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From: "Helmut Sennewald"
Subject: Re: How to interpret AC Analysis?
Date: Sun, 22 Sep 2002 21:50:44 +0200
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"carltons" schrieb im Newsbeitrag
> In article , John Woodgate
> I may be missing something in this thread, but AC analysis in Spice is
> always done with a 1v pk drive
it is not necessary to use 1V. You can use the voltage level
you want. e.g. 1mV. It is just convient to have 1V, because
the dB scale then gives the amplification factor directly.
> and an assumption of linearity.
That's the basic idea behind the .AC analysis.
> 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.
The .AC analysis assumes no DC-supply. It uses a linearized model
of all your components. Any DC-voltage supply is treated as a short(0V).
Any current source is treated as isolated. These sources are used
only for calculation of the DC-operating point which is necessary
to calculate the linearized models.
> If you want
> reality, then you put in correct models and do the analysis in the time
A filter designers will always use the .AC analysis first, but in
parallel he/she should think about slewrate limitations and output
voltage swing, especially of the active devices.
> mode or am I missing something here?
I agree with you that the .TRAN timing analysis is very important
if it comes to the reality check. Quite often, it is not possible
to overlook the stability of a circuit with the .AC analysis.
It is also nearly impossible to decide wether you have overshoot
or not in the time domain when you only have an .AC analysis.
The .TRAN analysis will give you the answer.
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