Effect of Rs in a CS amplifier with degeneration
HaneenSu, you forgot to mention the most important effect caused by such a resistor Rs: Negative feedback!

Here are the main effects of this (current controlled voltage) feedback:


Input resistance increase
Decrease of gain, but the reduced gain value is less dependent on transistor parameters (primarily determined by RD and RS)
Bandwidth increase
Improved linearity (less total harmonic distortion)
Small influence on output resistance.


Don`t forget that the dc drop across RD determines the DC operational point of the amplifier stage. An inductor has a DC resistance that is negligible.