1 Bipolar Junction Transistors (BJTs). Copyright 2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith2 Bipolar

1

Bipolar JunctionTransistors (BJTs)

Microelectronic Circuits - Fifth Edition Sedra/Smith 2Copyright 2004 by Oxford University Press, Inc.

• Bipolar Junction Transistors, BJT, a three terminal, non-linear electronic device.

• BJT’s are used as amplifiers in analog electronics.

• BJTs are used as switches in digital electronics.

• Invented in 1948 by Bardeen, Brattain, and Shockley at Bell Telephone Labs.

• Popular for three decades before being overtaken by Field Effect Transistors, FETs.


A simplified structure of the npn transistor.


A simplified structure of the pnp transistor


• Still used in harsh environment and high frequency applications, also Bi-CMOS applications.

• BJTs have the following four modes of operation.• The cutoff and saturation modes are used in digital

electronics to obtain a switch.• The active mode is used in analog electronics to obtain

an amplifier.

Mode EBJ CBJ

Cuttoff Reverse Reverse

Active Forward Reverse

Reverse-Active Reverse Forward

Saturation Forward Forward


Current flow in an npn transistor biased to operate in the active mode. (Reverse current

components due to drift of thermally generated minority carriers are not shown.)


• Ignore the very small drift currents.

• The EBJ is a forward biased pn junction.

• Holes are injected from the base to the emitter and electrons injected from the emitter to the base.

• These two components combine to form the emitter current, iE.

• A highly doped emitter and a lightly doped base keep the hole component of iE small.


Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor

operating in the active mode: vBE > 0 and vCB ≥ 0.


• The concentration np(0), using the Law of the Junction is:

0)(and)0( /0 Wnenn p

Vvpp

TBE

• The electron diffusion current, In, points in the positive x-direction.

direction)negativetheinflowsmeansnegative(,)0(

)0(,AreaEmitter,)(

xIW

nqDAI

W

n

dx

dnA

dx

dnDqAI

np

nEn

ppE

pnEn

• Recombination of electrons with holes in the base cause np(x) to deviate a small amount from ideal.


• The collector current, ic, points in the negative x-direction and is equal in magnitude to In.

WN

nqDAIwhereeIi

N

nne

W

nqDAi

ennW

nqDAi

Ii

A

inEs

VvsC

A

ip

VvpnEC

Vvpp

pnEC

nC

TBE

TBE

TBE

2/

2

0/0

/0

,

,

)0(,)0(

• The collector current is directly proportional to the emitter area, AE, and inversely proportional to the effective width of the base, W.


• The base current, iB, is composed of two components, iB1 and iB2 (iB= iB1+ iB2).

• The first component, iB1, is due to holes injected into the emitter from the base.

TBE Vv

pD

ipEB e

LN

nqDAi /

2

1

• The second component, iB2, is due to the recombination of holes and electrons in the base.

WnqAQeN

qWnAi

Q

Qi

pEnVv

Ab

iEB

bn

b

nB

TBE )0(,2

lifetimecarrierMinority,baseinchargecarrierMinoritywhere

21/

2

2

2


• The base current is the sum of these two components

gaincurrentEmitterCommon,2

where

2

2

2

2

12

//

2

/22

/22

/2

/2

21

nbpDn

Ap

CVv

sVv

nbpDn

ApsB

Vv

nbpDn

Ap

A

inEB

Vv

Ab

iE

pD

ipEB

Vv

Ab

iEVv

pD

ipEB

BBB

D

W

LND

WND

ieIe

D

W

LND

WNDIi

eD

W

LND

WND

WN

nqDAi

eN

qWnA

LN

nqDAi

eN

qWnAe

LN

nqDAi

iii

TBE

TBE

TBE

TBE

TBETBE


• The emitter current, iE, (using KCL) is equal to the base current plus the collector current.

gaincurrentBaseCommon,1

,

1

1

/

/

CVvsE

VvsE

CC

CE

BCE

ie

Ii

eIi

ii

ii

iii

TBE

TBE

• α = αF, β = βF forward active mode, αR and βR for the reverse active mode.


Summary of the BJT Current-Voltage Relationships in the Active Mode

11

)1(

1)1(

/

/

/

BEBC

EEBEC

VvsCE

VvsCB

VvsC

iiii

iiiii

eIi

i

eIi

i

eIi

TBE

TBE

TBE


Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode.


