Small signal low frequency FET model-page-1
ID = f(VGS, VDS) --------(1)
Total change in ID due to simultaneous change in VGS& VDS is approximately given my first two terms of the Taylor’s & series expansion of equn (1)
ΔID = id
ΔvGS = vgs
ΔvDS = vds
Drain resistance or output resistance
The electrical network which satisfies (3) is called small signal model of FET.