ee105 fet lect
TRANSCRIPT
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Chapter 7Field Effect Transistors
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Field Effect Transistors (FETs)OBJECTIVES:
describe the construction and basic operation of FETs
identify FET schematic symbols.
understand JFET parameters.
explain the information carried in FET data sheets.
identify the different FET DC bias circuits. determine the DC operating point of FET DC bias circuits.
identify the characteristics that FET amplifiers have in common withBJT amplifiers.
Explain the operation and characteristics of metal oxide
semiconductor field effect transistors (MOSFET) identify the different MOSFET DC bias circuits.
explain the advantages and disadvantages of FET amplifierscompared to BJT amplifiers.
apply troubleshooting principles to FET amplifiers.
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Introduction
1. Field effect transistors control current byvoltage applied to the gate.
2. The FETs major advantage over the BJT ishigh input resistance.
3. Overall, the purpose of the FET is the same asthat of the BJT.
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BJT vs JFET
BipolarJunctionTransistor
Current-based device
IBase controls ICollectorEmitter
JunctionFieldEffectTransistor
Voltage controlled device
VGate controls ISourceDrain
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The JFET Primary Characteristics
Junction field effect transistor controls current flow.
The JFET uses voltage to control the current flow.
You will recall, the transistor uses current flow through thebase-emitter junction to control current.
JFETs can be used as an amplifier just like the BJT.
VGGvoltage level controls current flow in the VDD, RDcircuit.
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The JFET - Labels
The terminals of a JFET are the source, gate, and drain.
A JFET can be either p channel or n channel.
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The JFET - Biasing
The source-drain current iscontrolled by a voltage field at thegate.
That field is developed by thereverse biased gate-source junction
(gate is connected to both sides). With more VGG(reverse bias) the
field grows larger.
This field or resistance limits the
amount of current flow through RD. With low or no VGG current flow
is at maximum.
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The JFET Current Control
(a) Mid-Bias for moderatecurrent flow
(b) Max-Bias for Pinch-off (nocurrent flow)
(c) Low-Bias for maximumcurrent flow
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JFET Characteristics and Parameters Ohmic
Lets first take a look at the effects with a VGSof 0V. ID increasesproportionally with increases ofVDD(VDSincreases as VDDisincreased). This is called the ohmic region (point A to B).
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JFET Characteristics and Parameters Pinch-Off
The point when ID ceases to increase regardless ofVDD increases
(constant current source) is called the pinch-off voltage (point B)
(Note: VGS = 0).
This current is called maximum drain current (IDSS).Breakdown(point C) is reached when too much voltage is applied. This isundesirable, so JFETs operation is always well below this value.
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JFET Characteristics and Parameters Drain CurvesFrom this set of curves you can see increased negative voltageapplied to the gate (-VGS) produces no change in ID. IDis limited
and the pinch-offvoltage (VP) is reduced. Note: VGS controls IDSSThis is the normal work zone for a JFET
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JFET Characteristics and Parameters Cutoff
We know that as VGS is increased IDwill decrease. The point that ID
ceases to increase is called cutoff. The amount ofVGS
required to do
this is called the cutoff voltage (VP). The field (in white) grows such
that it allows practically no current to flow through.
It is interesting to note that pinch-off voltage (VGS(off)) and cutoff
voltage (VP
) are the same value but opposite polarity.
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Pinch-OffVpvs CutoffVGS(off)
VGS(off) and VP are always = and opposite is sign
VP(inch off) - the value of VDS where ID becomes constantwith VGS = 0.
VP(inch off)alsooccurs for VDS
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Example
Vp = 4 V
VDS = 4 VID = I DSS =12 mA
VDD = V RD + VDS = (ID*RD) + VDS = 6.72 + 4 = 10.72 V
For the JFET below, VGSS (off) = - 4 V & IDSS =
12 mA. Determine the minimum value of VDD
required to put the device in the constant area
of operation. Given RD = 560
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JFET Characteristics and Parameters
The transfer characteristic curveillustrates the control VGShas on
IDfrom cutoff (VGS(off)) to pinch-off (VP). Note the parabolic shape. Theformula below can be used to determine drain current.
