Example1:
Determine gm for an nchannel 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 v_{GS} = 0.8 V and v_{GS} = 1.2 V; i_{D} = 8.5mA and i_{D} = 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 I_{DSS}. This locates the Q point in the linear region of the characteristic curves.
The relationship between i_{D} and v_{GS} 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 i_{D} / I_{DSS} and the horizontal axis is v_{GS} / V_{P}. The slope of the curve is g_{m}.
A reasonable procedure for locating the quiescent point near the center of the linear operating region is to select I_{DQ} ≈ I_{DSS} / 2 and V_{GSQ} ≈ 0.3V_{P}. Note that this is near the midpoint of the curve. Next we select v_{DS} ≈ V_{DD} / 2. This gives a wide range of values for v_{ds} that keep the transistor in the pinch –off mode.
The transductance at the Qpoint can be found from the slope of the curve of fig.2 and is given by
Example2
Determine g m for a JFET where I_{DSS} = 7 mA, V_{P} = 3.5 V and V_{DD} = 15V. Choose a reasonable location for the Qpoint.
Solution:
Let us select the Qpoint as given below:
The transconductance, g_{m}, is found from the slope of the curve at the point i_{D} / I_{DSS} = 0.5 and v_{GS} / V_{P} =0.3. Hence,
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