Điện tử cơ bản - Công thức tính phân cực tranzitor P2

Operation of the npn transistor in the saturation modeSaturation mode: both EBJ and CBJ are forward biased Carrier injection from both emitter and collector into base Base minority ca

CHAPTER 4 BIPOLAR JUNCTION TRANSISTORS (BJTs)

Chapter Outline

4.1 Device Structure and Physical Operation

4.2 Current-Voltage Characteristics

4.3 BJT Circuits at DC

4.4 Applying the BJT in Amplifier Design

4.5 Small-Signal Operation and Models

4.6 Basic BJT Amplifier Configurations

4.7 Biasing in BJT Amplifier Circuits

4 8 Discrete Circuit BJT Amplifiers

4.8 Discrete-Circuit BJT Amplifiers

4.1 Device Structure and Physical Operation

Physical structure of bipolar junction transistor (BJT)

Both electrons and holes participate in the conduction process for bipolar devices

BJT consists of two pn junctions constructed in a special way and connected in series, back to back.

The transistor is a three-terminal device with emitter, base and collector terminals.

From the physical structure, BJTs can be divided into two groups: npn and pnp transistors.

Modes of operation

The two junctions of BJT can be either forward or reverse-biased

The BJT can operate in different modes depending on the junction bias

The BJT operates in active mode for amplifier circuits

Switching applications utilize both the cutoff and saturation modes

Cutoff Reverse Reverse Active Forward Reverse Saturation Forward Forward

Operation of the npn transistor in the active mode

Electrons in emitter regions are injected into base due to the forward bias at EBJ

Most of the injected electrons reach the edge of CBJ before being recombined if the base is narrow

Electrons at the edge of CBJ will be swept into collector due to the reverse bias at CBJ

Emitter injection efficiency () = iEn / ( iEn + iEp)

Base transport factor (T) = iCn/ iEn

Common-base current gain () = iCn / iE = T < 1

Terminal currents of BJT in active mode:

iE(emitter current) = iEn(electron injection from E to B) + iEp(hole injection from B to E)

iC(collector current) = iCn(electron drift) + iCBO(CBJ reverse saturation current with emitter open)

iB(base current) = iB1(hole injection from B to E) + iB2(recombination in base region)

Terminal currents:

Collector current:

Base current:

 Hole injection into emitter due to forward bias:

 Eelectron-hole recombination in base:

 Total base current:

Emitter current:

T BE T

S V v B

i nB E B

nB E B

nB E Cn

W N

n qD A W n

qD A dx x dn qD A i

2

/ ) 0 ( /

i pE E E

pE E

L N

n qD A dx x dp qD A

i E n B

E n n

N

qWn A W

n q A Q

2 2

2 /

) 0 ( 2

1 /

E B nB

pE S B B B

i e

D

W L

W N

N D

D I i i

2 2

2

1 (







Large-signal model and current gain for BJT in active region

E B

nB pE

B

C

D

W L

W N

N D

D i

Common-base current gain:

The structure of actual transistors

In modern process technologies, the BJT utilizes a vertical structure

Typically, is smaller and close to unity while  is large

) 1 /( 

  



Operation of the npn transistor in the saturation mode

Saturation mode: both EBJ and CBJ are forward biased

Carrier injection from both emitter and collector into base

Base minority carrier concentraiton change accordingly  leading to reduced slope as vBCincreases

Collector current drops from the value in active mode for negative vCB

For a given vBE, iC drops sharply to zero at vCBaround 0.5 V and vCEaround 0.2 V

BJT in saturation: VCEsat= 0.2 V

Current gain reduces from  to forced:   saturation 

B

C forced

i i

np0

np0exp(vBE/VT) np0exp(vBC/VT)

vBCincreases

Ebers-Moll model

In EM model, the EBJ and CBJ are represented by two back to back diodes iDEand iDC

The current transported from one junction to the other is presented by F (forward) and R(reverse)

EM model can be used to describe the BJT in any of its possible modes of operation

EM model is used for more detailed dc analysis which can not be performed by the simplified models

