bjts

Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EESpring 2002Spring 2002

BB

CC

EE

The Bipolar Junction Transistor_______________________________slide 3The Bipolar Junction Transistor_______________________________slide 3

BJT Relationships – Equations________________________________slide 4BJT Relationships – Equations________________________________slide 4

DC DC and DC and DC _____________________________________________slides 5 _____________________________________________slides 5

BJT Example_______________________________________________slide 6BJT Example_______________________________________________slide 6

BJT Transconductance Curve_________________________________slide 7BJT Transconductance Curve_________________________________slide 7

Modes of Operation_________________________________________slide 8Modes of Operation_________________________________________slide 8

Three Types of BJT Biasing__________________________________slide 9Three Types of BJT Biasing__________________________________slide 9

Common Base______________________slide 10-11Common Base______________________slide 10-11

Common Emitter_____________________slide 12Common Emitter_____________________slide 12

Common Collector___________________slide 13Common Collector___________________slide 13

Eber-Moll Model__________________________________________slides 14-15Eber-Moll Model__________________________________________slides 14-15

Small Signal BJT Equivalent Circuit__________________________slides 16Small Signal BJT Equivalent Circuit__________________________slides 16

The Early Effect___________________________________________slide 17The Early Effect___________________________________________slide 17

Early Effect Example_______________________________________slide 18Early Effect Example_______________________________________slide 18

Breakdown Voltage________________________________________slide 19Breakdown Voltage________________________________________slide 19

Sources__________________________________________________slide 20Sources__________________________________________________slide 20

Table of ContentsTable of Contents


The BJT – Bipolar Junction TransistorThe BJT – Bipolar Junction Transistor


Note: It will be very helpful to go through the Analog Electronics Note: It will be very helpful to go through the Analog Electronics Diodes Tutorial to get information on doping, n-type and p-type materials.Diodes Tutorial to get information on doping, n-type and p-type materials.

The Two Types of BJT Transistors:The Two Types of BJT Transistors:

npnnpn pnppnp

nn pp nnEE

BB

CC pp nn ppEE

BB

CC

Cross SectionCross Section Cross SectionCross Section

BB

CC

EE

Schematic Schematic SymbolSymbol

BB

CC

EE

Schematic Schematic SymbolSymbol

• Collector doping is usually ~ 10Collector doping is usually ~ 1066

• Base doping is slightly higher ~ 10Base doping is slightly higher ~ 1077 – 10 – 1088

• Emitter doping is much higher ~ 10Emitter doping is much higher ~ 101515

BJT Relationships - EquationsBJT Relationships - Equations


BB

CCEE

IIEE IICC

IIBB

--

++

VVBEBE VVBCBC

++

--

++-- VVCECE

BB

CCEE

IIEE IICC

IIBB--

++

VVEBEB VVCBCB

++

--

++ --VVECEC

npnnpn

IIEE = I = IBB + I + ICC

VVCECE = -V = -VBCBC + V + VBEBE

pnppnp

IIEE = I = IBB + I + ICC

VVECEC = V = VEBEB - V - VCBCB

Note: The equations seen above are for the Note: The equations seen above are for the transistor, not the circuit.transistor, not the circuit.

DC DC and DC and DC


= Common-emitter current gain= Common-emitter current gain

= Common-base current gain= Common-base current gain

= I= ICC = I = ICC

IIBB I IEE

The relationships between the two parameters are:The relationships between the two parameters are:

= = = =

+ 1+ 1 1 - 1 -

Note: Note: and and are sometimes referred to as are sometimes referred to as dcdc and and dcdc

because the relationships being dealt with in the BJT because the relationships being dealt with in the BJT are DC.are DC.

BJT ExampleBJT Example


Using Common-Base NPN Circuit ConfigurationUsing Common-Base NPN Circuit Configuration

++__

++__

Given: IGiven: IBB = 50 = 50 A , I A , ICC = 1 mA = 1 mA

Find: IFind: IEE , , , and , and

Solution:Solution:

IIEE = I = IBB + I + ICC = 0.05 mA + 1 mA = 1.05 mA = 0.05 mA + 1 mA = 1.05 mA

= I= ICC / I / IBB = 1 mA / 0.05 mA = 20 = 1 mA / 0.05 mA = 20

= I= ICC / I / IEE = 1 mA / 1.05 mA = 0.95238 = 1 mA / 1.05 mA = 0.95238

could also be calculated using the value of could also be calculated using the value of with the formula from the previous slide. with the formula from the previous slide.

= = = 20 = 0.95238 = 20 = 0.95238

+ 1 21+ 1 21

IICC

IIEE

IIBB

VVCBCB

VVBEBE

EE

CC

BB

BJT Transconductance CurveBJT Transconductance Curve


Typical NPN Transistor Typical NPN Transistor 11

VVBEBE

IICC

2 mA2 mA

4 mA4 mA

6 mA6 mA

8 mA8 mA

0.7 V0.7 V

Collector Current:Collector Current:

IICC = = I IESES eeVVBEBE//VVTT

Transconductance: Transconductance: (slope of the curve)(slope of the curve)

ggmm = = I ICC / / V VBEBE

IIESES = The reverse saturation current = The reverse saturation current

of the B-E Junction.of the B-E Junction.

