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FET Lesson:
BJT is a current-controlled deviceFET is a voltage-controlled device
BJT: bipolar deviceFET: unipolar device
FET: high input impedanceBJT: higher sensitivity to changes in the applied signal
FET: more temperature stable than BJTFET: usually smaller in construction than BJT
ID = ISIG = 0 (gate current)
VDS > VP, JFET is a current-source device
IDSS means Drain-to-Source current with a Short-circuit connection from Gate to Source.- maximum drain current for JFET @ VGS = 0 V & VDS > |VP|
VGS(off) = -VP
Ohmic region: voltage-controlled resistance region- resistance is controlled by the applied gate-to-source voltage.
2
1
P
GS
OD
V
V
rr
ro = resistance with VGS = 0Vrd = resistance at a particular level of VGS
N-channel JFET: gate is p-type material, drain-to-source is n-typeP-channel JFET: gate is n-type material; drain-to-source is p-type
ID = IDSS( 1 VGS/VP)2
2 types of FET1. JFET (Junction FET)2. MOSFET (Metal Oxide Semiconductor FET)
- Depletion type- Enhancement type (source & drain are totally separated by a diff channel)
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The greater the applied reverse bias, the wider the depletion region.
IG (gate current) is zero..
Seen in datasheet: VGS(off) = VP
As voltage VDS is increased from 0 to few volts, the current will increase.
The relative straightness of the plot reveals that for the region of low values of VDS, theresistance is essentially constant.
As VDS increases & approaches a level referred to as VP (pinch-off voltage), a depletion regionwill widen causing a reduction in the channel width.
As VDS is increased beyond VP, the region of close encounter between the two depletion regionswill increase in length along the channel, but the level of ID remains essentially the same.
Therefore, once VDS > VP, JFET has the characteristics of a current source.
p
n
p
+
+
VGS
VDD
VDS = VP
D
G
S
Saturation region VGS = 0 V
ID
IDSS
VP
VDS
n-channel resistance
Increasing resistance due to narrowing channel
Ohmic region
VGS =2 V
VGS = VP
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IDSS- drain-to-source current with a short circuit connection from gate to source- maximum drain current for a JFET & is defined by the condition,
VGS = 0 V & VDS > VP
Voltage-controlled Resistor
Ohmic region:
- variable resistor as controlled by applied gate-to-source voltage
2
P
GS
Od
V
V1
rr
rO resistance with VGS = 0 Vrd resistance of a particular level of VGS
For an n-channel JFET with rO equal to 10k (VGS=0 V, VP= 6 V), then
rd = 40k @ VGS = 3 V
JFET Symbol
D
G
S
N-channel
D
G
S
P-channel
ID = IDSS VDS
+
LOAD
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Transfer Characteristics
For BJT,
IC
= f(IB)
IC = IB is the constant while IB is the controlled variable
For FET,
2
P
GSDSSD
V
V1II
- IDSS & VP are the constants- Raise to 2nd power means non-linear relation between ID & VGS.
Fixed Bias
Since IG 0 AVRG = IGRG
= (0 A)(RG)= 0 Volt
ID
IDSS
VGS VDS
ID
VGS = 0 V
VGS =1 V
VGS =2 V
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The circuit becomes
Using KVL,
VGG VGS = 0
VS = 0
VDS = VD VS
VD = VDS + VS
VGS = VG VS
VG = VGS + VS
Power Dissipation
DDSD IVP
Derating factor:
The dissipation rating decreases by constant value for every increase in temperature of
certain value above 25C. See datasheet for each FET.
VGS =VGG
VDS = VDD IDRD
VD = VDS
VG = VGS
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SMALL SIGNAL ANALYSIS
For BJT, amplification factor is beta ()
For FET, transconductance factor is gm
ID = gmVGS
GS
Dm
V
Ig
P
GS
P
DSSm
V
V1
V
2Ig
When VGS = 0 (max transconductance curve)
P
DSSmo
V
2Ig
P
GSmom
V
V1gg
Example:
For JFET having IDSS of 10 mA and VP of 5V,
a. Find gmob. Find gm @ VGS = 0.5V
Solution:
a.
