The low-pass filter shown in the figure uses two operational amplifiers with JFET at the input to provide a cutting action of 30 dB per octave. The values of C1 and C2 are calculated according to the frequency, according to the formulas given next to the diagram. Note that it is necessary to have a high gain and a high input impedance for the operational ones to compensate for the attenuation of the RC networks of this filter. It is also necessary to consider the frequency limits of the operational amplifiers used. In fact, these circuits are well suited to operation at frequencies that do not exceed a few tens of kilohertz. The power supplies used for the amplifiers must be symmetrical. Operational with JFET as those of the TLC and AC series are the most suitable for these circuits.
