The caption in the GIF has an error. You say R2=R3, but
the schematic shows R2=R1, which makes sense when one is
balancing the bias current.

If one uses a power op amp, and low values for R3, one should
be able to get reasonable currents in the 'simulated' capacitor.

Thomas McGahee wrote:
>
> Attached you will find CAPMULT.GIF which shows a simple
> circuit that can be used to multiply capacitance. This
> is a useful sub-circuit that can be used at the workbench
> when designing circuits that involve capacitors that
> have one lead connected to ground.
>
> This is based on AN29, page AN29-11 of the National
> Semiconductor Corporation Linear Applications book,
> January 1972. (There's still a lot of good stuff in those
> old books!)

That there is, which is why I never throw that kind of stuff
away. I still have Fairchild DTL data books since some of
the equipment around here still has those parts in it.

>
> It is particularly useful when designing timing circuits,
> as you can literally dial up the value you need by
> adjusting either R1 or R3.
>
> As shown, the circuit multiplies a 10 ufd capacitor
> by a factor of 10,000 to yield the equivalent of a
> 100,000 ufd capacitor. The "leakage" current is
> 8 microamps max over a -55 to 125 degree centigrade
> temperature range. Pretty good for 100,000 ufd!
>
> Note that the multiplied capacitance cannot discharge
> instantaneously like a real capacitor, and it only
> actually stores charge in C1, so there is a limit
> to how the multiplied capacitance can be used...
> but in simple RC and constant current charging
> circuits, it works like a charm.
>
> Note also that op-amp speed, etc., affect maximum
> frequency response, etc. You cannot use a 741
> for such a circuit. The LF356 is a high slew rate
> high input impedance opamp.
>
> Fr. Tom McGahee

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