FET BiasingFET Biasing

CHAPTER 6CHAPTER 6

IntroductionIntroduction

For the JFET, the relationship between input and output For the JFET, the relationship between input and output quantities is nonlinear due to the squared term in quantities is nonlinear due to the squared term in Shockley’s equation.Shockley’s equation.

Nonlinear functions results in curves as obtained for Nonlinear functions results in curves as obtained for transfer characteristic of a JFET.transfer characteristic of a JFET.

Graphical approach will be used to examine the dc Graphical approach will be used to examine the dc analysis for FET because it is most popularly used rather analysis for FET because it is most popularly used rather than mathematical approachthan mathematical approach

The input of BJT and FET controlling variables are the The input of BJT and FET controlling variables are the current and the voltage levels respectivelycurrent and the voltage levels respectively

JFETs differ from BJTs:

Nonlinear relationship between input (VGS) and output (ID) JFETs are voltage controlled devices, whereas BJTs are

current controlled

IntroductionIntroduction

Common FET Biasing Circuits• JFET

– Fixed – Bias – Self-Bias – Voltage-Divider Bias

• Depletion-Type MOSFET– Self-Bias– Voltage-Divider Bias

• Enhancement-Type MOSFET– Feedback Configuration– Voltage-Divider Bias

IntroductionIntroduction

General Relationships For all FETs:

For JFETs and Depletion-Type MOSFETs:

For Enhancement-Type MOSFETs:

AIG 0

SD II

2

P

GSDSSD )

V

V(1II

2)( TGSD VVkI

Fixed-Bias Configuration The configuration includes the ac levels Vi and Vo and The configuration includes the ac levels Vi and Vo and

the coupling capacitors.the coupling capacitors. The resistor is present to ensure that Vi appears at the The resistor is present to ensure that Vi appears at the

input to the FET amplifier for the AC analysis.input to the FET amplifier for the AC analysis.

Fixed-Bias Configuration For the DC analysis,For the DC analysis,

Capacitors are open circuitsCapacitors are open circuits and and

The zero-volt drop across RThe zero-volt drop across RGG permits replacing R permits replacing RGG by a short-circuit by a short-circuit

AIG 0 VRARIV GGGRG 0)0(

Fixed-Bias Configuration

Investigating the input loopInvestigating the input loop

IIGG=0A, therefore=0A, therefore

VVRGRG=I=IGGRRGG=0V=0V

Applying KVL for the input loop,Applying KVL for the input loop,

-V-VGGGG-V-VGSGS=0=0

VVGGGG= -V= -VGSGS

It is called It is called fixed-bias configurationfixed-bias configuration due to V due to VGGGG is a fixed is a fixed

power supply so Vpower supply so VGSGS is fixed is fixed

The resulting current,The resulting current,

2)1(P

GSDSSD V

VII

Investigating the graphical approach.Investigating the graphical approach. Using below tables, we Using below tables, we can draw the graphcan draw the graph

VVGSGS IIDD

00 IIDSSDSS

0.3V0.3VPP IIDSSDSS/2/2

0.50.5 IIDSSDSS/4/4

VVPP 0mA0mA

The fixed level of VThe fixed level of VGSGS has been superimposed as a has been superimposed as a

vertical line at vertical line at

At any point on the vertical line, the level of VAt any point on the vertical line, the level of VGG is -V is -VGGGG--- ---

the level of Ithe level of IDD must simply be determined on this vertical must simply be determined on this vertical

line.line.

The point where the two curves intersect is the common The point where the two curves intersect is the common

solution to the configuration – commonly referrers to as solution to the configuration – commonly referrers to as

the the quiescent quiescent or operating point.or operating point.

The quiescent level of IThe quiescent level of IDD is determine by drawing a is determine by drawing a

horizontal line from the Q-point to the vertical Ihorizontal line from the Q-point to the vertical IDD axis. axis.

GGGS VV

Output loopOutput loop

DDDDDS RIVV

VVS 0

SDDS VVV

SDSD VVV 0SV

DSD VV

SGGS VVV

SGSG VVV 0SV

GSG VV

ExampleExample

Determine VDetermine VGSGSQQ, I, IDDQQ, V, VDSDS, V, VDD, V, VGG, V, VSS

ExerciseExercise

Determine IDetermine IDDQQ, V, VGSGSQQ, V, VDSDS, V, VDD, V, VGG and V and VSS

Self Bias ConfigurationSelf Bias Configuration The self-bias configuration eliminates the need for two The self-bias configuration eliminates the need for two

dc supplies.dc supplies. The controlling VThe controlling VGSGS is now determined by the voltage is now determined by the voltage

across the resistor Racross the resistor RSS

For the indicated input loop:

Mathematical approach:

rearrange and solve.

