Re: Brief overdrive

On Mon, 08 May 2006 13:51:10 GMT, "Ian Iveson"
<IanIveson.home@xxxxxxxxxxxxxxxx> wrote:

Pooh Bear wrote

Actually feedback is largely irrelevant to slew rate.

As you say it's determined ( ususally by one dominant stage ) by the
ability
to
charge a capacitive node. You can't slew faster by applying feedback (
although you
can extend bandwidth - which is different ).

No, but you do need infinite gain.

Pardon ? Since when does any circuit have 'infinite gain' ?

I was responding to Chris's contention that SRL is independent of amplitude. For
that truly to be the case, then it must be possible even with an input signal of
infinitessimally small amplitude...for small signal analysis. It's a theoretical
point. If it does happen following such a signal, and the signal is a perfect
step, then it follows from the ensuing straight line response that there is no
limit to the steady state output. A straight line goes up to infinity as long as
its gradient is not zero. In a closed loop circuit, the gain would depend only
on the feedback proportion.

This is pretty close to what happens with opamps. I think the ideal opamp has
infinite gain. Obviously real opamps don't. Perhaps they are close enough,
however, so that SLR becomes a dominant effect in some circumstances. The
postulation of the perfect allows engineers to analyse. That's what the
principle of superposition is for.

To be useful in any circuit, feedback would
be a necessary addition.

Sorry, addition to what ?

To infinite gain.

Or so it seems to me. For the transient response to be truly a straight line,
there must be a discontinuity...an instantaneous change in dv/dt, requiring
infinite d2v/dt2.

I see what you mean, but how then do you generate an input signal with
infinite
d2V/dt2 ? Not possible. All siganls are band limited.

Of course. To clear up a matter of possible confusion, just because the
transient response displays an infinite d2V/dt2, doesn't mean that the input
signal does too. Not as a matter of definition, anyway.

However, in this case I was responding to Chris's point about response to a step
input, as square as can be no squarer. As it happens, the idealised
slew-rate-limited opamp would respond with an infinite d2V/dt2 no matter what.
To any real input, the total response would be triangular, at the frequency of
the input fundamental, and an amplitude depending entirely on that frequency, as
far as supply voltage would allow. A rail-to-rail trapezium.

For myself, I think that if the term "slew rate limiting" is applicable only to
the perfect case, then it has no application. If it applies to an imperfect
case, then why should it not be applied to a circuit which is not an opamp at
all? Also, to even approximate to this perfection, in the sense that an opamp
does, must surely involve clipping of some kind, somewhere. To put it in Chris's
terms, *all* SRL is "raggedy-assed".

What does "clipping" mean anyway? Can it happen to current, or only voltage?

cheers, Ian


I think you are allowing perfection to become an enemy of the useful
here. SRL is a clipping phenomenon. It is clipping of the input stage
current feeding the dominant pole capacitor in an op amp.

So what if D2V/dv2 isn't infinite. It just means that the transition
into slew rate limiting isn't a perfect corner - it has a little
roundness to it. Is this a reason to dismiss SRL out of hand? I don't
think so. It is still a vital term in the analysis of any circuit
involving capacitors and current sources.

In no way can SRL be described as "raggedy-assed" - it is far to
useful in the real world.

d

--
Pearce Consulting
http://www.pearce.uk.com
.

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