Similar to Bipolar junction transistor. JFET can also be used as an amplifier. The ac equivalent circuit of a JFET is shown in fig. 1.
Fig. 1
The resistance between the gate and the source RGS is very high. The drain of a JFET acts like a current source with a value of gm Vgs. This model is applicable at low frequencies.
From the ac equivalent model
The amplification factor µ for FET is defined as
When VGS = 0, gm has its maximum value. The maximum value is designated as gmo.
Again consider the equation,
As VGS increases, gm decreases linearly.
Measuring IDSS and gm, VGS(off) can be determined
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