• Scale current relations

AreasCollectorandEmitterand,

CEF

R

SC

SE

C

E

SSCRSEF

AAI

I

A

A

III

Cross-section of an npn BJT.


Model for the npn transistor when operated in the reverse active mode (i.e., with the CBJ

forward biased and the EBJ reverse biased).


The Ebers-Moll (EM) model of the npn transistor.


• This structure yields positive values of iC, iE, and iB in the forward-active mode and is valid for all modes.

DCRDEFB

DEFDCC

DCRDEE

iii

iii

iii

)1()1(

• The diode equations for iDE and iDC are as follows.

)1(

)1(/

/

TBC

TBE

VvSCDC

VvSEDE

eIi

eIi


• Substituting the diode equations into the expressions for iC, iE, and iB and using the scale current relations we obtain the following equations.

)1()1(

)1()1(

)1()1(

//

//

//

TBCTBE

TBCTBE

TBCTBE

Vv

R

SVv

F

SB

Vv

R

SVvSC

VvS

Vv

F

SE

eI

eI

i

eI

eIi

eIeI

i


• In the forward active mode vBE is between 0.6 V and 0.8 V, and vBC is negative. The exponential terms containing vBC will be negligibly small and we obtain

RFS

Vv

F

SB

RS

VvSC

FS

Vv

F

SE

IeI

i

IeIi

IeI

i

TBE

TBE

TBE

11

11

11

/

/

/

• Note that if we ignore the very small second term in these equations we obtain the relations derived earlier.


The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor enters the saturation mode of

operation for vCB < –0.4 V, and the collector current diminishes.


Concentration profile of the minority carriers (electrons) in the base of an npn transistor operating in

the saturation mode.

TBCTBE Vv

R

SVvSC e

IeIi //

Ignoring small terms:

0)( /0 TBC Vv

pp enWn

Law of the Junction:


Current flow in a pnp transistor biased to operate in the active mode.


Large-signal model for the pnp transistor operating in the active mode.


Circuit symbols for BJTs.


Voltage polarities and current flow in transistors biased in the active mode.


Figure 5.15 Circuit for Example 5.1.


Figure E5.10


Figure E5.11


The iC –vBE characteristic for an npn transistor.

TBE VvSC eIi /


Effect of temperature on the iC–vBE characteristic. At a constant emitter current

(broken line), vBE changes by –2 mV/°C.


• Under normal forward-active and saturation modes vBE is between 0.6 V and 0.8 V.

• Use vBE ≈ 0.7 V for rapid calculation.


The iC–vCB characteristics of an npn transistor, common-base characteristic.


• Not quite horizontal lines, some increase in iC as vCB is increased.

• CBJ will breakdown at BVCB0, typically greater than 50 V.

• The collector current in the saturation region (vCB < −0.4 V) can be obtained using the EM equations.

one.minustheIgnoring,1

)1(1

)1()1(

equationEM),1()1(

)1()1(

equationEM),1()1(

/

/

//

//

//

//

TBC

TBC

TBCTBC

TBCTBE

TBCTBE

TBCTBE

VvF

RSEFC

VvF

RSEFC

Vv

R

SVvSFEFC

Vv

R

SVvSC

VvSFEF

VvS

VvS

Vv

F

SE

eIii

eIii

eI

eIii

eI

eIi

eIieI

eIeI

i


Common-Emitter Characteristics.

(a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT. (b) The iC –vCE characteristics of a practical BJT.

Early Voltage


• The linear dependence of the collector current, iC, on the collector-emitter voltage, vCE, is called “The Early Effect”.

A

CEVvSC V

veIi TBE 1/

• The output resistance, ro

TBE

BE

VVS

Ao

vCE

Co

eI

Vr

v

ir

/

1

constant


Large-signal equivalent-circuit models of an npn BJT operating in the active mode in the common-emitter

configuration.


Figure 5.21 Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail.


• Large-signal or dc CE current gain, βdc, (hFE in data books)

BQ

CQdc I

I

• Incremental or ac CE current gain, βac, (hfe in data books)

constant

CEvB

Cac i

i

• βdc and βac differ by 10% to 20%


• Typical dependence of β on IC and on temperature in a modern integrated-circuit npn silicon transistor intended for operation around 1 mA.


