Bipolar Junction Transistor Basics

Dr. D G BorseDr. D G Borse

BB

CC

EE


The BJT – Bipolar Junction TransistorThe BJT – Bipolar Junction TransistorNote: Normally Emitter layer is heavily doped, Base layer is lightly Note: Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping.doped and Collector layer has Moderate doping.

The Two Types of BJT TransistorsThe 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 ~ 1099

• Base doping is slightly higher ~ 10Base doping is slightly higher ~ 101010 – 10 – 101111

• Emitter doping is much higher ~ 10Emitter doping is much higher ~ 101717


BJT Current & Voltage - EquationsBJT Current & Voltage - Equations

BB

CCEE

IIEE IICC

IIBB

--

++

VVBEBE VVBCBC

++

--

++-- VVCECE

BB

CCEE

IIEE IICC

IIBB--

++

VVEBEB VVCBCB

++

--

++ --VVECEC

n p nn p n

IIEE = I = IBB + I + ICC

VVCECE = -V = -VBCBC + V + VBEBE

p n pp n p

IIEE = I = IBB + I + ICC

VVECEC = V = VEBEB - V - VCBCB


Figure : Current flow (components) for an n-p-n BJT in the active region. NOTE: Most of the current is due to electrons moving from the emitter through base to the collector. Base current consists of holes crossing from the base into the emitter and of holes that recombine with electrons in the base.

- Electrons+ Holes

VVBEBE

VVCBCB++--

++

--nn++

nn

pp--

IIneneIIpepe

--I I coco

Bulk-recombination Bulk-recombination CurrentCurrent

IIncnc


Physical Structure• Consists of 3 alternate layers of n- and

p-type semiconductor called emitter (E), base (B) and collector (C).

• Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter.

• Carrier transport in the active base region directly beneath the heavily doped (n+) emitter dominates i-v characteristics of BJT.


- - - - - -- - - - - - - - -- - -

- - - - - - - -- - - - - - - -

- - - - - - -- - - - - -- - - - -

-- - - - - -- - - - - -- - - -- - -

- --

- - - - - - - - -- - - - - - - -

- -

--

- - - - -- - - - - - + - - + - -- + - - + - -

RecombinationRecombination

- ElectronsElectrons

+ Holes+ Holes

++

__

++

__

CC

BB

EE

nn

pp

nn

++

IIBB

IIcc

IIEE

VVBEBE

VVCBCB


Figure: An npn transistor with variable biasing sources (common-emitter configuration).

IIncnc

IIneneIIpepe

For CB Transistor IFor CB Transistor IEE= I= Inene+ I+ Ipepe

IIcc= I= Incnc- I- Icoco

And IAnd Icc= - = - ααIIE E + I+ ICoCo

CB Current Gain, CB Current Gain, αα ═ (I ═ (Icc- I- Icoco) .) .

(I(IEE- 0) - 0)

For CE Trans., IFor CE Trans., ICC = = ββIIbb + (1+ + (1+ββ) I) Icoco where where ββ ══ αα , ,

1- 1- α α is CE Gain is CE Gain

IICOCO

Bulk-Bulk-recombination recombination

currentcurrent


Common-Emitter Common-Emitter Circuit DiagramCircuit Diagram

++__VVCCCC

IICCVVCECE

IIBB

Collector-Current CurvesCollector-Current Curves

VVCECE

IICC

Active Active RegionRegion

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 be equal to 0.


BJT’s have three regions of operation:1) Active - BJT acts like an amplifier (most common use)2) Saturation - BJT acts like a short circuit3) Cutoff - BJT acts like an open circuit

BJT is used as a switch by switchingbetween these two regions.

rsat

Vo

_ +

C

B

E

Saturat ion Region Model

Vo

_ +

C

B

E

Active Region Model #1

dc IB

IB

Ro

Vo

_ +

C

B

E

Active Region Model #2

dc IB ICEO

RBB

VCE (V)

IC(mA)

IB = 50 A

IB = 0

30

5 10 15 20 0

0

IB = 100 A

IB = 150 A

IB = 200 A

22.5

15

7.5

Saturation Region

Active Region

Cutoff Region

C

E

B

When analyzing a DC BJT circuit, the BJT is replaced by one of the DC circuit models shown below.

DC Models for a BJT:


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.