ID= IDSS(1 - VGS/VGS(off))2 Note:(VGS= 0 to VGS(off)controlsID)
Square-law device: Parabolic curve of the JFET Transfer Characteristic Curve.See Datasheet, for FET, in text pg.339
VGS(OFF)
Vp
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Example
The partial datasheet for a 2N5459 JFET indicates that typically IDSS = 9 mA& VGS(off) = - 8 V maximum. Using these value, determine the drain current
for VGS = 0 V ,-1 V and -4 V.
ID= IDSS(1 - VGS/VGS(off))2 Note:(VGS= 0 to VGS(off)controlsID)
VGS = 0 ; Id = Idss = 9 mA
VGS = -1 V; Id= 6.89 mA
VGS = -4 ; Id = 2.25 mA
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Example
The following information is included on the datasheet for a2N5457 JFET : typically IDSS = 3 mA, VGS(off) = -6V (maximum) &gfs(max) = 5000 S. Using these values, determine the forwardtransconductance for VGS = -4 V & find ID.
gfs(max) = gm0gm = 1.66 mSId = 0.33 mA
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JFET Characteristics and Parameters
Input resistance for a JFET is high since the gate-source junction is
reverse-biased.RIN = lVGS/IGSSl
where: IGSSis the gate reverse current@ a certain gate-to-source voltage.
Drain-to-source resistance is the ratio of changes ofVDS to ID.Large changes in VDSproduce very small changes in ID.
rds = VDS/ID
However, the capacitive effectscan offset this advantage,particularly at high frequencies. (remember varactors !!)
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JFET Biasing
Just as we learned that the bipolar junctiontransistor must be biased for proper operation,the JFET must also be biased for operation.
Lets look at some of the methods for biasing
JFETs.
In most cases the ideal Q-point will be the middle
of the transfer characteristic curve, which is about
half of the IDSS.
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JFET Biasing Self Bias
Self-bias is the most common type of
biasing method for JFETs. No voltage isapplied to the gate. The voltage to
ground from here will always be 0V.
However, the voltage from gate to source
(VGS) will be positive for n channel and
negative for p channel keeping the
junction reverse-biased. This voltage can
be determined by the formulas below.
ID= ISfor all JFET circuits.
(n channel) VGS= +IDRS
(p channel) VGS= -IDRS
VGS
VGS
See Ex.7-6 for derivation of VGS.
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JFET Biasing - Q-Point
Setting the Q-point requires us todetermine a value ofRS that will
give us the desired ID and VGS.The formula below shows therelationship.
RS=| VGS/ID|
To be able to do that we must firstdetermine the VGSandIDfrom the
either the transfer characteristic
curve or more practically from the
formula below. The data sheet
provides the IDSSand VGS(off).
VGSis the desired voltage to setthe bias.
ID= IDSS(1 - VGS/VGS(off))2
Transfer characteristic curveSee Ex.7-7 & 7- 8
V
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Example
Find VDS & VGS for the figurebelow. For this particular JFET, the
internal values such as gm,VGS(off) & IDSS are such that thedrain current, ID of approximately5mA is produced.
VDD = 15 VRD = 1kRs = 220 ID = 5 mARG = 10 M
VDD = VDS + VRS +VRDVDS = VDDVRS VRD = 8.9 VVGS = VG VS =-1.1V
See Ex.7-7 & 7- 8
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JFET Biasing - Q-Point
Determine the value of Rs required to self
bias p channel JFET with :
VGS (off) = 15 V
VGS = 5 V
IDSS =25 mA
Ans : Rs = 450
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JFET Biasing Midpoint Biasing
Since midpoint biasing is mostcommon, lets determine how this isdone.
Step 1. The values ofRSand RDdetermine the approximate midpoint
bias. Half ofIDSSwould be IDmidpoint. The VGS to establish this
can be determined by the formula
below.
Step 2. VGSVGS(off)/3.4IDmidV
*See Appendix B:
2ID = IDSS when VGS = VGS(OFF)/3.4
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JFET Biasing
The value ofRS needed to
establish the computed VGS can
be determined by the previously
discussed relationship below.