VVTT = kT/q = 26 mV (@ T=300K) = kT/q = 26 mV (@ T=300K)

= the emission coefficient and is = the emission coefficient and is usually ~1usually ~1

Modes of OperationModes of Operation


• Most important mode of operationMost important mode of operation

• Central to amplifier operationCentral to amplifier operation

• The region where current curves are practically flatThe region where current curves are practically flat

Active:Active:

Saturation:Saturation: • Barrier potential of the junctions cancel each other out Barrier potential of the junctions cancel each other out causing a virtual shortcausing a virtual short

Cutoff:Cutoff: • Current reduced to zeroCurrent reduced to zero

• Ideal transistor behaves like an open switchIdeal transistor behaves like an open switch

* Note: There is also a mode of operation * Note: There is also a mode of operation called inverse active, but it is rarely used.called inverse active, but it is rarely used.

Three Types of BJT BiasingThree Types of BJT Biasing


Biasing the transistor refers to applying voltage to get the Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions.transistor to achieve certain operating conditions.

Common-Base Biasing (CB) :Common-Base Biasing (CB) : input input = V= VEBEB & I & IEE

output = Voutput = VCBCB & I & ICC

Common-Emitter Biasing (CE):Common-Emitter Biasing (CE): input input = V= VBEBE & I & IBB

outputoutput = V= VCECE & I & ICC

Common-Collector Biasing (CC):Common-Collector Biasing (CC): input input = V= VBCBC & I & IBB

output output = V= VECEC & I & IEE

Common-BaseCommon-Base


Although the Common-Base configuration is not the most Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of common biasing type, it is often helpful in the understanding of

how the BJT works. how the BJT works.

Emitter-Current CurvesEmitter-Current Curves

Sa

tura

tio

n R

egio

nS

atu

rati

on

Reg

ion

IIEE

IICC

VVCBCB

Active Active RegionRegion

CutoffCutoff

IIEE = 0 = 0

Common-BaseCommon-Base


Circuit Diagram: NPN TransistorCircuit Diagram: NPN Transistor

++ __ ++ __

IICC IIEE

IIBB

VVCBCB VVBEBE

EECC

BB

VVCECE

VVBEBEVVCBCB

Region of Region of OperationOperation

IICC VVCECE VVBEBE VVCBCBC-B C-B BiasBias

E-B E-B BiasBias

ActiveActive IIBB =V=VBEBE+V+VCECE ~0.7V~0.7V 0V0V Rev.Rev. Fwd.Fwd.

SaturationSaturation MaxMax ~0V~0V ~0.7V~0.7V -0.7V<V-0.7V<VCECE<0<0 Fwd.Fwd. Fwd.Fwd.

CutoffCutoff ~0~0 =V=VBEBE+V+VCECE 0V0V 0V0V Rev.Rev. NoneNone/Rev./Rev.

The Table Below lists assumptions The Table Below lists assumptions that can be made for the attributes that can be made for the attributes of the common-base biased circuit of the common-base biased circuit in the different regions of in the different regions of operation. Given for a Silicon NPN operation. Given for a Silicon NPN transistor.transistor.

Common-EmitterCommon-Emitter


Circuit DiagramCircuit Diagram

++__VVCCCC

IICCVVCECE

IIBB

Collector-Current CurvesCollector-Current Curves

VVCECE

IICC


IIBB

Saturation RegionSaturation RegionCutoff RegionCutoff Region

IIBB = 0 = 0

Region of Operation

Description

Active Small base current controls a large collector current

Saturation VCE(sat) ~ 0.2V, VCE increases with IC

Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0.

Common-CollectorCommon-Collector


Emitter-Current CurvesEmitter-Current Curves

VVCECE

IIEE


IIBB

Saturation RegionSaturation Region

Cutoff RegionCutoff RegionIIBB = 0 = 0

The Common-The Common-Collector biasing Collector biasing circuit is basically circuit is basically equivalent to the equivalent to the common-emitter common-emitter biased circuit except biased circuit except instead of looking at instead of looking at IICC as a function of V as a function of VCECE

and Iand IB B we are looking we are looking

at Iat IEE..