Output characteristics: npn BJT (typical)

VCE (V)

IC(mA)

IB = 50 A

IB = 0

30

5 10 15 20 0

0

IB = 100 A

IB = 150 A

IB = 200 A

22.5

15

7.5

Cdc FE

B

I = = h

I

Note: Two key specifications for the BJT are

Bdc and Vo (or assume Vo is about 0.7 V)

Note: The PE review text sometimes uses dc instead of dc.

They are related as follows:

Input characteristics: npn BJT (typical)

VBE (V)

IB(A)

200

0.5 1.0 0

0

VCE = 0

150

100

50

VCE = 0.5 V

VCE > 1 V

The input characteristics look like the characteristics of a forward-biased diode. Note that VBE varies only slightly,

so we often ignore these characteristics and assume:

Common approximation: VBE = Vo = 0.65 to 0.7V

dcdc

dc

= + 1

• Find the approximate values of

dc and dc from the graph.

dc

dc

- 1 dc


Figure: Common-emitter characteristics displaying exaggerated secondary effects.


Figure: Common-emitter characteristics displaying exaggerated secondary effects.


Various Regions (Modes) of Operation of BJT Various Regions (Modes) of Operation of BJT

• 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 short (behaves as on state Switch)causing a virtual short (behaves as on state Switch)

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


BJT Trans-conductance CurveBJT Trans-conductance CurveFor Typical NPN Transistor For 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 e eVVBEBE//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/qkT/q = 26 mV (@ T=300 = 26 mV (@ T=300ooK)K)

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


Three Possible Configurations of BJTThree Possible Configurations of BJT

Biasing the transistor refers to applying voltages to the Biasing the transistor refers to applying voltages to the transistor to achieve certain operating conditions.transistor to achieve certain operating conditions.

1. 1. Common-Base Configuration (CB)Common-Base Configuration (CB) : : input input = V= VEBEB & &

IIEE

output = Voutput = VCBCB & I & ICC

2. 2. Common-Emitter Configuration (CE):Common-Emitter Configuration (CE): input = V input = VBEBE & I & IBB

output= Voutput= VCECE & I & ICC

3. 3. Common-Collector Configuration (CC)Common-Collector Configuration (CC) :input = V :input = VBCBC & I & IBB

(Also known as Emitter follower)(Also known as Emitter follower) output = V output = VECEC & I & IEE


Common-Base BJT Configuration Common-Base BJT Configuration

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 BJT circuit in of the common-base BJT circuit in the different regions of operation. the different regions of operation. Given for a Silicon NPN transistorGiven for a Silicon NPN transistor..


Common-Base (CB) CharacteristicsCommon-Base (CB) Characteristics

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

operation of BJToperation of BJT

VVcc- I- Icc (output) Characteristic Curves (output) Characteristic Curves

Sa

tura

tio

n R

egio

nS

atu

rati

on

Reg

ion

IIEE

IIC C

VVCBCB


CutoffCutoff

IIEE = 0 = 0

0.8V0.8V 2V2V 4V4V 6V6V 8V8V

mAmA

22

44

66

IIEE=1mA=1mA

IIEE=2mA=2mA

Breakdown Reg.Breakdown Reg.


Common-Collector BJT Characteristics Common-Collector BJT Characteristics

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


n p n Transistor: Forward Active Mode Currents

Forward Collector current is

Ico is reverse saturation current

1expT

VBE

VcoI

CI

A910A1810 coI

VT = kT/q =25 mV at room temperature

Base current is given by

1expco

TVBE

V

FF

CI

BI I

50020 F

Emitter current is given by

1expT

VBE

V

F

coIB

IC

IE

I

0.11

95.0

F

FF

is forward common-emitter current gain

is forward common- base current gain

In this forward active operation region,

FB

IC

I

FE

IC

I

VVBEBE

IIEE==

IICC==

IIBB==


Various Biasing Circuits used for BJT

• Fixed Bias Circuit• Collector to Base Bias Circuit• Potential Divider Bias Circuit


The Thermal Stability of Operating Point SIco

The Thermal Stability Factor : SThe Thermal Stability Factor : SIcoIco

SSIcoIco = = ∂∂IIcc

∂∂IIcoco

This equation signifies that IThis equation signifies that Icc Changes S Changes SIcoIco times as fast as I times as fast as Icoco

Differentiating the equation of Collector Current IDifferentiating the equation of Collector Current IC C & rearranging & rearranging the terms we can writethe terms we can write

SSIco Ico ═ 1+═ 1+ββ

1- 1- ββ ( (∂∂IIbb//∂∂IICC))

It may be noted that Lower is the value of SIt may be noted that Lower is the value of S IcoIco better is the stability better is the stability

