In the figure we have a second order filter with very high input impedance, using an operational amplifier with a junction field effect transistor at the input. The cutoff frequency is given by the capacitors together with the resistors. If the reader wishes to change the cut-off frequency, he must change the values of the capacitors only. The power supply used in this circuit must be symmetrical and as the input impedance is very high, extra precautions must be taken with the signal input cables.
