Example-1:
Determine gm for an n-channel JFET with characteristic curve shown in fig. 1.
Fig. 1
Solution:
We select an operating region which is approximately in the middle of the curves; that is, between vGS = -0.8 V and vGS = -1.2 V; iD = 8.5mA and iD = 5.5 mA. Therefore, the transductance of the JFET is given by
Design of JFET amplifier:
To design a JFET amplifier, the Q point for the dc bias current can be determined graphically. The dc bias current at the Q point should lie between 30% and 70% of IDSS. This locates the Q point in the linear region of the characteristic curves.
The relationship between iD and vGS can be plotted on a dimensionless graph (i.e., a normalized curve) as shown in fig. 2 .
Fig. 2
The vertical axis of this graph is iD / IDSS and the horizontal axis is vGS / VP. The slope of the curve is gm.
A reasonable procedure for locating the quiescent point near the center of the linear operating region is to select IDQ ≈ IDSS / 2 and VGSQ ≈ 0.3VP. Note that this is near the midpoint of the curve. Next we select vDS ≈ VDD / 2. This gives a wide range of values for vds that keep the transistor in the pinch –off mode.
The transductance at the Q-point can be found from the slope of the curve of fig.2 and is given by
Example-2
Determine g m for a JFET where IDSS = 7 mA, VP = -3.5 V and VDD = 15V. Choose a reasonable location for the Q-point.
Solution:
Let us select the Q-point as given below:
The transconductance, gm, is found from the slope of the curve at the point iD / IDSS = 0.5 and vGS / VP =0.3. Hence,
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