Also, since Also, since ~ 1, and ~ 1, and = I = ICC/I/IEE that means that means

IICC~I~IEE

Eber-Moll BJT ModelEber-Moll BJT Model


The Eber-Moll Model for BJTs is fairly complex, but it is The Eber-Moll Model for BJTs is fairly complex, but it is valid in all regions of BJT operation. The circuit diagram valid in all regions of BJT operation. The circuit diagram below shows all the components of the Eber-Moll Model:below shows all the components of the Eber-Moll Model:

EE CC

BB

IIRRIIFF

IIEE IICC

IIBB

RRIIEERRIICC


Eber-Moll BJT ModelEber-Moll BJT Model

RR = Common-base current gain (in forward active mode) = Common-base current gain (in forward active mode)

FF = Common-base current gain (in inverse active mode) = Common-base current gain (in inverse active mode)

IIESES = Reverse-Saturation Current of B-E Junction = Reverse-Saturation Current of B-E Junction

IICSCS = Reverse-Saturation Current of B-C Junction = Reverse-Saturation Current of B-C Junction

IICC = = FFIIFF – I – IRR IIBB = I = IEE - I - ICC

IIEE = I = IFF - - RRIIRR

IIFF = I = IESES [exp(qV [exp(qVBEBE/kT) – 1]/kT) – 1] IIRR = I = ICC [exp(qV [exp(qVBCBC/kT) – 1]/kT) – 1]

If IIf IESES & I & ICSCS are not given, they can be determined using various are not given, they can be determined using various

BJT parameters.BJT parameters.


Small Signal BJT Equivalent CircuitSmall Signal BJT Equivalent CircuitThe small-signal model can be used when the BJT is in the active region. The small-signal model can be used when the BJT is in the active region.

The small-signal active-region model for a CB circuit is shown below:The small-signal active-region model for a CB circuit is shown below:

iiBBrr

iiEE

iiCCiiBB

BB CC

EE

rr = ( = ( + 1) * + 1) * VVTT

IIEE

@ @ = 1 and T = 25 = 1 and T = 25CC

rr = ( = ( + 1) * 0.026 + 1) * 0.026

IIEE

Recall:Recall:

= I= IC C / I/ IBB

The Early Effect (Early Voltage)The Early Effect (Early Voltage)


VVCECE

IICCNote: Common-Emitter Note: Common-Emitter ConfigurationConfiguration

-V-VAA

IIBB

GreenGreen = Ideal I = Ideal ICC

OrangeOrange = Actual I = Actual ICC (I (ICC’)’)

IICC’ = I’ = ICC V VCECE + 1 + 1

VVAA


Early Effect ExampleEarly Effect Example

Given:Given: The common-emitter circuit below with IThe common-emitter circuit below with IBB = 25 = 25A, A,

VVCCCC = 15V, = 15V, = 100 and V = 100 and VAA = 80. = 80.

Find: a) The ideal collector currentFind: a) The ideal collector current

b) The actual collector currentb) The actual collector current

Circuit DiagramCircuit Diagram

++__VVCCCC

IICCVVCECE

IIBB

= 100 = I= 100 = ICC/I/IBB

a)a)

IICC = 100 * I = 100 * IBB = 100 * (25x10 = 100 * (25x10-6-6 A) A)

IICC = 2.5 mA = 2.5 mA

b) Ib) ICC’ = I’ = ICC V VCECE + 1 + 1 = 2.5x10 = 2.5x10-3-3 15 + 1 15 + 1 = 2.96 mA= 2.96 mA

VVAA 80 80

IICC’ = 2.96 mA’ = 2.96 mA

Breakdown VoltageBreakdown Voltage


The maximum voltage that the BJT can withstand.The maximum voltage that the BJT can withstand.

BVBVCEOCEO = =The breakdown voltage for a common-emitter The breakdown voltage for a common-emitter

biased circuit. This breakdown voltage usually biased circuit. This breakdown voltage usually ranges from ~20-1000 Volts.ranges from ~20-1000 Volts.

BVBVCBOCBO = = The breakdown voltage for a common-base biased The breakdown voltage for a common-base biased

circuit. This breakdown voltage is usually much circuit. This breakdown voltage is usually much higher than BVhigher than BVCEOCEO and has a minimum value of ~60 and has a minimum value of ~60

Volts.Volts.

Breakdown Voltage is Determined By: Breakdown Voltage is Determined By:

• The Base WidthThe Base Width

• Material Being UsedMaterial Being Used

• Doping LevelsDoping Levels

• Biasing VoltageBiasing Voltage

SourcesSourcesDailey, Denton. Dailey, Denton. Electronic Devices and Circuits, Discrete and Integrated.Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New Prentice Hall, New

Jersey: 2001. (pp 84-153)Jersey: 2001. (pp 84-153)

11 Figure 3.7, Transconductance curve for a typical npn transistor, pg 90. Figure 3.7, Transconductance curve for a typical npn transistor, pg 90.

Liou, J.J. and Yuan, J.S. Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and SimulationSemiconductor Device Physics and Simulation. Plenum Press, . Plenum Press, New York: 1998.New York: 1998.

Neamen, Donald. Neamen, Donald. Semiconductor Physics & Devices. Basic Principles.Semiconductor Physics & Devices. Basic Principles. McGraw-Hill, McGraw-Hill, Boston: 1997. (pp 351-409)Boston: 1997. (pp 351-409)

Web SitesWeb Sites

http://www.infoplease.com/ce6/sci/A0861609.html


http://www.infoplease.com/ce6/sci/A0861609.html

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