Unit 4. Signal Conductance Parameters for Circuit Simulation

The basic low-frequency linear model for a MOS transistor has three conductance parameters: the transconductance parameter, g_m, the body-effect parameter, g_mb, and the output conductance parameter g_ds. They are the proportionality constants between incremental variables of current and voltage. For the linear model to be valid, the increment must be small compared to the dc (bias) value of the variable. To qualify as small, the increment must be sufficiently small, in each case, as to avoid unacceptable degrees of nonlinearity in the variable relationships. The conditions are explored in Unit 5 in connection with linearity of an amplifier gain function.

In the following, the three conductance parameters are explored, and in each case, an expression for obtaining the circuit value is developed. The discussion is based on a standard amplifier stage to provide for an association with electronic circuits.