Step 3. RS=|VGS/ID|
The value ofRDneeded can be
determined by taking half ofVDD
and dividing it by ID.
Step 4. RD= (VDD/2)/IDV
See Ex. 7-9
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Example
Select resistor values for RD and Rs below to
set up an approximate midpoint bias. For this
particular JFET, the parameter are IDSS = 12
mA & VGS (off) = -3V, VD= 6 V. Given VDD= 12
V and RG = 10 M
ID = 6 mA
VGs= -882 mV
RS = 147
RD = 1 K
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JFET Biasing - Summary
Remember the purpose of biasing isto set a point of operation (Q-point).In a self-biasing type JFET circuit theQ-point is determined by the givenparameters of the JFET itself andvalues ofRSand RD. Setting it at
midpoint on the drain curve is mostcommon.
One thing not mentioned in thediscussion was RG. Its value isarbitrary but it should be largeenough to keep the input resistancehigh.
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JFET Biasing Graphical Analysis
The transfer characteristic curve
along with other parameters can beused to determine the midpoint biasQ-point of a self-biased JFET circuit.
First determine the VGSat IDSS from
the formula below.VGS= -IDRS
#1VGS = -IDRs = (0)(470) = 0V
#2VGS = -IDRS=(10mA)(470)= -4.7V
Where the two lines intersect givesus the IDand VGS (Q-point) neededfor midpoint bias. Note that load lineextends from VGS(off)(ID= 0A) to VP(ID= IDSS)
#1
#2
Load Line
(ID = IDSS/2)Intersect
See Ex. 7-10
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JFET Biasing Voltage-Divider
Voltage-divider bias can also be used tobias a JFET. R1and R2are used to keep the
gate-source junction in reverse bias.
Operation is no different from self-bias.
Determining ID&VGS for a JFET voltage-
divider circuit with VDgiven can be
calculated with the formulas below.
VS=IDRS
VG
= (R2
/R1
+R2
)VDD
VGS=VG-VS
ID= (VDDVD)/RD
See Ex. 7-11
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Example
Determine ID
& VGS
for the JFET with voltagedivider bias below, given that for this particular
JFET the internal parameter values are such
that VD = 7 V.
VDD = 12 VRD = 3.3 k
Rs = 2.2 k
R1 = 6.8 M
R2 = 1 M
VS=IDRS
VG= (R2/R1+R2)VDD
VGS=VG-VS
ID= (VDDVD)/RD
ID=1.5 mA
Vs= 3.3 V
VG= 1.54 V
VGS = -1.76 V
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JFET Biasing Graphical Analysis
In using the transfer characteristiccurve to determine the approx. Q-point, we must establish the twopoints for the load line.
The 1st point is for ID= 0.
(Note: VGS = VG when ID = 0).
VGS =VG = (R2/R1+R2)VDD
The 2nd point is IDwhen VGS
is 0.ID=VG/RS
2nd pt.
1st pt.
Intersect
See Ex. 7-12
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JFET Biasing gm variable
Transfer characteristics can vary for JFETs ofthe same type (just like the of a transistor).
This will adversely affect the Q-point. The
voltage-divider bias is less affected by thisthan self-bias.
This is an undesirable problem that inextreme cases would require trying several of
the same type until you find one that workswithin the desired range of operation.
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Q-Point Instability
Shaded area between Q1 andQ2 illustrates the variability ofQ-point with changes in thetransfer characteristics (gm) ofa selection of replacement
JFETs.
Self-bias exerts no control overgm variability.
Voltage-divider bias offers someimprovement.
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The MOSFET
The metal oxide semiconductor field effect transistor (MOSFET) isthe second category of FETs. The chief difference is that there is no
actual pn junction as the p and n materials are insulated from eachother. MOSFETs are static sensitive devices and must be handled byappropriate means.
