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From: Fred Bloggs
X-Mailer: Mozilla 4.7 [en]C-CCK-MCD EBM-Compaq1 (Win95; U)
Subject: Re: How to interpret AC Analysis?
Date: Tue, 24 Sep 2002 03:01:40 GMT
NNTP-Posting-Date: Mon, 23 Sep 2002 20:01:40 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> I agree with you completely, but something in this thread just wasn't
> making much sense. AC analysis is fine by itself if you're sure that the
> circuit is stable and that the voltages in and out are going to make it
> linear. To believe otherwise is a waste of simulation time.
The OP was obviously using internally compensated amps in unity gain for
a resultant GBW nearly two decades more than his signal bandwidth, so an
AC analysis was acceptable for a preliminary estimation of performance.
The main advantage of AC analysis is that you can compute the /steady
state/ frequency response without wasting a lot of time for a transient
analysis to converge over many many cycles of input, and the AC analysis
is available whenever the operating point can be found which is several
orders of difficulty removed from getting the transient analysis to
converge, with accuracy, on many occasions. And there is absolutely
nothing wrong with the various unrealistic idealities of the AC analysis
as long as you understand what they are. Note also that the AC analysis
is an exact solution for linear networks like LC filters, unless you are
concerned with nonlinear effects 80dB down again. If the user sees no
problem with a 10KV output then it just goes to show you can't make
anything foolproof. You are correct in pointing out the misleading
aspect of the results for semiconductor modeling as the output magnitude
is a pure fiction since the so-called linearized model is only
asymptotically exact for an input of 0V.
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