Capacitive loading of op-amps

[Arthur Harrison]

AH:

> >While inventorying some Analog Devices ICs today, I came
> >across AD688 precision references.  The data sheet addresses
> >capacitive loading of the part's on-board op-amps. The technique
> >described is readily applicable to output-bypassed applications
> >such as that in Joachim's schematic.
> >
> >FWIW, see page 8 of 8 in the .pdf file at:
> >http://www.analog.com/pdf/ad688.pdf


JH:

>Fig. 14b - yes, that's the way to drive cap values that
>are inside the "forbidden" range. Overkill for buffering
>an auxilliary voltage like in Joachim's oscillator, but
>very helpful for audio outputs. (Where most people just
>add a series resistor.)

AH:

It also occurs to me that adding a large capacitor directly to the
output of an op amp will essentially cancel much of the loop's AC
gain, sort of rendering the circuit as a precision emitter follower
without a Vbe offset.  In such a case, the bulk of the load transient
rejection is furnished mainly by the capacitor itself. That is probably
fine in many cases, as Juergen stated.

For this simpler approach, however, another consideration is that a
huge slug of current will be drawn through the op amp's output transistors
upon power-up.  Most op-amps are capable of withstanding an
intermittent surge such as this, although it may cause a failure in some
cases when the capacitor is large, has an exceptionally low ESR
(e.g., tantalum), and the supply' s rate of rise is high.

The referenced circuit preserves the desired loop dynamics by
incorporating feedback comprised of the 1uF capacitor between the
op amp's negative input and output.

It would be interesting to compare each circuit's response to transient
loads.

-Art