AW: Capacitive loading of op-amps

[Haible Juergen]

	>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, 

Yes. Loop gain being more or less a LP function, it will
shift the cutoff frequency downward, i.e. less loop gain
at a certain frequency.
Now, loop gain (or let's speak about loop bandwidth, as
the DC gain is not affected by the capacitor on the
output !) isn't a benefit by itself. Why build a buffer with
high loop gain / bandwidth and tight feedback at all ?
(1) It will help for a well defined closed loop gain (i.e. relative
wide band voltage buffer), and
(2) it will also reduce the output resistance of the buffer.

You need (1) and (2) for amplifiers. But for a buffered
auxiliary voltage, you don't need (1), as the "amplifier
input" doesn't change at all. You still want (2), i.e. a stiff
voltage with low internal impedance over a wide frequency
range. The opamp with reduced loop gain (because of the
cap) will not provide such a good wide band low impedance
anymore. But who cares - that part is taken by the cap.
The opamp takes care of the DC and low frequency part.
So, for high frequencies, all is ok. (It even would be ok
with just a resistor divider and capacitor, no opamp)
For DC, everything's fine too. (cap is irrelevant here)
If there is no resonance peak in the closed loop gain
(i.e. external pole and internal pole from compensation
wide apart), everything should be nice and smooth  in 
between, too.

	>sort of rendering the circuit as a precision emitter follower
	>without a Vbe offset.  

Emitter follower is different. You don't have an external feedback
loop and no high output impedance of the "open loop" either.
Can't see how this would fit together.

	>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 current will be limited by the internal current limiting that's
build into most opamps. It's not like a single transistor, where
you must take care how long a certain high current can last
without burning something. The opamp will simply go into
limiting, and behave like a current source rather than a voltage
source until the cap is charged.
The only thing I'd worry about is what happens when the circuit
is switched *off*, i.e. supply voltage goes down, but the capacitor
is still charged. I never had problems with 10u caps and the usual
cheap opamps myself, but I think that there is some limit.

	JH.