U r doing the correct thing but give a small ac signal of 1mv mag instead of 1V ac. Then perform an ac analysis. The ac gain gain u will obtain will be same as the slope of the DC sweep that you have talked about.

Hi,

  Yes, the open loop gain measured by looking at the
slope of the dc analysis should match the results from low
frequency ac analysis. However, the test fixturing for the
dc test is complex. You might want to look at how ATE
machines measure dc open loop gain for the details.
Basically you need to put the op amp in a servo loop to
keep it in the linear region. There are also some other
simpler methods. The other issue is that you need to be
careful about the error tolerances in the dc analysis and
the dc stepping, since you are differentiating small errors
can create a lot of noise. BTW, you did not mention how
you are doing the ac measurement. If you are using
open loop measurements, there that could also be an
issue. Have you tried the approaches Ken suggests in
his book?  

                                                            Sheldon

Hi,

  Try section 3.4, Applications of AC Analyses, page 67
to 108. There is a good summary chart on page 107.  The
extensions for diffferential amplifiers are discussed in,
section 3.4.5, pages 113-119. BTW if memory serves,
stability analysis directly calculates loop gain, T. You can
use loop gain to calculate Open Loop Gain.
  Aopen,loop = Aclosed,loop + Tloop,gain

                                                   Best Regards,

                                                         ::)