The diode currents:

The terminal currents:

Application of the EM model

The forward active mode:

C E

B i i

S SC R SE

1

/V

v

I I

) 1 ( / 

 v BE V T SE

DE I e

SC

DC I e i

DE F DC

i    

DC R DE

V v F

/

T BC T

S V v SE

T BC T

SC V v S

T BC T

R SC V v F SE

i  ( 1   ) /  ( 1   ) /

The cutoff mode

 ICBO (CBJ reverse current with emitter open-circuited)

ICBO= (1RF)ISC

Both EBJ and CBJ are reverse-biased

In real case, reverse current depends on vCB

 ICEO(CBJ reverse current with base open-circuited)

ICEO= ICBO /(1F)

F is always smaller than unity such that ICEO> ICBO

CBJ current flows from (C to B) so CBJ is reverse-biased

EBJ current flows from (E to B) so EBJ is slightly forward-biased

The pnp transistor

Transistor structure:

 emitter and collector are p-type

 base is n-type

Operation of pnp is similar to that of npn

Operation of pnp in the active mode

E i i

Large-signal model and current gain for BJT in active region

Common-basecurrent gain

4.2 Current-Voltage Characteristics

Circuit symbols, voltage polarities and current flow

Terminal currents are defined in the direction as current flow in active mode

Negative values of current or voltage mean in opposite polarity (direction)

Summary of the BJT current-voltage relationships in the active mode

The values of the terminal currents for a BJT in active mode solely depend on the junction voltage of EBJ

The ratios of the terminal currents for a BJT in active mode are constant

The current directions for npn and pnp transistors are opposite.

T

BE V v S

C I e

T

BE V v S C

C I e

T

EB V v S C

pnp transistor npn transistor

Current-voltage characteristics of BJT

The Early effect

As CBJ reverse bias increases, the effective base width Weff reduces due to the increasing CBJ depletion

For a constant junction voltage vBE:

 The slope of nB(x) increases  iCincreases

 Charge storage Qn reduces  iBdecreases

 Current gain  and  increases

Early voltage (VA) is used for the linear approximation of Early Effect

/

A CE V

v S

C

A constant

v CE

C o

I

V v

i r

Common-base output characteristics Early effect breakdown

iCversus vCBplot with various iE as parameter is known as common-base output characteristics

The slope indicates that iCdepends to a small extent on vCB Early effect

i C increases rapidly at high vCB breakdown

BCJ is slightly forward-biased for 0.4V < vCB< 0

No significant change is observed in iC

The BJT still exhibits I-V characteristics as in the active mode

BCJ turns on strongly and the iCstarts to decrease for vBC< 0.4V

 I-V characteristics in the saturation mode and vCEsat is considered a constant ( 0.2 V)

Current gain (): large-signal  iC/iEand small-signal (incremental)   iC/iE

Common-emitter output characteristics (I)

iCversus vCEplot with various vBEas parameter

Common-emitter current gain is defined as  = iC/ iB

The BCJ turns on with a positive vBC at low vCE

BJT operates in saturation mode

The iC curve has a finite slope due to Early effect

The characteristics lines meet at vCE= VA

VAis called the Early Voltage (~ 50 to 100 V)

Common-emitter output characteristics (II)

Plot of iC versus vCEwith various iB as parameter

BJT in active region acts as a current source

with high (but finite) output resistance

The cutoff mode in common-emitter configuration

is defined as iB = 0

Current gain: large-signal dc iC/iBand ac  iC/iB

breakdown Early effect

Saturation of common-emitter configuration

In saturation region, it behaves as a closed switch with a small resistance RCEsat

The saturation IV curve can be approximated by a straight line intersecting the vCEaxis at VCEoff

The saturation voltage VCEsat VCEoff + ICsatRCEsat

Incremental  in saturation is lower than that in active region: forced  ICsat/ I B< 

Overdrive factor  / forced

Transistor breakdown

Transistor breakdown mechanism:

 Avalanche breakdown: avalanche multiplication mechanism takes place at CBJ or EBJ

