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An electric filter is a network designed to attenuate certain frequencies but pass others without attenuation. A filter circuit, therefore, possesses at least one pass band — a band of frequencies in which the output is approximately equal to the input (that is, attenuation is zero) and an attenuation band in which output is zero (that is, attenuation is infinite). The frequencies that separate the different pass and attenuation bands are called the cut-off frequencies.

Filters may be of any type such as electrical, mechanical, pneumatic, hydraulic, acous­tical etc. but the most commonly used filters are of the electrical type.

Electrical filters are used in practically all circuits which require separation of signals according to their frequencies. Applications include (but are certainly not limited to) noise rejection and signal separation in industrial and measurement circuits, feedback of phase and amplitude control in servo-loops, smoothing of digitally generated analog (D-A) signals, audio-signal shaping and sound enhancement, channel separation, and signal enhancement in communication circuits.

Classification of Filters

An electric filter .is usually a frequency-selective network that passes a specified band of frequencies and blocks or attenuates signals of frequencies outside this band. Filters may classified in a number of ways as follows :

Analog or digital  filters.

Passive or active  filters.

Audio-frequency (AF) or  radio-frequency (RF) filters.

Depending on the type of techniques used in the process of analog signals the filters may be analog or digital. Analog filters are designed to process analog signal using analog tech­niques, while digital filters process analog signals using digital techniques.

Depending on the type of elements used in their construction, filters may be passive or active. A passive filter is built with passive components such as resistors, capacitors and inductors. Active filters, on the other hand, make use of transistors or op-amps (providing voltage amplification, and signal isolation or buffering) in addition to resistors and capaci­tors. The type of elements used dictates the operating frequency range of the filter..

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

(i)                   low-pass,

(ii)                  high-pass

(iii)                 band-pass

(iv)                band stop and

(v)                  all pass.

The filter circuit may be so designed that some frequencies are passed from the input to the out­put of the filter with very little attenuation while others are greatly attenuated


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