FIELD-EFFECT TRANSISTORS (FETS)

8.1 The Junction Field-Effect Transistor (JFET)

PROBLEMS
8.1 8.1 An n-channel JFET has IDSS = 10 mA and Vp = 2 V. For small values of vDS,
determine the approximate drain-source resistance rDS when (a) vGS = 0 V and (b) vGS = 1
V.

8.2 8.2 For the circuit shown in Fig. P8.2, the JFET has IDSS = 12 mA and Vp = 4 V. Given
that RD = 3 k, RS = 0 , VDD = 15 V, and VGG = 2 V, find (a) iD and (b) vDS.

Fig. P8.2

8.3 8.3 For the circuit given in Fig. P8.2, the JFET has IDSS = 12 mA and Vp = 4 V. Given
that RS = 0 .and VDD = 15 V, find the value of RD for which vDS = 0,1 V when VGG = 0 V.
 Fig. P8.2

8.4 8.4 For the circuit shown in Fig. P8.2, the JFET has IDSS = 12 mA and Vp = 4 V. Given
that RD = 1 k, RS = 0 , VDD = 15 V, and VGG = 0 V, find (a) iD, (b) vDS, and (c) the region
of operation for the JFET.

Fig. P8.2
8.5 8.5 For the circuit given in Fig. P8.2, the JFET is in the active region and has IDSS = 12
mA and Vp = 4 V. Suppose that VDD = 12 V and VGG = 0 V. (a) Find iD when vGS = 2 V.
(b) Find vGS when iD = 9 mA. (c) Find RS when vGS = 3 V. (d) Find RD when vGS = 3 V
and vDS = 4,5 V. (e) Find vDS when RD = 2 k and vGS = 3 V.

Fig. P8.2
8.6 8.6 For the circuit given in Fig. P8.2, the JFET has IDSS = 16 mA and Vp = 4 V. Given
that VDD = 18 V, VGG = 0 V, and RD = RS = 500 , determine (a) vGS, (b) iD (c) vDS, and (d)
the region of operation for the JFET.

Fig. P8.2
8.7 8.7 For the circuit given in Fig. P8.2, the JFET has IDSS = 8 mA and Vp = 4 V. Given that
VDD = 18 V, VGG = 0 V, RD = 8 k, RS = 1 k, and the JFET operates in the ohmic region,
determine (a) vGS, (b) iD, and (c) vDS.

Fig. P8.2
8.8 8.8 Show that the equation for a JFET operating in the ohmic region (Equation 8.2 on p.
510) reduces to Eq. 8.4 for the case of operation on the border between the ohmic and
active regions, that is, for vDS = vGS Vp.

8.9 8.9 For the circuit given in Fig. P8.2, suppose that RD = RS = R, VDD = 15 V, and the JFET
has Vp = 3 V. (a) Assuming active-region operation, VGG = 5 V, and IDSS = 10 mA,
determine the value of R for which vGS = 0 V. (b) Assuming active-region operation, find
IDSS given that R = 400 , VGG = 5 V, and vGS = 0 V. (c) If IDSS, R, and the region of
 operation are not known, what is vGS when VGG = 5 V and vDS = 1 V? (D) If R is not
known, what is the region of operation when vDS = 2 V and vGS = 1,5 V?

Fig. P8.2

8.10 8.10 For the circuit shown in Fig. P8.10, the JFET has IDSS = 9 mA and Vp = 3 V. Given
that R1 = 150 k, R2 = 50 k, RD = RS = 7 k, and VDD = 20 V, assume active-region
operation and determine (a) vGS, (b) iD and (c) vDS. (Hint: Use Thvenins theorem.)

Fig. P8.10
8.11 8.11 For the circuit shown in Fig. P8.10, the JFET has IDSS = 12 mA and Vp = 4 V. Given
that R1 = 300 k, R2 = 100 k, RD = RS = R, VDD = 12 V, and vDS = 6 V, determine (a) vGS,
(b) iD and (c) R. (Hint: Use Thvenins theorem.)

Fig. P8.10
8.12 8.12 For the circuit shown in Fig. P8.10, the JFET has IDSS = 12 mA and Vp = 4 V. Given
that R1 = 300 k, R2 = 100 k, RD = RS = 1 k, and VDD = 12 V, determine (a) vGS, (b) iD,
(c) vDS, and (d) the region of operation for the JFET. (Hint: Use Thvenins theorem.)

Fig. P8.10
8.13 8.13 For the circuit shown in Fig. P8.10, the JFET has IDSS = 12 mA and V = 4 V. Given
that R1 = 300 k, R2 = 100 k, RD = RS = 2 k, and VDD = 12 V, determine (a) vGS, (b) iD,
(c) vDS, and (d) the region of operation for the JFET. (Hint: Use Thvenins theorem.)

