Signal Presence Detectors?

[Haible Juergen]

	>dont bybass the op amp o/p with a cap to anywhere.. you'll regret
it! they 
	>dont like cpacitive loads at all! any big cap >afew nf to anything
other 
	>than a high impedance point ie audio o/p, summing node, next op amp
stage 
	>etc really needs a few K res between the op amp and the cap..

That's just half of the story.

What opamp outputs don't like is capacitance withing a certain *range*.
As an example (exact values may differ), it can be safe to have below
50pF capacitance, and also ok to have more than 10uF.
For audio drivers, you'll choose a small cap, of course. (otherwise it
will be too slow.) For voltage regulation, you choose the big cap.
Opamp will be slow, but cap will make the regulated voltage "fast"
nevertheless.
Just avoid the instable in-between capacitance range.

(For theory, you want one dominant pole in your system. So either
make the opamp's internal compensation dominant, or the external
capacitive load * opamp's open loop output resistance. Normally
the former is done for amplifiers, and the latter is done for voltage 
regulators.)

JH.