Figure 1.  Circuit Diagram for a Low-Pass Filter

This is a circuit that only allows input signals with low frequencies to pass to the output, hence the name 'low-pass filter'.  All signals with high 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 decreases as the frequency of the signal through it increases. Thus, in the circuit above, C2 tends to shunt high-frequency signals towards the ground, causing them to be attenuated at the output.  On the other hand, the lower the frequency of the input signal, the higher 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 low-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 falls farther below fc, which is why this is a low-pass filter. 

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

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