When a high valued capacitor is needed and including one in you circuit is impractical, you may be able to use a capacitance multiplier as shown in the following schematic.

    The “synthetic” capacitor shown on the left will behave like the circuit shown on the right but there are some limitations. The total amount of current that can flow in or out of the circuit is limited by the op-amp’s ratings. Because of this, capacitance multipliers are useful in timing circuits and the like but cannot be used in applications where the energy stored in the capacitor is important (no spot welders).

    Note that the value of R1 appears in series with the multiplied capacitor value. For that reason the value of R1 should be kept small (on the order of 10Ω) to minimize its effect or the value of R1 should be included as part of the overall circuit design.

    Depending on the actual value of the capacitor used, the op-amp’s leakage current may come into play particularly for small values of C. The use of low leakage op-amps (ones with a FET input for instance) can help minimize the issue.