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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 V_GS and a current source of magnitude g_m 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 g_mb V_BS connected in parallel with the g_m V_GS current source.

The complete MOSFET small signal model including all capacitances is shown in Figure below.

Transconductance (g_m) :

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. g_m =

g_m = m_n C_ox (V_GS - V _TH)

In other words g_m represents the sensitivity of the device. For a high g_m, a small change in V_GS results in a large change in I_D.