How to simulate High Power Amplifier(HPA), like Solid State Power Amplifier (SSPA)
The model works fine.  You need to slow down and check your (or someone else's? ) steps.  I just worked through one pass of this, without pskmod, and only one value was high enough voltage to push the model into a nonlinear region.  Work through it step by step, you'll see that your x values are too low in magnitude to really get out of the linear region.  The x values have to approach A to get distortion.