Concept:
For a MOSFET in saturation, the current is given by:
\({{I}_{D}}=\frac{1}{2}{{μ }_{n}}{{C}_{ox}}\left( \frac{W}{L} \right){{\left( {{V}_{Gs}}-{{V}_{T}} \right)}^{2}}\left( 1+\lambda {{V}_{DS}} \right)\)
W = Width of the Gate
Cox = Oxide Capacitance
μ = Mobility of the carrier
L = Channel Length
Vth = Threshold voltage
Calculation:
The drain conductance (gd) is calculated as the rate of change of drain current with respect to the Drain to source voltage, i.e.
\(g_d= \frac{{\partial {{\rm{I}}_{\rm{D}}}}}{{\partial {{\rm{V}}_{{\rm{DS}}}}}} \)
\({{g}_{d}}=\frac{1}{2}{{μ }_{n}}{{C}_{ox}}\left( \frac{W}{L} \right){{\left( {{V}_{Gs}}-{{V}_{T}} \right)}^{2}}\lambda \)
Putting on the respective values, we get:
\({{g}_{d}}=\frac{1}{2}{\times 70\times 10^{-6}}\left( 4\right){{\left( {1.8-0.3} \right)}^{2}}\times0.09\)
gd = 28.35 μ Seimens