Hello,
could you plot the in and output waveform in transient operation? Your description is really unclear to me.
What is your design goal and what information do you want to retrieve with your S-Parameter simulation exactly?  

Cheers

Hi,
You cannot estimate gain using S21 because this is small signal parameter. Small-signal parameters are not valid with PAs because PAs work with large signals.  You should estimate power delivered to the load using time-domain or Harmonic Balance simulation. Practically, simulator can show some small-signal gain because small current may flow through a transistor. In fact, it is very easy to shift amplifier to class AB or class C operation. The best way to understand what biasing is to build a load line. For a class B it should be like in a figure I attached (VDD=20)

It does not make sense at all to estimate anything in a class B amplifier with small signal S-parameter simulation.

First, you have to define what you exactly mean by "zero". Do you mean 0dB (which is 1) or "nothing", or very small.
Actually, you will not have an ideal switch or transistor, where quiescent current  is zero at your biasing point. There is a "threshold voltage" , which actually is a transition from weak inversion with exponential behaviour to normal operation with quadratic characteristic of a transistor. There is no point, where the current is really zero, only very small with respect to "on" condition. Second, there will always be coupling of the voltage through parasitic gate-source capacitance, so the signal also won't be "zero".

Hi Purplewolf,

Small signal S21 for Class B would be 0 (not in db) if you had ideal components and there was no parasitic that connect from input to output. Real devices would have parasitics and so you would get some non-zero S21 even with zero bias, but it should be very small