\[I_D=\frac{1}{2}\mu C_{ox}\frac{W}{L}\left ( V_{GS}-V_T \right )^2\] \[I_D=\mu C_{ox}\frac{W}{L}\left [\left ( V_{GS}-V_T \right ) V_{DS}-\frac{1}{2}V_{DS}^{2} \right ]\]

\[g_m=\sqrt{2\mu C_{ox}\frac{W}{L}I_D}\] \[g_m=\frac{2I_D}{V_{GS}-V_T}\] \[g_m=\mu C_{ox}\frac{W}{L}\left ( V_{GS}-V_T \right )\]

\[r_o=\frac{1}{\lambda I_D}\]

\[r_{in}=\left [ 1 + \left ( g_{m1} + g_{mb1} \right )r_{o1} \right ]r_{o2}+r_{o1}\] \[r_{in}\approx g_{m1}r_{o1}r_{o2}\]