SDGS RIV

2

2

1

1

P

SDDSSD

P

GSDSSD

V

RIII

V

VII

Graphical approachGraphical approach Draw the device transfer characteristicDraw the device transfer characteristic Draw the network load lineDraw the network load line

UseUse to draw straight line. to draw straight line. First point,First point, Second point, any point from ISecond point, any point from IDD = 0 to I = 0 to IDD = I = IDSSDSS. Choose. Choose

the quiescent point obtained at the intersection of the the quiescent point obtained at the intersection of the straight line plot and the device characteristic curve.straight line plot and the device characteristic curve.

The quiescent value for IThe quiescent value for IDD and V and VGS GS can then be can then be determined and used to find the other quantities of determined and used to find the other quantities of interest.interest.

SDGS RIV 0,0 GSD VI

2

2

SDSSGS

DSSD

RIV

thenI

I

For output loopFor output loopApply KVL of output loopApply KVL of output loopUse IUse IDD = I = ISS

RDDDSDSD

SDS

DSDDDDS

VVVVV

RIV

RRIVV

)(

ExampleExampleDetermine VDetermine VGSGSQQ, I, IDDQQ,V,VDSDS,V,VSS,V,VGG and V and VDD..

ExampleExampleDetermine VDetermine VGSGSQQ, I, IDDQQ, V, VDD,V,VGG,V,VSS and V and VDSDS..

Voltage-Divider BiasVoltage-Divider Bias The arrangement is the same as BJT but the DC analysis is The arrangement is the same as BJT but the DC analysis is

differentdifferent In BJT, IIn BJT, IBB provide link to input and output circuit, in FET V provide link to input and output circuit, in FET VGSGS does does

the samethe same

Voltage-Divider Bias The source VThe source VDD DD was separated into two equivalent sources to permit was separated into two equivalent sources to permit

a further separation of the input and output regions of the network.a further separation of the input and output regions of the network.

IIG G = 0A ,Kirchoff’s current law requires that I= 0A ,Kirchoff’s current law requires that IR1R1= I= IR2R2 and the series and the series

equivalent circuit appearing to the left of the figure can be used to equivalent circuit appearing to the left of the figure can be used to find the level of Vfind the level of VGG. .

21

DD2G

RR

VRV

SDGGS

RSGSG

RIVV

VVV

0

Voltage-Divider Bias

VVG G can be found using the voltage divider rule : can be found using the voltage divider rule :

Using Kirchoff’s Law on the input loop: Using Kirchoff’s Law on the input loop:

Rearranging and using ID =IS: Rearranging and using ID =IS:

Again the Q point needs to be established by Again the Q point needs to be established by

plotting a line that intersects the transfer curve.plotting a line that intersects the transfer curve.

Procedures for plottingProcedures for plotting

1. Plot the line: By plotting two points: VGS = VG, ID =0 and VGS = 0, ID = VG/RS

2. Plot the transfer curve by plotting IDSS, VP and calculated values of ID.

3. Where the line intersects the transfer curve is the Q point for the circuit.

Once the quiescent values of IOnce the quiescent values of IDQDQ and V and VGSQGSQ are determined, the are determined, the

remaining network analysis can be found.remaining network analysis can be found.

Output loop:Output loop:

2121 RR

VII DDRR

)( SDDDDDDS RIRIVV

DDDDD RIVV

SDS RIV

Effect of increasing values of REffect of increasing values of RSS

ExampleExampleDetermine IDetermine IDDQQ, V, VGSGSQQ, V, VDD, V, VSS, V, VDS DS and Vand VDGDG..

ExampleExampleDetermine IDetermine IDDQQ, V, VGSGSQQ, V, VDS, DS, VVDD and V and VSS

Depletion-type MOSFET bias circuits are similar to JFETs. The only difference is that the depletion-Type MOSFETs can operate with positive values of VGS and with ID values that exceed IDSS.

Depletion-Type MOSFETs

The DC AnalysisThe DC Analysis Same as the FET calculationsSame as the FET calculations

Plotting the transfer characteristics of the devicePlotting the transfer characteristics of the device Plotting the at a point that VPlotting the at a point that VGSGS exceeds the 0V or more positive values exceeds the 0V or more positive values Plotting point when VPlotting point when VGSGS=0V=0V andand IIDD=0A=0A The intersection between Shockley characteristics and linear The intersection between Shockley characteristics and linear

characteristics defined the Q-point of the MOSFETcharacteristics defined the Q-point of the MOSFET

The problem is that how long does the transfer characteristics have to The problem is that how long does the transfer characteristics have to be draw?be draw? We have to analyze the input loop parameter relationship.We have to analyze the input loop parameter relationship. As RAs RSS become smaller, the linear characteristics will be in narrow slope become smaller, the linear characteristics will be in narrow slope

therefore needs to consider the extend of transfer characteristics for therefore needs to consider the extend of transfer characteristics for example of voltage divider MOSFET,example of voltage divider MOSFET,

The bigger values of VThe bigger values of VPP the more positive values we should draw for the the more positive values we should draw for the transfer characteristicstransfer characteristics

