In previous section we have discussed about the ideal quadratic I-V characteristics and capacitance model of the MOSFET. These models are helpful when the signals levels are high. For the small voltage levels in the signal is concerned small signal models are used. For analog circuit applications the MOSFET is operated in saturation region of operation. The simple MOSFET model is shown in Figure below which consists of a voltage source VGS and a current source of magnitude gm V GS between drain and source.
If the channel length modulation effect is incorporated in the small signal model then this effect is represented by a resistor (ro) connected between Source and Drain in parallel with current source as shown in Figure below.
When both channel length modulation and body bias effect is considered in the circuit design then the small signal model is shown in Figure below. The body bias effect is represented by an extra current source of magnitude gmb VBS connected in parallel with the gm VGS current source.
The complete MOSFET small signal model including all capacitances is shown in Figure below.
When MOSFET is operating in saturation, drain current is produced in response to Gate-source overdrive voltage. The transconductance (g m) is a figure of merit which is defined as the ratio of change in the drain current divided by the change in the gate to source voltage.
i.e. gm =
gm = mn Cox (VGS - V TH)
In other words gm represents the sensitivity of the device. For a high gm, a small change in VGS results in a large change in ID.