Definition of Parameters:
Substituting the equation (4) in equation (3) we get,
Differentiate the equation (2), we get
In this equation,
∆VD=0, because VD is a constant
This leads us to the following equation (7):
From this equation, the change in the variable value vDS is equal to the variation in the AC value vds.
Since vds is a variable value we will get the equation (8)