These are depletion MOSFETs (D-MOSFET). Note thedifferences in construction with the E-MOSFETs on next slide)
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THE MOSFET
MOSFETs have characteristics similar to JFETs and
additional characteristics that make then very useful
There are 2 types of MOSFETs:
Depletion mode MOSFET (D-MOSFET) Operates in Depletion mode the same way as a
JFET when VGS 0
Operates in Enhancement mode like E-MOSFET
when VGS > 0
Enhancement Mode MOSFET (E-MOSFET) Operates in Enhancement mode
IDSS = 0 until VGS > VT (threshold voltage)
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D-MOSFET
These are depletion MOSFETs (D-MOSFET). Interestingly, they
can also be biased to operate as enhancement mode D-MOSFETS
Note the differences in construction with the E-MOSFETs on next slide)
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The D-MOSFET Basic OperationA D-MOSFET may be biased to operate in two modes:
the Depletion mode or the Enhancement mode
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The D-MOSFET Depletion Mode
The D-MOSFET can beoperated in depletionor enhancementmodes. To be operated
in depletion mode, thegate is made morenegative effectivelynarrowing the channelor depleting the
channel of electrons.
Note the solid lines
indicating D_MOSFET.
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The D-MOSFET Depletion Mode Operation
The transfer characteristics are similar to the JFET
In Depletion Mode operation:
When VGS = 0V, ID = IDSS
When ID = 0 ; VGS = VGS(off)
When VGS(off) = -Vp
The formula used to plot the Transfer Curve, is:
2
GS
D DSS
P
VI = I 1 -
V
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The D-MOSFET Enhancement Mode
To be operated in theenhancement mode thegate is made morepositive, attracting moreelectrons into the channel
for better current flow.Remember we are using nchannel MOSFETs fordiscussion purposes. For pchannel MOSFETs,polarities would change.
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The D-MOSFET Enhancement Mode
2
GS
D DSS
P
VI = I 1 -
V
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E-MOSFET has no physical channel
The enhancement MOSFET (E-MOSFET) has no structural channel. Thechannel is induced thru biasing. For an n-channel device, a +VG inducesa channel to form (must exceed a threshold voltage).
NoChannel
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The E-MOSFET
The E-MOSFET orenhancementMOSFET canoperate in only the
enhancementmode. With apositive voltage onthe gate the psubstrate is made
more conductive.
Note: Broken line indicates E_MOSFET
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Power MOSFETs
The lateral double diffused MOSFET (LDMOSFET) and theV-
groove MOSFET (VMOSFET) are specifically designed for highpower applications.
Dual gate MOSFETs have two gates, which helps controlunwanted capacitive effects at high frequencies.
LDDMOSFET
VMOSFET
Dual Gate MOSFET
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MOSFET Characteristics and Parameters
Since most of the characteristics and parameters
of MOSFETs are the same as JFETs we will coveronly the key differences.
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D-MOSFET Characteristics and Parameters
For the D-MOSFET we have to also consider its enhancement mode.Calculating IDwith given parameters in the enhancement modeanddepletion mode is the same. Note this equation is no different for IDthan JFETs and the transfer characteristics are similar except for itseffect in the enhancement mode.
ID= IDSS(1 - VGS/VGS(off))2
Remember n and p channel polarity differences.See Ex. 7-13
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Example
For a certain D MOSFET, IDSS= 10 mA and VGS(0ff)= -8 V
a) Is this an n channel or p channel ?b) Calculate ID at VGS = -3V ;ans: 3.9mA
c) Calculate ID at VGS= 3 V ;ans:18.9mA
ID= IDSS(1 - VGS/VGS(off))2
Remember n and p channel polarity differences.See Ex. 7-13
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E-MOSFET Characteristics and Parameters
The E-MOSFET for all practical purposes does not conduct until VGS
reaches the threshold voltage (VGS(th)). IDwhen it is when conductingcan be determined by the formulas below. The constant Kmust first
be determined. ID(on)is a data sheet given value.
K= ID(on)/(VGS- VGS(th))2
ID=K(VGS-VGS(th))2
See Ex. 7-14
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Example
The data sheet for a given 2N 7008 E MOSFET gives ID(on) = 500mA
(minimum) at VGS = 10 V & VGS(th) =1V. Determine the drain currentfor VGS = 5 V
K= ID(on)/(VGS- VGS(th))2 ; ans=6.17mA/v 2
ID=K(VGS-VGS(th))2 ; ans= 98.8 mA
See Ex. 7-14
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MOSFET Biasing
The three ways to bias a MOSFET are zero-bias, voltage-dividerbias, and drain-feedback bias.
