Lecture -39: FET a amplifier

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

GOTO >> 1 || 2 || 3 || Home