another phenomenon in FFT analysis (Hspice): for a high gain amplifier (about 60dB), the input signal amplitude is very small. when doing fft analysis, the results indicate that the harmonic distortion will be improved with the increasing input signal strength in some range. But this violates the reality. because for a linear amplifier, Y(t)=a0+a1*x(t)+a2*x(t)²+a3*x(t)³, then HD3 (defined as the ratio of 3ord HD to fundamental) can be expressed as ¼*|a3/a1|*x(t)² , so with increased x(t), HD3 is lower. How to explain this?