Engineering Principles V11

Clive W. Humphris

PASSIVE FILTERS: RC Low Pass Passive Filter.  

Typical examples of filter circuits are used for AC smoothing in a power supply , separating the frequencies within a loudspeaker enclosure, i.e bass treble and mid range or the signal processing in a radio or television receiver. Filters act as an electronic gateway by allowing some frequencies to pass whilst others are rejected.

A low pass filter has a pass band situated below fc which is the cut-off frequency. Frequencies above this point being gradually attenuated. At fc the reactance of C will equal the series resistance, thus forming a voltage divider network. At frequencies below fc the reactance of C will be high and a greater part of the input will be available as output.

For frequencies greater than fc, Xc falls relative to R thereby reducing Vout. Filter response is expressed in decibels as the output is relative to the input, using the formula for voltage.

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