VVbebe,, ββ


The Fixed Bias Circuit

15 V

C

E

B

15 V

200 k 1 k

The Thermal Stability Factor : SThe Thermal Stability Factor : SIcoIco


∂∂IIcoco

General Equation of General Equation of SSIco Ico Comes out to beComes out to be

SSIcoIco ═ 1 + ═ 1 + ββ

1- 1- ββ ( (∂∂IIbb//∂∂IICC))

VVbebe, , ββ

Applying KVL through Base Circuit we Applying KVL through Base Circuit we can write, can write, IIb b RRbb+ V+ Vbebe= V= Vcccc

Diff w. r. t. IDiff w. r. t. ICC, we get (, we get (∂∂IIbb / ∂I / ∂Icc) = 0) = 0

SSIcoIco= (1+= (1+ββ) is very large) is very large

Indicating high un-stabilityIndicating high un-stability

IIbb

RRbb

RRCC

RRCC


The Collector to Base Bias Circuit

The General Equation for Thermal The General Equation for Thermal Stability Factor,Stability Factor,


∂∂IIcoco

Comes out to beComes out to be

SSIcoIco ═ 1 + ═ 1 + ββ

1- 1- ββ ( (∂∂IIbb//∂∂IICC))

VVbebe, , ββ

Applying KVL through base circuit Applying KVL through base circuit

we can write (Iwe can write (Ibb+ I+ ICC) R) RCC + I + Ib b RRbb+ V+ Vbebe= V= Vcccc

Diff. w. r. t. IDiff. w. r. t. ICC we get we get

((∂∂IIbb / ∂I / ∂Icc) = - R) = - RC C // (R (Rbb + R + RCC))

Therefore, Therefore, SSIcoIco ═ (1+ ═ (1+ ββ) )

1+ 1+ [[ββRRCC//(R(RCC+ R+ Rbb))]]

Which is less than (1+Which is less than (1+ββ), signifying better ), signifying better thermal stabilitythermal stability

VCC

RC

C

E

B

RF

IIcc

IIbb

VVBEBE++

-- IIEE


The Potential Devider Bias Circuit

VCC

RC

C

E

B

VCC

R1

RE R2

The General Equation for Thermal Stability The General Equation for Thermal Stability Factor,Factor, S SIco Ico ═ 1 + ═ 1 + ββ

1- 1- ββ ( (∂∂IIbb//∂∂IICC))

Applying KVL through input base circuit Applying KVL through input base circuit

we can write Iwe can write IbbRRThTh + I + IE E RREE+ V+ Vbebe= V= VThTh

Therefore, ITherefore, IbbRRThTh + (I + (ICC+ I+ Ibb) R) REE+ V+ VBEBE= V= VThTh

Diff. w. r. t. IDiff. w. r. t. ICC & rearranging we get & rearranging we get

((∂∂IIbb / ∂I / ∂Icc) = - R) = - RE E // (R (RThTh + R + REE))

Therefore, Therefore,

This shows that SThis shows that SIIcoco is inversely proportional is inversely proportional to Rto RE E andand It is less than (1+It is less than (1+ββ), signifying better thermal ), signifying better thermal stabilitystability

VCC

RC

C

E

B

RE

RTh

VTh _ +

Thevenin Thevenin Equivalent CktEquivalent Ckt

IICC

IIbb

IICC

IIbb

IICC

Thevenins Thevenins Equivalent Equivalent

VoltageVoltage

Self-bias ResistorSelf-bias ResistorRRthth == R R11*R*R2 2 && Vth Vth == Vcc R Vcc R22

RR11+R+R2 2 RR11+R+R22

ThRR

R

E

EIcoS

1

1


A Practical C E Amplifier Circuit

VCC

RC

C

E

B

VCC

R1

RE R2

Rs Ci

RL

Co

CE vi

vo

+

+

vs

+

_ _

_

io

ii

Common Emitter (CE) Amplifier

Input Signal SourceInput Signal Source


BJT Amplifier (continued)

An 8 mV peak change in vBE gives a 5 A change in iB and a 0.5 mA change in iC.

The 0.5 mA change in iC gives a 1.65 V change in vCE .

If changes in operating currents and voltages are small enough, then IC and VCE waveforms are undistorted replicas of the input signal.

A small voltage change at the base causes a large voltage change at the collector. The voltage gain is given by:

The minus sign indicates a 1800 phase shift between input and output signals.

