The above circuit diagram illustrates a simple

You can use the buttons provided in the above experiment to alter the values of the applied frequency and the circuit components. You should find that the circuit tends to 'smooth over' quick changes, but passes low-frequency waves almost unaffected.

Of course, the meanings of 'low' and 'high' frequencies are relative. In this case they depend upon the filter's

The action of the circuit can also be described in terms of a related quantity, the

The circuit's behaviour can be understood as arising due to the finite time taken to change the capacitor's charge when we alter the applied input voltage. This process is illustrated in the diagram below.

A quick change in input voltage initially leaves the capacitor voltage unaffected. Hence it produces a voltage difference across the resistor, and so a current flows. This current tends to charge up the capacitor, moving its voltage towards that at the input end of the resistor. As the two voltages move closer the current falls and the rate of change reduces. The overall effect is an output voltage which moves in an exponential curve towards the input level. The rate of change of voltage is determined by the exponential factor

The experiment on this page shows what happens when we apply a square-wave. This keeps 'changing its mind' about the input voltage and never gives the circuit a chance to reach a steady level and settle down. Experiment with changing the frequency and the component values and see what happens to the size and shape of the output waveform. Then choose values for the resistor and capacitor (i.e. fix on a value for the time constant) and plot a graph of how Vpk(out)/Vpk(in) varies with the half-cycle time,

If you examine the output waveform when

using HTMLEdit3 on a StrongARM powered RISCOS machine.

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