Transistor Biasing for AC Signal Amplification

[Hendrixisgod]

Let me preface this by saying that I understand biasing for both BJTs and MOSFETs to get the transistors in the correct modes of operation for amplification.  My question is simple: in a transistor amplification circuit that involves an AC source (followed by a capacitor to prevent leakage), do you need to take into account the amplitude of the AC voltage applied to the base in calculations?  In other words, will the gate/base of the MOSFET/BJT "see" a maximum voltage of the DC input + the AC input?  Could the transistor change states with the variance of the AC input?

On that note, would this essentially clip the top of the AC wave as the transistor changed away from a mode suitable for amplification?  I understand that this would be asymmetrical if so, but could there be any advantage to that?  Such as inverting a signal and clipping in the same manner a second time?

[R.G.]

Let me preface this by saying that I understand biasing for both BJTs and MOSFETs to get the transistors in the correct modes of operation for amplification.  My question is simple: in a transistor amplification circuit that involves an AC source (followed by a capacitor to prevent leakage), do you need to take into account the amplitude of the AC voltage applied to the base in calculations? 

Yes. The control node (base or gate) sees the sum of the DC bias and the instantaneous signal applied.

In other words, will the gate/base of the MOSFET/BJT "see" a maximum voltage of the DC input + the AC input?  Could the transistor change states with the variance of the AC input?

Yes, and yes.
But remember that AC inputs vary both UP and DOWN. So the input goes up for a while, then down, pulling the base/gate with it.

On that note, would this essentially clip the top of the AC wave as the transistor changed away from a mode suitable for amplification?  I understand that this would be asymmetrical if so, but could there be any advantage to that?  Such as inverting a signal and clipping in the same manner a second time?

For gain stages, moving the base/gate up moves the collector/drain down. All the common, simple gain stages invert.

An AC input signal pulls the input higher and lower, which moves the output terminal lower and higher by an amount depending on the gain. If the signal times gain tries to move the output further from the DC bias point than it has available power supply to move, it clips. Note that this happens for both positive and negative going signal halves on the input.