2061802060008.0

18065.1~

~~

bevcev

vA


A Practical BJT Amplifier using Coupling and Bypass Capacitors

• AC coupling through capacitors is used to inject an ac input signal and extract the ac output signal without disturbing the DC Q-point

• Capacitors provide negligible impedance at frequencies of interest and provide open circuits at dc.

In a practical amplifier design, C1 and C3 are large coupling capacitors or dc blocking capacitors, their reactance (XC = |ZC| = 1/C) at signal frequency is negligible. They are effective open circuits for the circuit when DC bias is considered.

C2 is a bypass capacitor. It provides a low impedance path for ac current from emitter to ground. It effectively removes RE (required for good Q-point stability) from the circuit when ac signals are considered.


D C Equivalent for the BJT Amplifier (Step1)

• All capacitors in the original amplifier circuit are replaced by open circuits, disconnecting vI, RI, and R3 from the circuit and leaving RE intact. The the transistor Q will be replaced by its DC model.

DC Equivalent Circuit


A C Equivalent for the BJT Amplifier (Step 2)

• Coupling capacitor CC and Emitter bypass capacitor CE are replaced by short circuits. • DC voltage supply is replaced with short circuits, which in this case is connected to ground.

RR11IIIIRR22=R=RBB

RRinin

RRoo


A C Equivalent for the BJT Amplifier (continued)

100kΩ4.3kΩ3

R C

RR

30kΩ10kΩ2

R 1

RB

R

• By combining parallel resistors into equivalent RB and R, the equivalent AC circuit above is constructed. Here, the transistor will be replaced by its equivalent small-signal AC model (to be developed).

All externally connected capacitors are All externally connected capacitors are assumed as short circuited elements for ac assumed as short circuited elements for ac

signalsignal


A C Analysis of CE Amplifier1) Determine DC operating point and

calculate small signal parameters

2) Draw the AC equivalent circuit of Amp.

• DC Voltage sources are shorted to ground

• DC Current sources are open circuited

• Large capacitors are short circuits

• Large inductors are open circuits

3) Use a Thevenin circuit (sometimes a

Norton) where necessary. Ideally the

base should be a single resistor + a single

source. Do not confuse this with the DC

Thevenin you did in step 1.

4) Replace transistor with small signal model

5) Simplify the circuit as much as necessary.

Steps to Analyze a Transistor Amplifier

6) Calculate the small signal parameters and gain etc.

Step 1Step 1

Step Step 22

Step Step 33

StepStep 44

StepStep 55 ππ--model model

usedused


Hybrid-Pi Model for the BJT

• The hybrid-pi small-signal model is the intrinsic low-frequency representation of the BJT.

• The small-signal parameters are controlled by the Q-point and are independent of the geometry of the BJT.

Transconductance:

qKT

TV

CI

mg TV ,

Input resistance: Rin

mgo

CI

TVor

Output resistance:

CI

CEV

AV

or

Where, VWhere, VAA is Early Voltage is Early Voltage (V(VAA=100V for npn)=100V for npn)


Hybrid Parameter Model

hi

hrVohohfIiVi

Ii 2

2'

Io

Vo

1

1'

11 12

21 22

i i o i i r o

o i o f i o o

V h I h V h I h V

I h I h V h I h V

Linear Two Linear Two port Deviceport DeviceVVii

IIii IIoo

VVoo


11 12

21 22

0 0

0 0

i i

o ii o

o o

o ii o

V Vh h

V II V

I Ih h

V II V

h-Parameters

h11 = hi = Input Resistanceh12 = hr = Reverse Transfer Voltage Ratioh21 = hf = Forward Transfer Current Ratioh22 = ho = Output Admittance


The Mid-frequency small-signal models

b

e

hoe

hie

hrevce hfeib vbe

ib ic

vce

c

e

+ _

+ +

_ _

h-parameter model

b

e

rd gmv vbe

ib ic

vce

c

e

+ +

_ _

hybrid- model

r v

+

_

b

e

ib vbe

ib ic

vce

c

e

+ +

_ _

re model

re

fe ac o

Alternate names:

h = = =

m C C

o fe doe

ore ie

m

38.92g = I (Note: Uses DC value of I )

nwhere n = 1 (typical, Si BJT)

1 = h r =

h

h = 0 r = h = g

e BB

o fe

o e ie

re

oe doe

26 mVr = (Note: uses DC value of I )

I

= h

r = h

h = 0

1h = 0, or use r =

h

Three Small signal Models of CE TransistorThree Small signal Models of CE Transistor


