • There are two pass bands while one stop band. • The two ranges are 0<f<Fl & Fh<f<infinity.


 BAND PASS FILTER 

There are two types of band pass filters which are classified as per Quality factor: 1. Narrow band pass filter (Q>10) 2. Wide band pass filter (Q<10) BW = f2 – f1 Q = f0 / BW Av R1 C2 C1 R2 R fb Stage 2 Stage 1 R1 response R2 response BW Ri + Vin _ + VO _ f f1 fo f2

 BAND REJECT FILTER

                This can also be either as a Narrow or wide band reject filter. The narrow band reject filter is commonly called a notch filter & is useful for the rejection of a single frequency, such as 50 Hz power line frequency hum. Av (dB) low-pass high-pass C1 {-3dB R1 R1  Rfb R2 Ri + C2 + Vin VO _ _ f f1 f2

 ALL PASS FILTER

An all-pass filter passes all frequency components of the i/p signal without any attenuation & provides desired phase shifts at different frequencies of the I/P signal.• When signals are transmitted over transmission lines, such as telephone wires, they undergo change in phase. These phase changes can be compensated by all-pass filters.• Thus, they are also called as delay equalizers or phase corrector.

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