• An expanded view of the common-emitter characteristics in the saturation region.


• βac is high in the active region and low in the saturation region.

• IC,sat < βFIB

• βForced = IC,sat /IB < βF

• Overdrive factor: βF / βForced


Figure 5.24 (a) An npn transistor operated in saturation mode with a constant base current IB. (b) The iC–vCE characteristic curve corresponding to iB = IB. The curve can be approximated by a straight line of slope 1/RCEsat. (c) Equivalent-circuit representation of the saturated transistor. (d) A simplified equivalent-circuit model of the saturated transistor.


• Saturation Resistance, RCEsat (~ 3 to 30 ohms)

CsatC

BBIiIiC

CECEsat i

vR

• Model of the saturated transistor

V2.0satsatoffsat CECCECE RIVV

• Using the EM equations for the saturated BJT one obtains the following expression for RCEsat.

)10/(1 BFCEsat IR


Figure 5.25 Plot of the normalized iC versus vCE for an npn transistor with F = 100 and R = 0.1. This is a plot of Eq. (5.47), which is derived using the Ebers-Moll model.


Figure E5.18


Table 5.3


Table 5.3 (Continued)


Figure 5.26 (a) Basic common-emitter amplifier circuit. (b) Transfer characteristic of the circuit in (a). The amplifier is biased at a point Q, and a small voltage signal vi is superimposed on the dc bias voltage VBE. The resulting output signal vo appears superimposed on the dc collector voltage VCE. The amplitude of vo is larger than that of vi by the voltage gain Av.


• Amplification using the common-emitter (CE) configuration.

)()()()( tvVtvVtvtv BEBEbeBEiI

• Lowercase letters with capital subscripts refer to the total signals, dc signal plus the ac signal (small –signal).

• Capital letters with capital subscripts refer to the dc or constant component of the signal.

• Lowercase letters with lowercase subscripts refer to the ac signal or small–signal component.


• The output voltage of the amplifier using the CE configuration biased to operate at the quiescent point, Q, in the active region.

CCCCcCO

VVsCCC

VvsCO

VVvsCCCO

VvsCCCCCCCO

CEoOoO

IRViRv

eIRVeIRveIRVv

eIRViRVv

VvVvv

TBETi

TBEi

TI

//

/)(

/


Small-signal Amplification gain, Av

T

CCv

T

VVSC

v

Vv

VvSCCC

Iv

VvI

Ov

V

IRA

V

eIRA

eIRVdv

dA

dv

dvA

TBE

BEI

TI

BEI

/

/


Figure 5.27 Circuit whose operation is to be analyzed graphically.


Figure 5.28 Graphical construction for the determination of the dc base current in the circuit of Fig. 5.27.


Figure 5.29 Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE in the circuit of Fig. 5.27.


Figure 5.30 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB (see Fig. 5.27).


Figure 5.31 Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE.


Figure 5.32 A simple circuit used to illustrate the different modes of operation of the BJT.


• Switching using the CE configuration as an inverter.

saturation closed,SwitchV,8.0,V2.0

cutoff.open,SwitchV,5.0,

sat ICE

ICCC vV

vVv

• The BJT will be cutoff with vI < 0.5 V.

• The BJT at the edge of saturation (EOS).

CCCCCBCB

BEIB iRVvii

R

Vvi

,,

• The BJT at the edge of saturation (EOS) will have vCB = −0.4 V and vBE = 0.7 V, therefore vC will be 0.3 V (using KVL).


• The collector and base currents at the edge of saturation (EOS).

)EOS(

)EOS()EOS( ,3.0 C

BC

CCC

Ii

R

VI

• The corresponding value of vI required to drive the transistor to the edge-of-saturation.

BEBBI VRIV )EOS()EOS(

• The ratio of IB to IB(EOS) is known as the overdrive factor.


Figure 5.33 Circuit for Example 5.3.


Figure 5.34 Analysis of the circuit for Example 5.4: (a) circuit; (b) circuit redrawn to remind the reader of the convention used in this book to show connections to the power supply; (c) analysis with the steps numbered.


Figure 5.35 Analysis of the circuit for Example 5.5. Note that the circled numbers indicate the order of the analysis steps.


Figure 5.36 Example 5.6: (a) circuit; (b) analysis with the order of the analysis steps indicated by circled numbers.