BJT Mid-frequency Analysis using the hybrid- model:

b

e

rd gmv vi

ii io

vo

c

e

+ +

_ _

mid-frequency CE amplifier circuit

r v

+

_

RC RL RTh vs

+

_

is

RS

A common emitter (CE) amplifier VCC

RC

C

E

B

VCC

R1

RE R2

Rs Ci

RL

Co

CE vi

vo

+

+

vs

+

_ _

_

io

ii

The mid-frequency circuit is drawn as follows:

• the coupling capacitors (Ci and Co) and the

bypass capacitor (CE) are short circuits

• short the DC supply voltage (superposition)• replace the BJT with the hybrid- model

The resulting mid-frequency circuit is shown below.

si

iv

s

i

i

o

s

o

svCLoLLmi

ov RZ

ZA

v

v

v

v

v

vARRrRRg

v

vA where, , ,''

R where, 21

RRrRI

vZ

ThThi

ii

, Co

ovo

oo

Rri

vZ

i

i

oi i

iA

An a c Equivalent CircuitAn a c Equivalent Circuitrroo


Details of Small-Signal Analysis for Gain Av (Using Π-model)

33

RCRC

Ro

rL

R R ,

ivbe

v

bev

ov

ivo

v

vA

LbemRvgv

LR

oI

o

RsRs

RsRs

LR

orR

CR

bev

mg

ov

3

rB

RS

R

rB

R

LR

mg

vA

rB

RS

R

rB

Ri

v

bev

From input circuitFrom input circuit


C-E Amplifier Input Resistance

• The input resistance, the total resistance looking into the amplifier at coupling capacitor C1, represents the total resistance presented to the AC source.

rRRrBRR

rBR

21xixv

in

)(xixv


C-E Amplifier Output Resistance

• The output resistance is the total equivalent resistance looking into the output of the amplifier at coupling capacitor C3. The input source is set to 0 and a test source is applied at the output.

CRorC

RR

mgorC

R

xixv

out

bevxvxv

xi

But vbe=0.

since ro is usually >> RC.


High-Frequency Response – BJT Amplifiers

Capacitances that will affect the high-frequency response:• Cbe, Cbc, Cce – internal capacitances

• Cwi, Cwo – wiring capacitances• CS, CC – coupling capacitors• CE – bypass capacitor


Frequency Response of AmplifiersThe voltage gain of an amplifier is typically flat over the mid-frequency range, but drops drastically for low or high frequencies. A typical frequency response is shown below.

LM(Avi) = 20log(vo/vi) [in dB]

BW

3dB

20log(Avi(mid))

f

fLOW fHIGH

LM Response for a General Amplifier

For a CE BJT: (shown on lower right)• low-frequency drop-off is due to CE, Ci and Co • high-frequency drop-off is due to device capacitances Cp and Cm (combined to form Ctotal)• Each capacitor forms a break point (simple pole or zero) with a break

frequency of the form f=1/(2pREqC), where REq is the resistance seen by the capacitor

• CE usually yields the highest low-frequency break which establishes fLow.


Amplifier Power Dissipation

• Static power dissipation in amplifiers is determined from their DC equivalent circuits.

PDV

CEICV

BEIB

Total power dissipated in C-B and E-B junctions is:

where

Total power supplied is:

BIIII

CI

CCV

SP

12 where ,

2

BEVCB

VCE

V

ER

FEQR

BEV

EQV

BI

RRCC

VI

1 and

211

The difference is the power dissipated by the bias resistors.



Figure 4.36a Emitter follower.


Figure Emitter follower.

Very high input ResistanceVery high input Resistance

Very low out put ResistanceVery low out put Resistance

Unity Voltage gain with no phase shiftUnity Voltage gain with no phase shift

High current gainHigh current gain

Can be used for impedance matching or a Can be used for impedance matching or a circuit for providing electrical isolationcircuit for providing electrical isolation

An Emitter Follower (CC Amplifier) AmplifierAn Emitter Follower (CC Amplifier) Amplifier


Figure 4.36b Emitter follower.


Figure 4.36c Emitter follower.


Capacitor Selection for the CE Amplifier

Zc1

jC Capacitive Reactance XcZc

1C

where 2f

Xc1R

Br Make X

c10.01 R

Br

for < 1% gain error.

Xc2 0 Make X

c21 for <1% gain error.

Xc3R

3 Make X

c30.01 R

3

for <1% gain error.

