see

http://www.national.com/apnotes/apnotes_all_1.html


> 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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