Figure 5.37 Example 5.7: (a) circuit; (b) analysis with the steps indicated by circled numbers.


Figure 5.38 Example 5.8: (a) circuit; (b) analysis with the steps indicated by the circled numbers.


Figure 5.39 Example 5.9: (a) circuit; (b) analysis with steps numbered.


Figure 5.40 Circuits for Example 5.10.


Figure 5.41 Circuits for Example 5.11.


Figure E5.30


Figure 5.42 Example 5.12: (a) circuit; (b) analysis with the steps numbered.


Figure 5.43 Two obvious schemes for biasing the BJT: (a) by fixing VBE; (b) by fixing IB. Both result in wide variations in IC and hence in VCE and therefore are considered to be “bad.” Neither scheme is recommended.


Figure 5.44 Classical biasing for BJTs using a single power supply: (a) circuit; (b) circuit with the voltage divider supplying the base replaced with its Thévenin equivalent.


Figure 5.45 Biasing the BJT using two power supplies. Resistor RB is needed only if the signal is to be capacitively coupled to the base. Otherwise, the base can be connected directly to ground, or to a grounded signal source, resulting in almost total -independence of the bias current.


Figure 5.46 (a) A common-emitter transistor amplifier biased by a feedback resistor RB. (b) Analysis of the circuit in (a).


Figure 5.47 (a) A BJT biased using a constant-current source I. (b) Circuit for implementing the current source I.


Figure 5.48 (a) Conceptual circuit to illustrate the operation of the transistor as an amplifier. (b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis.

Small-signal circuit.


Figure 5.49 Linear operation of the transistor under the small-signal condition: A small signal vbe with a triangular waveform is superimposed on the dc voltage VBE. It gives rise to a collector signal current ic, also of triangular waveform, superimposed on the dc current IC. Here, ic = gmvbe, where gm is the slope of the iC–vBE curve at the bias point Q.

Q-(VBE, IC ),

dc bias point or quiescent point


• DC currents and voltages, capital letters with capital subscripts.

• DC voltages: VBE, VC, VCE, and VCC.

• DC currents: IC, IE, and IB.

• AC (small-signal) currents and voltages, lowercase letter with lowercase subscripts.

• AC voltages: vbe, vc, and vce

• AC currents: ic, ie, and ib

• Total currents and voltages, lowercase letters with capital subscripts.

• Total voltages: vBE = VBE + vbe, vC = VC + vc, and vCE = VCE + vce.

• Total currents: iC = IC + ic, iE = IE +ie, and iB = IB + ib.


• Small-signal approximation, valid for vbe < 10 mV.

cCbeVI

CVv

CC

axVvCC

VVSC

VvVVSC

VvVSC

VvSC

iIvIIi

axaxeeIi

eIIeeIi

eIi

eIi

T

C

T

be

Tbe

TBETbeTBE

TbeBE

TBE

1

1,1,

,/

///

/

/


• Transconductance, gm, slope of the iC − vBE characteristic at the quiescent point.

bemCC

VvIiBE

C

T

Cm

vgIi

v

i

V

Ig

BEBECC

,

• Small-signal input resistance between the base and emitter, rπ, looking into the base.

B

T

C

T

mb

be

bem

beT

Cb

bBB

beT

CCcCcCCB

I

V

I

V

gi

vr

vg

vV

Ii

iIi

vV

IIiIiIii


• Small-signal resistance between base and emitter, looking into the emitter, re.

mE

T

C

T

me

bee

bem

beT

Ce

eEE

beT

CCcCcCCE

gI

V

I

V

gi

vr

vg

vV

Ii

iIi

vV

IIiIiIii

1

• Relationship between re and rπ

eeb

e

eebbe

rri

ir

ririv

1


• Voltage gain, Av

T

CCCm

be

cv

CbemCcc

cCCcCCCCC

CcCCCCCCCC

V

RIRg

v

vA

RvgRiv

vVRiRIVv

RiIVRiVv

)(

)(

)(


Figure 5.50 The amplifier circuit of Fig. 5.48(a) with the dc sources (VBE and VCC) eliminated (short circuited). Thus only the signal components are present. Note that this is a representation of the signal operation of the BJT and not an actual amplifier circuit.