The key objective in design is to make the capacitive reactance much smaller at the operating frequency f than the associated resistance that must be coupled or bypassed.


Summary of Two-Port Parameters forCE/CS, CB/CG, CC/CD


A Small Signal h-parameter Model of C E - Transistor

= h= h1111

VVcece*h*h1212


A Simple MOSFET Amplifier

The MOSFET is biased in the saturation region by dc voltage sources VGS and VDS = 10 V. The DC Q-point is set at (VDS, IDS) = (4.8 V, 1.56 mA) with VGS = 3.5 V.

Total gate-source voltage is: gsvGS

VGS

v

A 1 V p-p change in vGS gives a 1.25 mA p-p change in iDS and a 4 V p-p changein vDS. Notice the characteristic non-linear I/O relationship compared to the BJT.


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 Modelbelow 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 VoltageThe 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


Potential-Divider Bias Circuit with Emitter FeedbackMost popular biasing circuit.Problem: dc can vary over a wide range for BJT’s (even with the same part number)

Solution: Adding the feedback resistor RE. How large should RE be? Let’s see.

Substituting the active region model into the circuit to the left and analyzing the circuit yields the following well known equation:

VCC

RC

C

E

B

VCC

R1

RE R2

VCC

RC

C

E

B

RE

RTh

VTh _ +

2Th CC Th 1 2

1 2

RV = V and R = R R

R + R

dc Th o CEO Th EC

Th dc E

CEO dc CBO

V - V + I R + R I =

R + + 1 R

where I = + 1 I

ICEO has little effect and is often

neglected yielding the simpler relationship:

dc Th oC

Th dc E

V - V I =

R + + 1 R

Test for stability: For a stable Q-point w.r.t. variations in dc choose:

Th dc ER << + 1 R Why? Because then

dc Th o dc Th o Th oC dc

Th dc E dc E E

V - V V - V V - V I = (independent of )

R + + 1 R + 1 R R

Voltage divider biasing circuit with emitter feedback

Replacing the input circuit by a Thevenin equivalent circuit yields:


PE-Electrical Review Course - Class 4 (Transistors)

Example : Find the Q-point for the biasing circuit shown below.The BJT has the following specifications:

dc = 100, rsat = 100 (Vo not specified, so assume Vo = 0.7 V)15 V

C

E

B

15 V

200 k 1 k

Example : Repeat Example 3 if RC is changed from 1k to 2.2k.



Example Determine the Q-point for the biasing circuit shown.The BJT has the following specifications:

dc varies from 50 to 400, Vo = 0.7 V, ICBO = 10 nA

Solution:

Case 1: dc = 50 C

E

B

18 V

30 k

15 k

10 k

8 k

18 V

Case 2: dc = 400 Similar to Case 1 above. Results are: IC = 0.659 mA, VCE =

6.14 V Summary:



BJT Amplifier Configurations

and Relationships:

Using the hybrid- model.

VCC

RC

C

E

B

VCC

R1

RE R2

Rs Ci

RL

Co

CE vi

vo

+

+

vs

+

_ _

_

io

ii

Common Emitter (CE) Amplifier

'o L' '

vi m L m L 'o L

'L d C L d C L E L

'i Th E Th o L

m

Th S o d C d C E

o

i i ivs vi vi vi

s i s i s i

CE CB CC

1 + RA -g R g R

r + 1 + R

R r R R r R R R R

1Z R r R r R r + 1 + R

g

r + R RZ r R r R R

1 +

Z Z ZA A A A

R + Z R + Z R + Z

i i iI vi vi vi

L L L

P vi I vi I vi I

Th 1 2

Z Z ZA A A A

R R R

A A A A A A A

where R = R R

VCC

RC

E

R2

RE

Rs Ci

RL

Co

C2

vi vo

+

+

vs

+

_

_ _

io ii

Common Base (CB) Amplifier

R1

C

B

VCC

C

E

B

VCC

R1

RE R2

Rs Ci

vi

+

vs

+

_

_

RL

Co

vo

+

_

io

ii

Common Collector (CC) Amplifier (also called “emitter-follower”)

Note: The biasing circuit is

the same for each amplifier.


Figure 4.16 The pnp BJT.


Figure 4.17 Common-emitter characteristics for a pnp BJT.


Figure 4.18 Common-emitter amplifier for Exercise 4.8.


Figure 4.19a BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata temperature of 300K.


Figure 4.19b BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata temperature of 300K.


Figure 4.19c BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata temperature of 300K.


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Bipolar Junction Transistor Basics