sheldon
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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
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