What is a Filter?

A circuit’s ability to selectively filter a single frequency or range of frequencies from a mix of other frequencies is sometimes desirable. A filter, or simply a filter, is a type of circuit designed to select frequencies.

In high-performance stereo systems, where certain ranges of audio frequencies must be amplified or suppressed for optimal sound quality and power efficiency, filter circuits are frequently required.

You might be know about balancers, which permit the amplitudes of a few recurrence reaches to be changed in accordance with suit the audience’s taste and acoustic properties of the listening region.

You may also be familiar with crossover networks, which prevent speakers from hearing certain frequency ranges.

A crossover circuit is connected between the tweeter and the stereo’s output terminals to block low-frequency signals while only passing high-frequency signals to the speaker’s connection terminals because a tweeter (high-frequency speaker) is ineffective at reproducing low-frequency signals like drum beats.

This improves performance and efficiency of the audio system. Equalizers and crossover networks are two examples of filters that are intended to filter particular frequencies.

The “conditioning” of voltage waveforms that are not sinusoidal in power circuits is yet another practical application of filter circuits.

Power conditioning is necessary for certain electronic devices because they are sensitive to harmonics in the power supply voltage.

A filter circuit that only allows the fundamental waveform frequency to pass through and blocks all (higher-frequency) harmonics should be feasible if a distorted sine wave voltage behaves like a series of harmonic waveforms added to the fundamental frequency.

This lesson will focus on the layout of a few basic filter circuits. I will extensively utilize SPICE as an analysis tool, displaying Bode plots (amplitude versus frequency) for the various kinds of filters, to alleviate the reader’s math burden.

However, if the student is willing to devote a significant amount of time to reworking circuit calculations for each frequency, these circuits can be analyzed at multiple frequency points using repeated series-parallel analysis, similar to the previous example with two sources (60 and 90 Hz).


In mixed-frequency signals, a filter is an AC circuit that separates some frequencies from others.
Filter circuits are widely used in crossover networks and audio equalizers.
A Bode plot is a graph that shows the frequency and amplitude or phase of a waveform on one axis.

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