From: John Woodgate
Subject: Re: How to interpret AC Analysis?
Date: Sun, 22 Sep 2002 22:37:56 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 23 Sep 2002 06:37:28 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that carltons
wrote (in ) about 'How to interpret AC Analysis?', on Sun, 22 Sep 2002:
>I may be missing something in this thread,
Yes, like 'the whole point', I'm afraid. It's about the meaning of '-3
dB frequency', if you like.
>but AC analysis in Spice is
>always done with a 1v pk drive and an assumption of linearity. For
>example, you put in 1v into a gain stage of 10,000 and you get out a 10KV
>pk sine wave. It also assumes an infinite supply voltage.
Yes, but none of that is relevant.
> If you want
>reality, then you put in correct models and do the analysis in the time
>mode or am I missing something here?
Yes, 'the whole point' again, I'm afraid.
>If you also are having a problem
>with convergence of a solution you may want to change the basic parameters
>such as time step, the solution method of gear versus trap, RESTol, etc.
All true, but irrelevant.
This is a **very basic and simple** matter. When you are looking for
the -3 dB frequency of a filter, that '-3 dB' is relative to the
**output voltage in the pass-band**, not to the input voltage. And the
output voltage in the pass-band changes if you increase the gain of a
buffer inside the filter.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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