[SOLVED] Why Zin gets smaller as gate width grows

[hunas2127]

Is it because of the increase in Cgs?

 

[D.A.(Tony)Stewart]

The input impedance Z_in of a MOSFET is heavily influenced by the gate capacitance (C_g), which includes the gate-to-source (C_gs) and gate-to-drain (C_gd) capacitances. These capacitances scale directly with the gate width (W). As W increases, C_g increases proportionally because the capacitance is a function of the gate area (W × L, where L is the gate length). At a given frequency, the impedance due to capacitance is Z_C = 1/(jωC), where ω is the angular frequency. A larger C means a smaller impedance magnitude, so Z_in decreases as W grows.