2.2 Simplified AC Model of the JFET

An AC model of the JFET is shown in Figure 11.2.1. This is essentially the same model as was used for DC analysis. Once again, we have a voltage-controlled current source situated in the drain. The reverse-biased junction shows up as a very large resistance, ππΊπ.

It is worth mentioning that this model is suitable only for low frequencies. At higher frequencies, device capacitances can play a major role in the response of the amplifier. There are three device capacitances not shown in the Figure that shunt each pair of terminals: πΆπΊπ, πΆπ·πΊ and πΆπ·π. On a data sheet, the βlumpedβ capacitances are often given. These are πΆππ π , the capacitance looking into the gate with the source and drain shorted to ground: πΆππ π =πΆπΊπ+πΆπ·πΊ; and πΆππ π , the capacitance seen from the drain with the gate and source shorted to ground: πΆππ π =πΆπ·π+πΆπ·πΊ. As we shall see, these capacitances can have a sizable impact on amplifier characteristics such as πππ.

The value of transconductance, ππ, will prove to be of particular interest. It is roughly of equal importance to πβ²π in a BJT.[1]

1. In fact, we can say that 1/πβ²π is ππ for a BJT.