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Basic High-Pass RC filters

high pass chart

A high-pass filter passes high frequencies and blocks low frequencies.

A High Pass Filter is the exact opposite to the low pass filter circuit as the two components have been interchanged with the filters output signal now being taken from across the resistor

Where as the low pass filter only allowed signals to pass below its cut-off frequency point, ƒc, the passive high pass filter circuit only passes signals above the selected cut-off point, ƒc eliminating any low frequency signals from the waveform.

High-pass filters have many applications such as blocking DC from circuitry sensitive to non-zero average voltages or radio frequency devices. They are used as part of an audio crossover to direct high frequencies to a tweeter while attenuating bass signals which could interfere with, or damage, the speaker. When such a filter is built into a loudspeaker cabinet it is normally a passive filter that also includes a low-pass filter for the woofer and so often has both a capacitor and inductor.

Passive filters are made up of passive components such as resistors, capacitors and inductors and have no amplifying elements (transistors, op-amps, etc) so have no signal gain, therefore their output level is always less than the input.

High-pass and low-pass filters are also used in digital image processing to perform image modifications, enhancements, noise reduction, etc., using designs done in either the spatial domain or the frequency domain.[6] The unsharp masking, or sharpening, operation used in image editing software is a high-boost filter, a generalization of high-pass.

To create a passive high-pass filter, we need a circuit that consists of a resistor and either a capacitor or an inductor

  • The cutoff frequency of a high-pass filter indicates the frequency region in which the filter is transitioning from high attenuation to low attenuation.

The upper cutoff frequency for an RC high pass filter is determined by the circuit resistor and capacitor values. By using the Formula,

high pass chart

ƒc = Cutoff frequency

R = Resistance in Ohms

C= Capacitance in Farads

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