AC Model

The small signal AC model of the JFET we need is obtained by linearizing (110) at the operating point; this means that for small AC signals, we have

i_d = g_m v_gs .

(114)

**Figure 128:** JFET AC equivalent circuit.
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This model also applies to MOSFETs, using the appropriate value of transconductance g_m.

The AC model is only valid when the transistor is operating in its active region.

ANU Engineering - ENGN2211