射级跟随器的振荡问题

Source follower has an interesting frequency response: its low frequency output impedance equals to 1/gm but at high frequency approximates to Rs, which appears to have a negative resistance at the intermediate frequencies, or "inductive".
This "virtue inductance" would resonant with Cgs of the output transistors, creating oscillation.
To inhibit this oscillation, the easiest way is to add an input resistor, which increases the damping of this RLC circuit, reduces the oscillation problem.

   Thanks for your explanation.
   Now it's clear since I found that this kind of oscillating is time-limited with finite energy.Using that resistance can consume the energy of the oscillating to keep a stable output.

Can I ask a question that why at low frequency the output impedance of source follower  equals to 1/gm and at high frequency approximates to Rs?
Even though, how do you know at the intermediate frequencies it becomes inductive? Shouldn't it be a number between 1/gm and Rs?

Whatever, many thanks for your help.

源级跟随器具有上述大电感特性,射级跟随器应该类似.