Fig. P8.10
8.14 8.14 Both JFETs in the circuit shown in Fig. P8.14 have IDSS = 4 mA and Vp = 2 V. Given
that both JFETs are in the active-region, R1 = 0 , and VDD = Vp = 10 V, find (a) iD (b) vGS,
(c) vDS, and (d) confirm the regions of operation.

Fig. P8.14
8.15 8.15 Both JFETs in the circuit shown in Fig. P8.14 have IDSS = 4 mA and Vp, = 2 V. Place
a 1-k resistor in series with the drain of the lower JFET. Given that both JFETs are in
the active region, RS = 1 k and VDD = VSS = 10 V, find (a) iD, (b) vGS, (c) vDS, and (d)
confirm the regions of operation.

Fig. P8.14
8.16 8.16 Both JFETs in the circuit shown in Fig. P8.14 have IDSS = 16 mA and Vp = 4 V.
Given that RS = 0 , VDD = 12 V, VSS = 0 V, the upper JFET is in the active region, and the
lower JFET is in the ohmic region, find (a) vGS, (b) iD, and (c) vDS.

Fig. P8.14
8.17 8.17 Both JFETs in the circuit shown in Fig. P8.14 have IDSS = 16 mA and Vp = 4 V.
Instead of connecting the gate of the upper JFET to the reference as shown, connect a
short circuit between the gate of the upper JFET and the drain of the lower JFET. Given
that RS = 1 k, VDD = 6 V, VSS = 0 V, the lower JFET is in the active region, and the upper
JFET is in the ohmic region, find (a) iD and (b) vDS.

Fig. P8.14
8.18 8.18 An n-channel depletion MOSFET has IDSS = 10 mA and Vp = 2 V. For small values
of vDS, determine the approximate drain-source resistance rDS when vGS is (a) 1 V and (b) 2
V.

8.19 8.19 For the circuit given in Fig. P8.19, the depletion MOSFET has IDSS = 8 mA and Vp =
2 V. Given that RS = 0 , VDD = 16 V, and VGG = 1 V, determine the value of RD for
which the depletion MOSFET will operate on the border between the active and the
ohmic regions.

Fig. P8.19
8.20 8.20 For the MOSFET circuit shown in Fig. P8.19, the depletion NMOS transistor has
IDSS = 8 mA and Vp = 2 V. Given that RD = RS = 250 , VDD = 16 V, and VGG = 2 V, find
(a) vGS, (b) iD (c) vDS, and (d) the region of operation for the MOSFET.

Fig. P8.19

8.21 8.21 For the MOSFET circuit shown in Fig. P8.19, the depletion NMOS transistor has
IDSS = 8 mA and Vp = 2 V. Suppose that VDD = 16 V and VGG = 2 V. (a) If RD = RS = R,
 then what value of R results in active-region operation and vGS = 1 V? (b) Find RD and RS
such that vDS = vGS = 1 V.

Fig. P8.19

8.22 8.22 For the circuit shown in Fig. P8.19, the depletion MOSFET has IDSS = 10 mA and Vp
= 4 V. Given that RD = 1 k, RS = 0 , and VDD = 15 V, find vDS when VGG is (a) 0 V and
(b) 5 V.

Fig. P8.19
8.23 8.23 For the MOSFET circuit shown in Fig. P8.23, the depletion NMOS transistor has
IDSS = 4 mA and Vp = 4 V. Given that VDD = 10 V and RD = 1 k, find (a) the region of
operation, (b) vGS, (c) vDS, and (d) iD.

Fig. P8.23
8.24 8.24 For the MOSFET circuit shown in Fig. P8.23, the depletion NMOS transistor has
IDSS = 4 mA and Vp = 4 V. Given that VDD = 10 V and vDS = 6 V, find (a) vGS, (b) the
region of operation, (c) iD, and (d) RD.

Fig. P8.23
8.25 8.25 For the MOSFET circuit shown in Fig. P8.23, the depletion NMOS transistor has
IDSS = 4 mA and Vp = 4 V. Given that VDD = 10 V, and iD = 16 mA, find (a) the region of
operation, (b) vGS, (c) vDS, and (d) RD.

Fig. P8.23

8.26 8.26 For the depletion-MOSFET circuit shown in Fig. P8.26, M1 has IDSS = 8 mA and Vp =
4 V, whereas M2 has IDSS = 16 mA and Vp = 4 V. When VDD = 11 V and VGG = 0 V, then
M1 is in the active region and M2 is in the ohmic region. Find (a) vDS2, (b) vDS1, and (c)
confirm the regions of operation.
 Fig. P8.26

8.27 8.27 For the depletion-MOSFET circuit shown in Fig. P8.26, M1 has IDSS = 8 mA and Vp =
4 V, whereas M2 has IDSS = 16 mA and Vp = 4 V. When VDD = 11 V and VGG = 1 V, then
M1 is in the active region and M2 is in the ohmic region. Find (a) iD, (b) vDS2, (c) vDS1, and
(d) confirm the regions of operation.

