to have an estimatin of your THD make a dft of your output to see how high are your armonic frequencies.

what i did is use the "prints" in Cadence calculator to export the spectrum points, then use Matlab to calculate SNR

Neurocat,

   If the input is a sine wave, one alternative would
be to use the Spectre fourier component to calculate
the THD. In general the fourier component is very
accuracte and simplifies the THD measurement. The
only limitations are that it doesn't work for sampled
data applications, that is, for ADCs. In addition, you
need to be careful about calculating SNR from a dft.
The issue is that normally transient analysis does not
include device noise unless you specifically add[using behavioral models] so the noise floor of the spectrum
is set by the numerical noise in the simulation.  

  If you do want to calculate SNR from the large signal
performance, a reasonable goal since this circuit is a
class-AB amplifier, try transient noise analysis. Don't
forgot to subtract the numerical noise floor from the
original spectrum(by RSS).

  BTW, as a thought experiment if you are using
Spectre, run your transient simulation using different
relref options: sigglobal, alllocal, pointlocal. You should
see that the amplitude of the fundamental and the
harmonics are constant, however, the amplitude of
the "noise" falls of significantly as the accuracy of the
simulation increases.

                                                        Best Regards,

                                                           Sheldon