 Base punch-through effect: the base width reduces to zero at high CBJ reverse bias

In CB configuration, BVCBO is defined at iE = 0

The breakdown voltage is smaller than BVCBOfor iE > 0

In CE configuration, BVCEOis defined at iB=0

The breakdown voltage is smaller than BVCEOfor i B> 0

Typically, BVCEOis about half of BVCBO

Breakdown of the BCJ is not destructive as long as the power dissipation is kept within safe limits

Breakdown of the BCJ is not destructive as long as the power dissipation is kept within safe limits

Breakdown of the EBJ is destructive because it will cause permanent degradation of 

4.3 BJT Circuits at DC

BJT operation modes

The BJT operation mode depends on the voltages at EBJ and BCJ

The I-V characteristics are strongly nonlinear

Simplified models and classifications are needed to speed up the hand-calculation analysis

Active Forward Reverse

Cutoff Reverse Reverse

Saturation Forward Forward

Inverse Reverse Forward

Equivalent circuit models

DC analysis of BJT circuits

Step 1: assume the operation mode

Step 2: use the conditions or model for circuit analysis

Step 3: verify the solution

Step 4: repeat the above steps with another assumption if necessary

Example 4.4

Example 4.5

Example 4.9

Example 4 11

4.4 Applying the BJT in Amplifier Design

BJT voltage amplifier

A BJT circuit with a collector resistor RCcan be used as a simple voltage amplifier

Base terminal is used the amplifier input and the collector is considered the amplifier output

The voltage transfer characteristic (VTC) is obtained by solving the circuit from low to high vBE

Biasing the circuit to obtain linear amplification

The slope in the VTC indicates voltage gain

BJT in active mode can be used as voltage amplification

Point Q is known as bias point or dc operating point

IC= ISexp(VBE/VT)

The signal to be amplified is superimposed on VBE

vBE(t) = VBE + vbe(t)

The time-varying part in vCE(t) is the amplified signal

The circuit can be used as a linear amplifier if:

 A proper bias point is chosen for gain

 The input signal is small in amplitude

The small-signal voltage gain

The amplifier gain is the slope at Q:

Voltage gain depends on ICand RC

Maximum voltage gain of the amplifier

C T

C V

v BE

CE

V

I dv

CE CC C T

C

V

V V

V V R V

I

Determining the VTC by graphical analysis

Provides more insight into the circuit operation

Load line: the straight line represents in effect the load

iC= (VCCVCE)/RC

The operating point is the intersection point

Locating the bias point Q

The bias point (intersection) is determined by properly choosing the load line

The output voltage is bounded by VCC(upper bound) and VCEsat (lower bound)

The load line determines the voltage gain

The bias point determines the headroom or maximum upper/lower voltage swing of the amplifier

The bias point determines the headroom or maximum upper/lower voltage swing of the amplifier

4.5 Small-Signal Operation and Models

The collector current and the transconductance

The total quantities (ac + dc) of the collector current:

Small-signal approximation: vbe << VT

The transconductance indicates the incremental change of iC versus change of vBE

T be T

be T BE T

C V v V V S V v S

C

be BE BE

e I e

e I e

I

i

v V

v

/ /

/ /

C C T

be C

c C

V

I I V

v I

i I

C m

V

I v

i g

The transconductance gm is determined by its dc collector current IC

General, BJTs have relatively high transconductance compared with FETs at the same current level

The base current and the input resistance at the base

The total quantities (ac + dc) of the base current:

Small-signal approximation:

Resistance r is the small-signal input resistance between base and emitter (looking into the base)

T be T

be T BE T

B V v V V S V v S C

B B T

be B

b B

V

I I V

v I

i I

be

I

V g i

v



The emitter current and the input resistance at the emitter

The total quantities (ac + dc) of the emitter current:

E

i I I i I

m m E T e

be

e

g g I

V i

v

be T

E be T

C be m c

V

I v V

I v g i

Output resistance accounting for Early effect

Use the collector current equation with linear vCE dependence:

