# gm and ro in a NewFET

Discussion in 'General Electronics Chat' started by kbcheong, Apr 7, 2012.

1. ### kbcheong Thread Starter New Member

Mar 24, 2012
5
0
How to derive gm and ro in a small signal models in term of large model parameters K, VT and bias voltages VGS and VDS, given that in a bias region iD = K(VGS-VT)(VDS^2)

2. ### crutschow Expert

Mar 14, 2008
13,006
3,232
Sounds like homework. If so it should be in that section.

3. ### Vahe Member

Mar 3, 2011
75
9
Take the partial derivative of the drain current with respect to the drain-source voltage and invert it to find the output resistance.
$r_o = \left( \frac{\partial i_d}{\partial v_{ds} \right) ^{-1}$

Take the partial derivative of the drain current with respect to the gate-source voltage to find the transconductance.
$g_m = \frac{\partial i_d}{\partial v_{gs}$

All partial derivatives should be evaluated at the operating point -- that is, you should plug in the dc voltage/current values to evaluate the output resistance and the transconductance.