For D-MOSFET, zero biasing as the name implies has no appliedbias voltage to the gate. The input voltage swings it intodepletionand enhancement mode.VGS = 0, ID = IDSS therefore, no amplification, input isolation only.
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Example
Determine the drain to source voltage for the circuit shown belowwhere VGS(off) = -8 V and IDSS = 12mA ,RD = 620 & VDD= 18 V
VDS = VDD - IDSS RD; ans= 10.6V
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E- MOSFET Biasing
For E-MOSFETs, no zero biasing. Voltage-
divider bias used to set the VGSgreaterthan the threshold voltage (VGS(th)). ID
can be determined as follows. To
determine VGS, normal voltage divider
methods can be used. The following
formula can now be applied.
K= ID(on)/(VGS -VGS(th))2
ID = K(VGS-VGS(th))2
VDScan be determined by application ofOhms law and Kirchhoffs voltage law tothe drain circuit.
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E-MOSFET Biasing
With drain-feedback biasthere is no voltage dropacross RGmaking VGS= VDS.With V
GSgiven determining
ID can be accomplished bythe formula below.
ID= VDD- VDS/RD
l
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Example
Determine VGS & VDS for the E MOSFET
circuit below. Assume ID(on) = 200mA at
VGS = 4V & VGS(th) = 2 V. VDD = 24V ,
R1= 100 k , R2 =15 k & RD = 200.
VGS = [R2/ (R1+R2) ] VDD
VDS = VDDID * RD
K= ID(on)/(VGS -VGS(th))2
ID = K(VGS-VGS(th))2
Summary Table
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January 2004 ELEC 121 55
Summary Table
JFET D-MOSFET E-MOSFET
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Troubleshooting
As always, having a thorough knowledge of the devicesmakes it easier to utilize them for troubleshooting circuits.We will discuss some of the common faults associatedwith FET circuits.
Experience in troubleshooting is the best teacher, andhaving basic theoretical knowledge is extremely helpful.
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Troubleshooting
IfVD= VDDin a self-biased
JFET circuit, it could be one ofseveral opens. It is a clearindication ofno drain current.Use of senses to check forobvious failures is the first andeasiest step. Replace the FETonly if associated componentsare known to be good.
IfVDis less than normal in aself-biased JFET circuit, an
open in the gate circuit ismore than likely the problem.The low drain voltage wouldbe indicative ofmore draincurrent flowing than normal.
less
T o bleshooting
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Troubleshooting
In a zero-biased D-MOSFETor drain-feedback biasedE-MOSFET, an open in thegate circuit is more difficult to
detect. It may seem to bebiased properly with dcvoltages but will fail to workproperly when an ac signal isapplied.
This is a classic FET fault!!
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Troubleshooting
With a voltage-divider biasedE-MOSFET, circuitfaults are moreeasily detected.
With an open R1there is no draincurrent, so theVD= VDD. Withan open R2full
VDD is applied tothe gate turningit on fully. VD= 0
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Summary
JFETs are unipolar devices.
JFETs have three terminals: source, gate, and drain.
JFETs have a high input resistance since the gate-source junction is reverse-biased.
IDSS for all FETs is the maximum amount ofcurrentflow in the drain circuit when VGSis 0V.
All FETs must be biased for proper operation.Midpoint is most common for use in amplifiers.
Unwanted capacitance associated with FETs can bedealt with by using dual gate-type FETs.
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Summary
E-MOSFETs have no physical channel. A channel isinduced with VGSgreater than VGS(th).
There are special MOSFET designs forhighpower applications.
E-MOSFETs have no IDSSparameter.
MOSFETs differ in construction in that the gate isinsulated from the channel.
D-MOSFETs can operate in both depletion andenhancement modes. E-MOSFETs can only operate in
the enhancement mode.