4mS5
10mA2
V
2Ig
P
DSSmo
b. @VGS = 0.5V
5V
0.5V14mS
V
V1gg
P
GSmom
= ___________
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2
P
GSDSSD
V
V1II
P
GS
DSS
D
VV1
II
P
GSmom
VV1gg
DSS
Dmom
I
Igg
FET Input Impedance
Zi =
Typical values:
JFET: 1000 MMOSFET: 1015
FET Output Impedance
ZO = rD
OS
dy
r1
D
DSd
I
Vr
VGS is constant
IDSS
VDS
ID
VGS = 0 V
VGS =1 V
VGS =2 V
VDS
ID
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FET Equivalent Circuit
Note: Vgs code used @AC levelVGS code used @DC level
JFET FIXED BIASED
Equivalent circuit:
rd
gm
Vgs
G
S
D
VgsRG RDZ
i ZO
Vin Vout
Input Impedance
Gi RZ
dDO rRZ
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dDgsmO rRVgV
igs VV
dDimO rRVgV
i
dDim
i
OV
V
rRVg
V
VA
dDmV rRgA
Example:
Find gm, rd, Zi, ZO, AV.
a.
mmhos.V
V
V
mA
V
V
V
Ig
P
GS
P
DSSm 8751
8
21
8
1021
2
b. kmhosy
rOS
d 2540
11
c. MRZ Gi 1
d. 8521851252 .kkrRZ dDO
e. 472385185218518751 ..m.ZgrRgA OmdDmV
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JFET SELF-BIAS
A. Bypassed
RG
RS
RD
C1
C3
C2
Vout
+VDD
Vin
Equivalent circuit:
rd
gm
Vgs
G
S
D
VgsRG RD
Zi ZO
Vin Vout
Note: same with Fixed Bias equivalent circuit.
Input Impedance
Gi RZ
dDO rRZ
dDgsmO rRVgV
igs VV
dDimO rRVgV
i
dDim
i
OV
V
rRVg
V
VA
dDmV rRgA
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2
P
GSDSSDq
V
V1II
SDGS RIV
2
P
SDDSSDq
V
RI-1II
2
222
1
P
SDq
P
SDq
DSS
Dq
V
RI
V
RI
I
I
01122
2
2
Dq
DSSP
SDq
P
S I
IV
RI
V
R
In quadratic equation,
2
2
P
S
V
Ra
DSSP
S
IV
Rb
12 1c
roots
a
acbbxIDq
2
42
Possible root values:
Notes: q-pt must be within the transfer curve.
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Example:
Answers:Idq = 12.783mA, 2.816mA
Since IDSS is only 10 mA, IDq = 2.816 mA.
VGSq = -2.816 V
gm = 0.00176887 mhos
rd = 50 k
Zi = 1 M
ZO = 1155.91766
AV = 2.04467342
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B. Unbypassed
Input Impedance
Gi RZ
d
SDSm
DO
r
RRRg
RZ
1
d
SDSm
DmV
rRRRg
RgA
1
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Example:
Answers:Idq = 11.4187mA, 3.1527mA
IDq = 3.1527mA
VGSq = -3.1527 V
gm = 0.00221455 mhos
rd = 50 k
Zi = 1 M
ZO = 999.833284
AV = 2.2141811
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JFET Voltage Divider Bias
R2 RS
RD
C1
C2 Vout
+VDD
Vin
R1
C3
Zi ZO
AC equivalent circuit:
dDgsmO rRVgV
igs VV
dDimO rRVgV
i
dDim
i
OV
V
rRVg
V
VA
dDmV rRgA
Zi = R1 || R2
ZO = rd || RD
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JFET Source Follower (Common-Drain) Configuration
AC Equivalent Circuit:
Gi RZ
mSdO1/gRrZ
Ogsi VVV
Oigs VVV
SdOimO RrVVgV SdOmSdimO RrVgRrVgV
SdimSdmO RrVgRrg1V
Sdm
Sdm
i
OV
Rrg1
Rrg
V
VA
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MOSFET
Channel formation in the N-Channel enhancement-type MOSFET
VDG = VDS - VGS
As VGS is increased beyond the threshold level, the density of free carriers in the inducedchannel will increase, resulting in an increased level of drain current.
However, if we hold VGS to a constant value while increasing the VDS, the drain current will reacha saturation level.
VDS sat = VGS VT
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For values of VGS less than the threshold level the drain current of an enhancement-typeMOSFET is 0 mA.
For levels of VGS > VT,
ID = k(VGS VT)2
2
TonGS
onD
VV
Ik
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P-Channel Enhancement-type MOSFET
Symbols:
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Sample Data Sheet
Using the datasheet, determine k.
22
310
3
VV
mA
VV
Ik
TonGS
onD
k = 0.061 x 10-3A/V2
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MOSFET also used in analog circuits.
MOSFET can be used as precision resistors w/c can have higher controlled resistance than BJT.
Main advantage of BJT over MOSFET is the ability to handle larger current in a smaller space.
In logic circuits, BJT has an advantage over MOSFET. BJTs are able to drive more gates (largerfanout) because they can output more current than MOSFET. Many chips use MOSFET asinput while BiCMOS (BJT-FET mixed) as outputs.
In high speed switching, BJT doesnt have larger capacitance from the gate which whenmultiplied by the resistance of the channel gives the intrinsic time constant of the process. Athigher frequency, MOSFET operates slower.