Fig. P8.26
 Fig. P8.26
8.28 8.28 For the depletion-MOSFET circuit shown in Fig. P8.26, M1 has IDSS = 8 mA and Vp =
4 whereas M2 has IDSS = 16 mA and Vp = 4 V. Given that VDD = 11 V, find VGG such that
M1 is in the active region and M2 is on the border between the active and the ohmic
regions.

Fig. P8.26
8.29 8.29 For the depletion-MOSFET circuit shown in Fig. P8.26, M1 has IDSS = 8 mA and Vp =
4 V, whereas M2 has IDSS = 16 mA and Vp = 4 V. When VDD = 11 V and VGG = 2 V, then
M1 is in the ohmic region and M2 is in the active region. Find (a) iD (b) vDS1, (c) vDS2, and
(d) confirm the regions of operation.

Fig. P8.26
8.30 8.30 For the depletion-MOSFET circuit shown in Fig. P8.26, M1 has IDSS = 8 mA and Vp =
4 V, whereas M2 has IDSS = 16 mA and Vp = 4 V. When VDD = 11 V and VGG = 10 V, then
M1 is in the ohmic region and M2 is in the active region. Find (a) iD (b) vDS1, (c) vDS2, and
(d) confirm the regions of operation.

Fig. P8.26

8.31 8.31 An n-channel enhancement MOSFET has K = 0,25 mA/V2, and Vt = 2 V. For small
values of vDS, determine the approximate drain-source resistance rDS when vGS is (a) 4 V,
(b) 6 V, and (c) 10 V.
8.32 8.32 For the circuit given in Fig. P8.32, the enhancement MOSFET has K = 0,15 mA/V2,
Vt = 2 V, and operates in the active region. Suppose that VDD = 12 V. (a) Find VGG when RS
= 0 and iD = 5,4 mA. (b) Find iD when vGS = 4 V. (c) Find vGS when iD = 2,4 mA. (d)
Find RD when vGS = 3 V and vGG = 4,5 V. (e) Find RD when vGS = 3 V, vDS = 7,5 V, and VGG
= 4,5 V. (f) Find vDS when RD = 4 k, vGS = 3 V, and VGG = 4,5 V.

Fig. P8.32

8.33 8.33 For the circuit given in Fig. P8.32, the enhancement MOSFET has K = 0,25 mA/V2
and Vt = 2 V. Given that RS = 0 and VDD = 16 V, determine the value of RD for which the
enhancement MOSFET will operate on the border between the active and the ohmic
regions when VGG is (a) 4 V and (b) 10 V.
 Fig. P8.32

8.34 8.34 For the circuit given in Fig. P8.32, the enhancement MOSFET has K = 0,25 mA/V2
and Vt = 2 V. Given that RD = 1 k, RS = 0 , VDD = 16 V, and VGG = 4 V, find (a) iD (b)
vDS, and (c) the region of operation for the MOSFET.

Fig. P8.32
8.35 8.35 For the circuit given in Fig. P8.32, the enhancement MOSFET has K = 0,25 mA/V2
and v2 = 2 V. Given that RD = R = 1 k, VDD = 20 V, and VGG = 10 V, find (a) vGS, (b) 1 (c)
vDS, and (D) the region of operation for the MOSFET.

Fig. P8.32

8.36 8.36 For the circuit given in Fig. P8.32, the enhancement MOSFET has K = 0,25 mA/V2
and v2, = 2 V. Given that RD = R = 1 k, VDD = 9 V, and VGG = 10 V, find (a) vGS, (b) 1 (c)
vDS, and (D) the region of operation for the MOSFET.

Fig. P8.32
8.37 8.37 For the MOSFET circuit shown in Fig. P8.37, the enhancement NMOS transistor
has K 0,2 mA/V2 and Vt = 1 V. Given that VDD = 10 V and vDS = 6 V, find (a) vGS, (b) the
region of operation, (c) iD, and (d) RD.

Fig. P8.37

8.38 8.38 For the MOSFET circuit shown in Fig. P8.37, the enhancement NMOS transistor
has K = 0,2 mA/V2 and Vt = 1 V. Given that VDD = 10 V and iD2 = 3,2 mA, find (a) the
region of operation, (b) vGS, (c) vDS, and (d) RD.
 Fig. P8.37

8.39 8.39 For the MOSFET circuit shown in Fig. P8.37, the enhancement NMOS transistor
has K = 0,2 mA/V2 and Vt = 1 V. Given that VDD = 10 V and RD = 1 k, find (a) the region
of operation, (b) vGS, (c) vDS, and (d) iD.

Fig. P8.37