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Filters Tutorial

A filter circuit is like a sieve. It allows some things through and holds back others. In this case we are talking about AC frequencies. Some frequencies pass through the filter while others are rejected.

Filter Circuit - Input Selected Frequencies out Diagram

The characteristics of a filter can be shown on a graph called a FREQUENCY RESPONSE CURVE.


VOLTAGE OUT is plotted against FREQUENCY.

Figure 2 shows a LOW PASS filter response curve giving output at low frequencies but none at higher frequencies.

Filters - Low Pass - High Pass - Band Pass - Band Stop Diagram

Figure 3 shows a selection of filter characteristics.

Simple filters can be made from capacitors and resistors.

Capacitors and Resistors - Low Pass Filter - Hight Pass Filter Diagram

Filters have many applications.

Block Diagram Symbols of Filters - A Filter - Low Pass - Hight Pass - Band Pass - band Stop - Interference Filter

In audio frequency amplifiers, CROSSOVER filters to direct low frequencies to the WOOFER and high frequencies to the TWEETER speakers.

In SCRATCH filters to remove unwanted high frequency noise.

In NOTCH filters to remove whistles due to two radio stations being too close together in frequency.

In Hum filters to remove low frequency noise due to the mains supply.

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