  The above circuit diagram illustrates a simple 'RC' high-pass filter. This page provides a 'Java' experiment which you can use to explore its properties when the applied signal is a sinewave. This lets you discover the Frequency Response of the circuit.

The red line shows the input waveform and the blue line shows the output. 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 passes 'high' frequencies fairly well, but attenuates 'low' frequencies. Hence it is useful as a filter to block any unwanted low frequency components of a complex signal whilst passing higher frequencies frequencies. Circuits like this are used quite a lot in electronics as a 'D.C. Block' - i.e. to pass a.c. signals but prevent any D.C. voltages from getting through.

The basic quantities which describe this circuit are similar to those used for the Low Pass Filter. In effect, this circuit is just a simple low-pass filter with the components swapped over. The action of the circuit can also be described in terms of a related quantity, the Turn Over Frequency, f0, which has a value As with the low-pass filter, the circuit's behaviour can be understood as arising due to the time taken to change the capacitor's charge when we alter the applied input voltage. It always takes a finite (i.e. non-zero) time to change the amount of charge stored by the capacitor. Hence it takes time to change the potential difference across the capacitor. As a result, any sudden change in the input voltage produces a similar sudden change on the other side of the capacitor. This produces a voltage across the resistor and causes a current to flow thorough it, charging the capacitor until all the voltage falls across it instead of the resistor. The result is that steady (or slowly varying) voltages appear mostly across the capacitor and quick changes appear mostly across the resistor. Since we're using the voltage across the resistor as out output the main properties of the circuit are therefore

The Voltage Gain: The Phase Delay: Try using the above experimental system to collect results and plot a graph of how the voltage gain, Av, (and the phase change) depend upon the input frequency and check that your result agrees with the above formulae. Compare this with a low-pass filter that uses the same component values and you should see that they give 'opposite' results. Note that, in the high-pass filter, the output waveform 'leads' the input waveform - i.e. it peaks before the input. Once you are happy that this is correct try using the experimental system to choose circuit values that give a turn over frequency of around, say, 2kHz. Then, if you get a chance, try building a real circuit and see if it behaves like the computer experiment.

More generally, you can use this page to 'design' a high pass filter. However, remember that, like most computer experiments, it may not be perfectly accurate, so your real results may differ a little bit! Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.