SDGGS

RSGSG

RIVV

VVV

0

Depletion-Type MOSFETs

Analyzing the MOSFET circuit for DC Analyzing the MOSFET circuit for DC analysisanalysis

How to analyze dc How to analyze dc analysis for the shown analysis for the shown network?network? It is a …. Type networkIt is a …. Type network Find VFind VG G or V or VGSGS

Draw the linear Draw the linear characteristicscharacteristics

Draw the transfer Draw the transfer characteristicscharacteristics

Obtain VObtain VGSQGSQ and I and IDQDQ from from

the graph intersectionthe graph intersection

1. Plot line for VGS = VG, ID = 0 and ID = VG/RS, VGS = 0

2. Plot the transfer curve by plotting IDSS, VP and calculated values of ID.

3. Where the line intersects the transfer curve is the Q-point.Use the ID at the Q-point to solve for the other variables in the voltage-divider bias

circuit. These are the same calculations as used by a JFET circuit.

When RS change…the linear characteristics will change..

1. Plot line for VGS = VG, ID = 0 and ID = VG/RS, VGS = 0

2. Plot the transfer curve by plotting IDSS, VP and calculated values of ID.

3. Where the line intersects the transfer curve is the Q-point.

Use the ID at the Q-point to solve for the other variables in the voltage-divider bias

circuit. These are the same calculations as used by a JFET circuit.

The transfer characteristic for the enhancement-type MOSFET is very different from that of a simple JFET or the depletion-typeMOSFET.

Enhancement-Type MOSFET

Transfer characteristic for E-MOSFETTransfer characteristic for E-MOSFET

andand

2)( )( ThGSGSD VVkI

2)()(

)(

)( ThGSonGS

onD

VV

Ik

Feedback Biasing Arrangement

IG =0A, therefore VRG = 0V

Therefore: VDS = VGS

Which makes DDDDGS RIVV

1. Plot the line using VGS = VDD, ID = 0 and ID = VDD / RD and VGS = 0

2. Plot the transfer curve using VGSTh , ID = 0 and VGS(on), ID(on); all given in the

specification sheet.3. Where the line and the transfer curve intersect is the Q-Point.4. Using the value of ID at the Q-point, solve for the other variables in the bias

circuit.

Feedback Biasing Q-Point

DC analysis step for Feedback Biasing DC analysis step for Feedback Biasing Enhancement type MOSFETEnhancement type MOSFET

Find k using the datasheet or specification given;Find k using the datasheet or specification given;

ex: Vex: VGS(ON)GS(ON),V,VGS(TH)GS(TH)

Plot transfer characteristics using the formula Plot transfer characteristics using the formula

IIDD=k(V=k(VGSGS – V – VTT))22. Three point already defined that is I. Three point already defined that is ID(ON)D(ON), ,

VVGS(ON)GS(ON) and V and VGS(TH)GS(TH)

Plot a point that is slightly greater than VPlot a point that is slightly greater than VGSGS Plot the linear characteristics (network bias line)Plot the linear characteristics (network bias line) The intersection defines the Q-pointThe intersection defines the Q-point

ExampleExampleDetermine IDetermine IDDQQ and V and VDSDSQQ for network below for network below

Again plot the line and the transfer curve to find the Q-point.Using the following equations:

21

DD2G

RR

VRV

)( DSDDDDS

SDGGS

RRIVV

RIVV

Input loop :

Output loop :

Voltage-Divider Biasing

1. Plot the line using VGS = VG = (R2VDD)/(R1 + R2), ID = 0 and ID = VG/RS

and VGS = 0

2. Find k

3. Plot the transfer curve using VGSTh, ID = 0 and VGS(on), ID(on); all given in the specification sheet.

4. Where the line and the transfer curve intersect is the Q-Point.

5. Using the value of ID at the Q-point, solve for the other variables in the bias circuit.

Voltage-Divider Bias Q-Point

ExampleExampleDetermine IDetermine IDDQQ and V and VGSGSQQ and V and VDS DS for for

network belownetwork below

== -

-

=-=

- + )(

=

= -+

= - )( +

==

-)( ++ -

==

=-==

= -

= -= -

=+-=

= - +( )

== -

=

=+-

TroubleshootingTroubleshooting

N-channel VN-channel VGSQGSQ will be 0V or negative if properly checked will be 0V or negative if properly checked

Level of VLevel of VDSDS is ranging from 25%~75% of V is ranging from 25%~75% of VDDDD. If 0V . If 0V

indicated, there’s problemindicated, there’s problem Check with the calculation between each terminal and Check with the calculation between each terminal and

ground. There must be a reading, Rground. There must be a reading, RGG will be excluded will be excluded

For p-channel FETs the same calculations and graphs are used, except that the voltage polarities and current directions are the opposite. The graphs will be mirrors of the n-channel graphs.

P-Channel FETs

• Voltage-Controlled Resistor

• JFET Voltmeter

• Timer Network

• Fiber Optic Circuitry

• MOSFET Relay Driver

Practical Applications