Figure 5.51 Two slightly different versions of the simplified hybrid- model for the small-signal operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltage-controlled current source (a transconductance amplifier), and that in (b) represents the BJT as a current-controlled current source (a current amplifier).


Figure 5.52 Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance re rather than the base resistance r featured in the hybrid- model.


Figure 5.53 Example 5.14: (a) circuit; (b) dc analysis; (c) small-signal model.


Figure 5.54 Signal waveforms in the circuit of Fig. 5.53.


Figure 5.55 Example 5.16: (a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.


Figure 5.56 Distortion in output signal due to transistor cutoff. Note that it is assumed that no distortion due to the transistor nonlinear characteristics is occurring.


Figure 5.57 Input and output waveforms for the circuit of Fig. 5.55. Observe that this amplifier is noninverting, a property of the common-base configuration.


Figure 5.58 The hybrid- small-signal model, in its two versions, with the resistance ro included.


Figure E5.40


Table 5.4


Figure 5.59 Basic structure of the circuit used to realize single-stage, discrete-circuit BJT amplifier configurations.


Figure E5.41


Table 5.5


Figure 5.60 (a) A common-emitter amplifier using the structure of Fig. 5.59. (b) Equivalent circuit obtained by replacing the transistor with its hybrid- model.


)unilateralsince(,

)normally(,||

)since(,||

inin

in

in

RRrR

rRrRR

rRRRi

vR

i

BB

ibibBi

i

• Input resistance, Rin

• Input Voltage, vi

sig

sig

sigin

in

)assuming(,||

||

division)voltageusing(,

Rr

rvv

rRRrR

rRvv

RR

Rvv

sigi

BB

Bsigi

sigi


• Output voltage, vo

)||||(

)since(),||||(

LCoimo

iLComo

RRrvgv

vvRRrvgv

• Voltage Gain, Av

)||||( LComv

i

ov

RRrgA

v

vA


Cmvo

CoComvo

Comvo

Ri

ovo

RgA

RrtypicallyRrgA

RrgA

v

vA

L

)(),||(

)||||(

• Output resistance, Rout

• Open-circuit voltage Gain, Avo

),unilateralsince(,

)typically(,||

outoCout

CooCout

RRRR

RrrRR


• Overall voltage gain, Gv

)casethefor(,)||||(

)||||(||

||

sig

sig

rR

Rr

RRrG

RRrgRrR

rRG

Av

v

v

v

v

v

v

vG

BLCo

v

LComB

Bv

vsig

i

i

o

sig

i

sig

ov


• Overall voltage gain, Gv, is highly dependent on β if Rsig >> r π, an undesirable property.

• If Rsig << rπ then the overall voltage gain is almost independent of β.

)(),||||( sig rRandrRrRRgG BoLCmv


• Short-circuit current gain, Ais

is

mmis

BBmis

mi

osis

ii

iimos

A

grrgA

rRrRgA

Rgi

iA

Riv

vvvgi

,

,||

,

in

in

• The common-emitter configuration provides large current and voltage gains, but Rin is relatively low and Rout is relatively high.


Figure 5.61 (a) A common-emitter amplifier with an emitter resistance Re. (b) Equivalent circuit obtained by replacing the transistor with its T model.


• Input resistance, Rin

))(1(

1,

||in

eeib

ee

ie

eb

b

iib

ibBi

i

RrR

Rr

viand

iiwhere

i

vR

RRi

vR

• The input resistance looking into the base, Rib, is (β + 1) times the total resistance in the emitter. This is known as the resistance-reflection rule.

eme

e

e

ee

eib

eib

Rgr

Rr

Rr

RR

RR

11

)1(

))(1(

)without(

)includedwith(


• The voltage gain, Av

)1since(,)||(

)||(

)||(

)||(

)||(

ee

LC

ee

LC

i

ov

LCee

i

LCe

LCco

Rr

RR

Rr

RR

v

vA

RRRr

v

RRi

RRiv


• The open-circuit voltage gain, Avo, found by setting RL = ∞

)1(,1

)(,1

1

eemem

Cm

emee

Cm

ee

C

e

ee

Cvo

rrgRg

Rg

rgrR

RgrR

R

r

Rr

RA

• The open-circuit voltage gain, Avo, is reduced by the factor (1 + gmRe). The factor by which Rib is increased. This allows the designer to trade-off gain for input resistance.


• The output resistance, Rout, is Rout = RC.