The output resistance ro is included to represent Early Effect of the BJT

The resulting ro is typically a large resistance and can be neglected to simplify the analysis

e r

v S

C

V

v e

I

i BE/ T 1

C

A constant

v CE

C o

I

V v

i r

BJT small-signal models

Two models are exchangeable and does not affect the analysis result

The hybrid- model

 Typically used as the emitter is grounded

Neglect ro

The T model

 Typically used as the emitter is not grounded

Neglect ro

4.6 Basic BJT Amplifier Configuration

Three basic configurations

Characterizing amplifiers

The BJT circuits can be characterized by a voltage amplifier model (unilateral model)

Common-Emitter (CE) Common-Base (CB) Common-Collector (CC)

The electrical properties of the amplifier is represented by Rin, Ro and Avo

The analysis is based on the small-signal or linear equivalent circuit where dc components are not included

Voltage gain:

Overall voltage gain:

vo o L L i

o

R R

R v

v A







vo so L L sig in

in v

sig in in sig

o

R R

R R

R

R A

R R

R v

v G

The common-emitter (CE) amplifier

Characteristic parameters of the CE amplifier

 Input resistance:

 Output resistance:

 Open-circuit voltage gain:

 Voltage gain:

 Overall voltage gain:

CE amplifier can provide high voltage gain

Input and output are out of phase due to negative gain



r

R in 

C o C

C m o

C m

||

||

sig m

o L C m sig

R r

r g r

R R g R r

r G

Lower ICincreases Rinat the cost of voltage gain

Output resistance is moderate to high

Small RCreduces Roat the cost of voltage gain

The common-emitter (CE) with an emitter resistance

Characteristic parameters (by neglecting ro)

C m e

e

C m vo

R g

R g r

R

R g A

1

L C m L

C

g

 Overall voltage gain:

Emitter degeneration resistance Reis adopted

Input resistance is increased by adding (1+)Re

Gain is reduced by the factor (1+gmRe)

The overall gain is less dependent on 

It is considered a negative feedback of the amplifier

e m C

L e m v

R g R

R R g

e m

L C m sig C

L L e m

C m sig v

R g

R R g R r

r R

R

R R g

R g R r

r G

The common-base (CB) amplifier

Characteristic parameters of the CE amplifier (by neglecting ro)

 Input resistance:

 Output resistance:

 Open-circuit voltage gain:

 Voltage gain:

 Overall voltage gain:

CE amplifier can provide high voltage gain

Input and output are in-phase due to positive gain

vo g R

A  )

e

R r

r G





Input resistance is very low

A single CB stage is not suitable for voltage amplification

Output resistance is moderate to high

Small RCreduces Roat the cost of voltage gain

The amplifier is no longer unilateral if ro is included

The common-collector (CC) amplifier

Characteristic parameters of the CC amplifier (by neglecting ro)

 Input resistance:

 Output resistance:

 Open-circuit voltage gain:

 Overall voltage gain:

CC amplifier is also called emitter follower

Input resistance is very high

) )(

1 ) /(  

 L L e

A

1 )

)(

1 (

) 1

L L sig in

in v

R r R

R r

R

R R R

R G





Output resistance is very low

The voltage gain is less than but can be close to 1

CC amplifier can be used as voltage buffer

It is noted that, in the analysis, the amplifier is not unilateral

4.7 Biasing in BJT Amplifier Circuits

DC bias for BJT amplifier

The amplifiers are operating at a proper dc bias point

Linear signal amplification is provided based on small-signal circuit operation

The DC bias circuit is to ensure the BJT in active mode with a proper collector current IC

The classical discrete-circuit bias arrangement

A single power supply and resistors are needed

Thevenin equivalent circuit:

RCis chosen to ensure the BJT in active (VCE> VCEsat)

A two-power-supply version of the classical bias arrangement

Two power supplies are needed

Similar dc analysis

BJT operating point:

) / 1 1 ( /    



E B

BE BB C

R R

I

) / 1 1 ( /    





E B

BE EE C

R R

V V I