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The JFET shown in fig has parameters IDSS=8mA and VP=4V. Determine VG,IDQ,VGSQ and VDSQ.

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Mumbai University > EXTC > Sem 3 > Analog Electronics 1

Marks: 10 M

Year: May 2014

1 Answer
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1. Calculation for Rth and VG

Fig1 (a) determining $R_{th}$ (b) determining $V_G$

Fig1 (a) determining Rth (b) determining VG

  • Rth=R1||R2

    =140k||60k

    =42k

  • Calculation for VG

    VG=VDD×R2R1+R2

    =20×60k140k+60k

    =6V

  • The reduced circuit as shown in Fig2

Fig2 thevenin’s equivalent circuit

Fig2 thevenin’s equivalent circuit

2. Calculation for IDQ and VGSQ

IS=ID+IG

Since IG0

Hence ISID

  • Applying KVL to gate-source node we get

    VGVGSIDRS=0

    VGS=VGIDRS..(1)

    Now, ID=IDSS(1VGSVp)2(2)

  • Substituting eq(1) in eq(2)

    ID=IDSS(1VGRS×IDVp)2

    ID=8m(162k×ID4)2

  • After simplification,

    2ID=1m(102kID)2

    0=0.140ID+4kI2D2ID

  • Solving above quadratic equation

    We get, ID1=3.65mA and ID2=6.85mA

    • Put above two values in eq(1)

      ID2=6.85mA and VGS2=7.7V since |VGS2|>|Vp| FET is in pinch off region and ID=0 at pinch-off region.

      Hence, IDQ=ID1=3.65mA and VGSQ=1.3V

3. Calculation for VDSQ

  • Now, Applying KVL to drain-source channel

    VDDVDSIDQ(RS+RD)=0

    203.65×4.7=VDS

    VDSQ=2.845V

IDQ=3.65mAandVGSQ=1.3VVDSQ=2.845VVG=6V

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