What are Filters

In terms of electronics, filters are circuits that allow a desired band of frequencies to pass through while attenuating the rest of them when an electronic signal is applied at their input terminals. Basically filters are of four types:-

Low Pass filter: – allows a band of frequencies ranging from zero to f to pass through without any attenuation where f is the cut off frequency. High pass filter: – allows a band of frequencies ranging from f to infinity to pass through without attenuation, f being the cut off frequency again. Band Pass filter: – allows a band of frequencies ranging from f1 to f2 to pass through without attenuation where f1 is the lower cut off while f2 is the higher cut off frequency. Band Stop filter: – attenuates a band of frequencies ranging from f1 to f2 while allowing the rest of the frequencies to pass through.

Filters are of passive and active types. Passive filters consist of electrical elements like resistors, inductors, capacitors etc. combined in a circuit. These utilize the frequency selective properties of inductors and capacitors for the purpose of filtering.We know that the impedance of an inductor increases with increase in frequency of the signal while that of a capacitor decreases. Thus by using these elements in a circuit arrangement, frequency selective attenuation can be obtained. The cut-off values of the filter circuit depend upon the value of the circuit elements being used. Also the capacitive filters are preferred over the inductive filters because of the inductors being costlier and much more bulky.

In active filters, the passive electrical elements are accompanied by an IC circuit for the purpose of signal processing. These filters are particularly useful in low power signal applications where amplification is needed and heat losses in the resistors of a passive filter are not affordable by the system. The major IC that is used in these filters is IC-741 operation amplifier.

On either side of the cut off frequency, the amplitude of the signal at the output end rolls to zero with a slope instead of a rapid descend. This slope decides the order of the filter. For first order filter it is-20 db/decade, for second order it is -40 db/decade and so on.

Filters are utilized in many communication applications for the purpose of tuning to a particular set of frequencies. These are also commonly used in clipping and clamping circuits, dc power supplies, digital to analog conversion circuits etc. An efficient filtering system is the foremost requirement for many of these systems.

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