390 SEMICONDUCTOR DEVICES7.4.5For ap-channel JFET in its active region, specify
the polarities of voltages and the directions of
conventional currents.
7.4.6Consider the common-source JFET circuit shown
in Figure P7.4.6 with fixed bias. Sketch the si-
nusoidal variations of drain current, drain volt-
age, and gate voltage superimposed on the direct
values at the operating point. Assume reasonable
common-source drain characteristics.
7.4.7A self-biasedn-channel JFET used in the circuit
of Example 7.4.1 has the characteristics given in
Figure P7.4.7 and a supply voltageVDD= 36
V,RS=1k, andRD=9k. Determine the
operating point and the values ofVGSQ,IDQ,and
VDSQ.
*7.4.8In thep-channel version of the circuit of Example
7.4.1, with the JFET havingVP=4 V andIDSS=
−5 mA, findRDandRSto establishIDQ=− 2
mA andVDSQ=−4 V whenVDD=−12 V.
7.4.9Ann-channel JFET in the circuit configuration
shown in Example 7.4.1 is operating atIDQ=
6 mA andVGSQ =−1 V whenVDS =5V.
DetermineRSandVDDif: (a)RD=2k, and
(b)RD=4k.
7.4.10Ann-channel JFET is given to haveVP =3V
andIDSS=6 mA.
(a) Find the smallest value ofvDSwhenvGS=− 2
V if the operation is to be in the active region.
(b) Determine the correspondingiDfor the small-
estvDS.
7.4.11Given that a siliconn-channel JFET hasVP= 5
V andIDSS=12 mA, check whether the device
is operating in the ohmic or active region when
vGS=− 3 .2 V andiD= 0 .5 mA.
7.4.12For ann-channel JFET withVA=350 V,IDSS=
10 mA, andVP=3V,findVDSthat will cause
iD=11 mA whenvGS=0.
7.4.13Ann-channel JFET withVA=300 V,VP= 2
V, andIDSS=10 mA is to be operated in the
active mode. DetermineiDwhenvDS=10 V and
vGS=− 0 .5V.
7.4.14SketchgmversusvGSfor a JFET withIDSS= 10
mA,VP=3V,VA=100 V, andvDS=10 V. See
what happens ifVA→∞. Also sketchroversus
vGS.
7.4.15The drain current of a JFET in the ohmic region is
approximated by
iD=IDSS[
2(
1 +
vGS
VP)(
vDS
VP)
−(
vDS
VP) 2 ]Assuming smallvDS, find the channel resistance
rDSforvGS=−2 V if the JFET’s parameters are
IDSS=25 mA andVP=3V.
7.4.16Ann-channel enhancement MOSFET operates in
the active region with very largeVA,vGS=6V,
VT=4 V, andiD=1 mA. CalculateK.
7.4.17Consider the MOSFET circuit with variable volt-
age shown in Example 7.4.2, withRD=2k
andVDD=12 V. The static characteristics of
then-channel enhancement MOSFET are given
in Figure P7.4.17.
(a) Draw the load line and find the operating point
ifvGS=4V.
(b) Sketch the resulting transfer curves (i.e.,iD
andvDSas a function ofvGS) showing cutoff,
active, and saturation regions.
(c) For relatively undistorted amplification, the
MOSFET circuit must be restricted to sig-
nal variations within the active region. Let
vGS(t)= 4 + 0 .2 sin ωtV. SketchiD(t)
andvDS(t), and estimate the resulting voltage
amplificationAv.
(d) LetvGS(t)= 6 sinωt, whereωis slow
enough to satisfy the static condition. Sketch
iD(t) andvDS(t) obtained from the transfer
curves. Comment on the action of the MOS-
FET in the switching circuit.
7.4.18Find idealized expressions for the active and
ohmic states and sketch the universal character-
istics of ann-channel enhancement MOSFET op-
erated below breakdown.
7.4.19Let the circuit in Example 7.4.2 haveVDD=12 V
andRD=2k, and let the MOSFET have very
largeVA,VT= 2 .5V,andIDSS= 8 .3 mA.
(a) DeterminevGSandvDSwheniD=4 mA.
(b) DetermineiDandvDSwhen the gate voltage is
6V.
7.4.20Consider the MOSFET connected as a two-
terminal device, as shown in Figure P7.4.20. Dis-
cuss its states of operation.
7.4.21Find the parameter valuesVTandIDSSfor ap-
channel MOSFET withiD=0 whenvGS≤− 3
V, andiD=5 mA whenvGS=vDS=−8V.You
may neglect the effect ofvDSoniD.
7.4.22In a depletion MOSFET for whichVP=3 V and
IDSS=11 mA, the drain current is 3 mA whenvDS
is set at the largest value that will maintain ohmic-
region operation. FindvGSifVAis very large.