• The amplifier is unilateral so Ro = Rout

• The short-circuit current gain, Ais

ee

eeis

eeibibB

ee

ibB

i

eis

inii

eRoos

i

os

Ri

ois

Rr

RrA

RrRRR

Rr

RR

v

iRA

Rvi

iii

i

i

i

iA

L

L

))(1(

:then))(1(sinceandif

,)||(in

0

0


• The overall voltage gain, Gv

))(1(

)||(

))(1(sinceandassuming,||Since

)||(

sig

sigsigsigsig

ee

LC

eeibibBibBin

ee

LC

in

inv

i

i

oio

RrR

RRGv

RrRRRRRR

Rr

RR

RR

RA

v

v

v

v

v

v

v

vGv

• The gain is lower than that of the CE amplifier because of the term (β+1)Re in the denominator. However, the gain is also less sensitive to changes in β, a desirable property.


• The CE amplifier with emitter resistor can handle larger input signal without nonlinear distortion, since only a fraction of the input signal at the base appears between the base and emitter.

emee

e

i RgRr

r

v

v

1

1

• Thus, for the same vπ, the signal at the input terminal, vi, can be greater than for the CE amplifier by the factor (1+gmRe)

• Recall that vπ = vi for the CE amplifier


• The input resistance Rib is increased by the factor (1 + gmRe).

• The voltage gain from base to collector, Av, is reduced by the factor (1 + gmRe).

• For the same nonlinear distortion, the input signal vi can be increased by the factor (1 + gmRe).

• The overall voltage gain is less dependent on the value of β.

• The high-frequency response is significantly improved.

• The reduction in gain is the tradeoff for the other improvements.

Summary of the characteristics of the CE amplifier with emitter resistor.


Figure 5.62 (a) A common-base amplifier using the structure of Fig. 5.59. (b) Equivalent circuit obtained by replacing the transistor with its T model.


• The common-base (CB) amplifier, ignoring Early effect.

• The input resistance, Rin = Ri, since unilateral if ro is ignored.

ein rR

• The voltage gain, Av

)||()||(

so),||(

therefore,and),||(

LCmLCei

ov

LCe

io

e

ieLCeo

RRgRRrv

vA

RRr

vv

r

viRRiv

• Same as CE amplifier without minus sign.


• The output resistance, Rout = Ro, ignoring Early effect.

Cout RR

• The short-circuit current gain Ais.

e

e

i

eis i

i

i

iA

• The overall voltage gain, Gv.

e

LC

LCme

ev

i

i

oiov

e

ei

rR

RR

RRgrR

rA

v

v

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v

v

v

v

vG

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r

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• Very low input resistance.

• Short-circuit current gain nearly equal to one.

• Open-circuit voltage gain positive with same magnitude as CE.

• Relatively high output resistance, compared to input resistance.

• Excellent high-frequency performance.

• A significant application of the CB amplifier is as a current buffer.

Summary of Common-Base amplifiers


Figure 5.63 (a) An emitter-follower circuit based on the structure of Fig. 5.59. (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model augmented with ro. (c) The circuit in (b) redrawn to emphasize that ro is in parallel with RL. This simplifies the analysis considerably.


Figure 5.64 (a) An equivalent circuit of the emitter follower obtained from the circuit in Fig. 5.63(c) by reflecting all resistances in the emitter to the base side. (b) The circuit in (a) after application of Thévenin theorem to the input circuit composed of vsig, Rsig, and RB.


Figure 5.65 (a) An alternate equivalent circuit of the emitter follower obtained by reflecting all base-circuit resistances to the emitter side. (b) The circuit in (a) after application of Thévenin theorem to the input circuit composed of vsig, Rsig / ( 1 1), and RB / ( 1 1).


Figure 5.66 Thévenin equivalent circuit of the output of the emitter follower of Fig. 5.63(a). This circuit can be used to find vo and hence the overall voltage gain vo/vsig for any desired RL.


• The common-collector amplifier exhibits a high input resistance.

• The common-collector amplifier exhibits a low output resistance.

• The CC amplifier has a voltage gain smaller than, but very close to unity.

• The CC amplifier is often used as the last stage in a multi-stage amplifier (voltage buffer).

• The CC amplifier is also used to connect a high-resistance source to a low-resistance load.

