Figure 1.  Circuit Diagram for a High-Pass Filter

This is a circuit that only allows input signals with high frequencies to pass to the output, hence the name 'high-pass filter'.  All signals with low frequencies are attenuated (weakened or decreased in level) by this circuit.

The circuit is an active filter that uses a 741 operational amplifier IC, with R1=R2=R and C1=C2=C.  The reactance of a capacitor increases as the frequency of the signal through it decreases. Thus, in the circuit above, C1 and C2 tend to resist or block low-frequency signals, causing them to be attenuated at the output.  On the other hand, the higher the frequency of the input signal, the lower are the reactances of C1 and C2, and the closer the circuit resembles a non-inverting amplifier.  The gain G of this amplifier is approximately given by: G = R3/R4.

The cut-off frequency fc of the high-pass filter circuit in Figure 1 is approximately given by fc = 1/(2πRC). At fc, the output is 0.707 times the maximum output level.  The output level increases as the input frequency increases farther above fc, which is why this is a high-pass filter. 

See Also:   Op Amp Low Pass Filter;  Passive High-Pass Filters

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