Typical examples of filter circuits are used for AC smoothing in a power supply, separating the frequencies within a loudspeaker enclosure, ie, 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. TEST YOUR UNDERSTANDING: Passive filters only comprise capacitors, resistors and inductors they have no active circuitry to boost or reduce gain or loss. The function of each type of filter is in its name. As with our other AC signal circuits find suitable values either online or using standard electronics text books and explore each filter by inputting values from this source. Only changing one value at a time will enable you to see its effect.