Summary of the common-collector properties


Summary and Comparisons• The CE configuration is the best suited for realizing

the bulk of the gain required in an amplifier.• Including a resistor in the emitter lead of the CE

amplifier provides a number of performance improvements at the expense of gain reduction.

• The low input resistance of the CB amplifier makes it useful only in specific applications, such as high-frequency applications and as a current buffer.

• The CC is used as a voltage buffer for connecting a high-resistance source to a low-resistance load and as the output stage in a multistage amplifier.


Table 5.6


Figure 5.67 The high-frequency hybrid- model.


Figure 5.68 Circuit for deriving an expression for hfe(s) ; Ic/Ib.


Figure 5.69 Bode plot for uhfeu.


Figure 5.70 Variation of fT with IC.


Table 5.7


Figure 5.71 (a) Capacitively coupled common-emitter amplifier. (b) Sketch of the magnitude of the gain of the CE amplifier versus frequency. The graph delineates the three frequency bands relevant to frequency-response determination.


Figure 5.72 Determining the high-frequency response of the CE amplifier: (a) equivalent circuit; (b) the circuit of (a) simplified at both the input side and the output side; (c) equivalent circuit with C replaced at the input side with the equivalent capacitance Ceq; (d) sketch of the frequency-response plot, which is that of a low-pass STC circuit.


Figure 5.73 Analysis of the low-frequency response of the CE amplifier: (a) amplifier circuit with dc sources removed; (b) the effect of CC1 is determined with CE and CC2 assumed to be acting as perfect short circuits;


Figure 5.73 (Continued) (c) the effect of CE is determined with CC1 and CC2 assumed to be acting as perfect short circuits; (d) the effect of CC2 is determined with CC1 and CE assumed to be acting as perfect short circuits;


Figure 5.73 (Continued) (e) sketch of the low-frequency gain under the assumptions that CC1, CE, and CC2 do not interact and that their break (or pole) frequencies are widely separated.


Figure 5.74 Basic BJT digital logic inverter.


Figure 5.75 Sketch of the voltage transfer characteristic of the inverter circuit of Fig. 5.74 for the case RB 5 10 k, RC 5 1 k, 5 50, and VCC 5 5 V. For the calculation of the coordinates of X and Y, refer to the text.


Figure 5.76 The minority-carrier charge stored in the base of a saturated transistor can be divided into two components: That in blue produces the gradient that gives rise to the diffusion current across the base, and that in gray results from driving the transistor deeper into saturation.


Figure E5.53


Figure 5.77 The transport form of the Ebers-Moll model for an npn BJT.


Figure 5.78 The SPICE large-signal Ebers-Moll model for an npn BJT.


Figure 5.79 The PSpice testbench used to demonstrate the dependence of dc on the collector bias current IC for the Q2N3904 discrete BJT (Example 5.20).


Figure 5.80 Dependence of dc on IC (at VCE 5 2 V) in the Q2N3904 discrete BJT (Example 5.20).


Figure 5.81 Capture schematic of the CE amplifier in Example 5.21.


Figure 5.82 Frequency response of the CE amplifier in Example 5.21 with Rce = 0 and Rce = 130 .


Figure P5.20


Figure P5.21


Figure P5.24


Figure P5.26


Figure P5.36


Figure P5.44


Figure P5.53


Figure P5.57


Figure P5.58


Figure P5.65


Figure P5.66


Figure P5.67


Figure P5.68


Figure P5.69


Figure P5.71


Figure P5.72


Figure P5.74


Figure P5.76


Figure P5.78


Figure P5.79


Figure P5.81


Figure P5.82


Figure P5.83


Figure P5.84


Figure P5.85


Figure P5.86


Figure P5.87


Figure P5.96


Figure P5.97


Figure P5.98


Figure P5.99


Figure P5.100


Figure P5.101


Figure P5.112


Figure P5.115


Figure P5.116


Figure P5.124


Figure P5.126


Figure P5.130


Figure P5.134


Figure P5.135


Figure P5.136


Figure P5.137


Figure P5.141


Figure P5.143


Figure P5.144


Figure P5.147


Figure P5.148


Figure P5.159


Figure P5.161


Figure P5.162


Figure P5.166


Figure P5.167


Figure P5.171

Documents

1 Bipolar Junction Transistors (BJTs). Copyright 2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith2 Bipolar