Andre,
makes me wonder if there is any way to start a
transient simulation, stop at some predefined
point in time and use that result for the ac
simulation.
No this isn't possible in LTspice. It's pretty
hard to implement. What you can do, to help
with your confidence in the solution from a .ac
analysis, is to do a .step set of runs that
varies some aspect of the dc operating point
and see if the .ac small signal transfer function
looks the same for all those slightly different
.op points.
I had that problem too, but in my
designs i almost only rely on transient
simulation (for the exact same reason that you
mentioned above and because large signals
change the operating point anyways).
Yes, e.g., power amplifier stability is really
difficult to do reliably in small signal .ac
analysis. The open loop gain/phase varies
wildly with output stage operating point.
One method that helps in this situation is to
drive the amp to one end or the other with a DC
input source and insert a floating AC source
in the loop in front of a high impedance
point for an .ac analysis. The open
loop transfer function can be obtained from
the ratio of voltages to either side of the
floating source.
But ultimately, the .tran analysis comes
out at the ultimate SPICE test of stability.
Best Regards,
--Mike
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Mike Engelhardt