bjtbjt as as anan amplifieramplifier &&aaswitch,switch...

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BJTBJT asas anan AmplifierAmplifier && aa Switch,Switch, LargeLargeSignalSignal Operation,Operation, GraphicalGraphical Analysis,Analysis,BJTBJT atat DCDC BiasingBiasing BJTBJT SmallSmall SignalSignal

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BJTBJT atat DC,DC, BiasingBiasing BJT,BJT, SmallSmall SignalSignalOperationOperation Model,Model, HybridHybrid PiPi--Model,Model,TT ModelModel..

Lecture # 7Lecture # 7

Direction of Current Flow & Operation for Direction of Current Flow & Operation for Amplifier ApplicationAmplifier Application

Copyright 2004 by Oxford University Press, Inc.2

TBE VvSC

CEBConnBCBC

BEBEonBEonBE

eIi

VvVVVv

VvVVVv

3.0 Typically, 4.0;

7.0 Typically, 5.0;

0

TEB VvSC

ECCBonnCBCB

EBEBonEBonEB

eIi

VvVVVv

VvVVVv

3.0 Typically, 4.0;

7.0 Typically, 5.0;

0

2

Large Signal Equivalent ModelLarge Signal Equivalent Model

TBE VvSB e

Ii

TEB VvSB e

Ii


TBE

TBE

VvS

AO

A

CEVvSC

eI

Vr

V

veIi

1

TEB

TEB

VvS

AO

A

ECVvSC

eI

Vr

V

veIi

1

EbersEbers--Moll ModelMoll Model

)1(

)1(

TBC

TBE

Vv

VvSEDE

eIi

eIi

)1(

)1(

TCB

TEB

Vv

VvSEDE

eIi

eIi


)1( SCDC eIi )1( SCDC eIi

Area

Area

EBJ

CBJ

I

I

III

R

F

SE

SC

SSCRSEF

3

Operation in Saturation ModeOperation in Saturation Mode

VVvVVVv

VVvVVVv

BCBConnBCBC

BEBEonBEonBE

6.05.0 Typically, 4.0;


0

VVvVVVv

VVvVVVv

CBCBonnCBCB

EBEBonEBonEB



0


VVVv CEsatCE 2.01.0 VVVv CEsatCE 2.01.0

Fforced

BforcedCsat II

Beta forced means that the transistor is operating in saturated mode. The ratio of Beta forward and the forced beta is called overdrivefactor, the more transistor goes into saturation the voltage VCE will also be lower.

Transfer Characteristics of CE AmplifierTransfer Characteristics of CE Amplifier

TI

TITBE

VvSCCCCEO

VvS

VvSC

eIRVvv

eIeIi

The resistance RC has two functions,firstly, to establish the desired DC biasvoltage at the collector and secondconverting the current signal into voltage.

CCCCCEO iRVvv

C

CEsatCCCsat

d

R

VVI


(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.

T

CECCv

VvI

Ov

V

VVA

dv

dvA

BEI

4

Example 5.2Example 5.2


?

ˆ , 0

BE

ivT

CECCv

C

CECCC

v

VAVV

VVA

R

VVIIC = 1 mA

VBE = 0.6908 VAv = -272 V/VVo = 1.36 V∆ VBE = 12 mV∆ VBE = -105.5 mV

Exercise 5.19Exercise 5.19

CCCCC

T

CCv

VVS iN ti

RIVV

V

RIA


v

CCE

CCE

A

VVAmplitudeInput

VVSwingNegative

5

Graphical AnalysisGraphical Analysis

CCCCCE RiVv


. & between iprelationsh lineae a shows this1

CCECECC

CCC ivv

RR

Vi

This linear relationship can be represented by a straight line.

Graphical AnalysisGraphical Analysis

BBBBBE RiVv CCCCCE RiVv


BBBEBB RivV CE

CC

CCC

CCCCC

vRR

Vi

1

6

Bias Point LocationBias Point Location


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. above).

Bias Point LocationBias Point Location


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.

7



To use it as switch we utilize the cutoff and saturation modes of operations. A simple circuit used to illustrate the different modes of operations of the BJT.

Operation as a SwitchOperation as a Switch

iRVv

vv

iiR

Vvi

CCCCC

BC

BCB

BEiB

holds. thisas long as biased reversed is CBJ so 4.0

,

R

VVI

V

Vv

VRIV

VvI

I

R

VI

iRVv

C

CEsatCCCsat

CE

EOSIi

BEBEOSBEOSI

BEEOSC

EOSB

C

CCEOSC

CCCCC

)(

)()(

)()(

)(

whichdesiredatoperatetoBJTforcecanonesaturationinthatnotedbeshouldIt

as slightly, changeonly but saturation

intodeeper BJT sendich current wh base theincrease beyond Increasing

).saturation of edge toBJT thedrives vi(where

7.0 assume when we

saturation of Edge 3.0


factordriveoverI

I

I

I

EOSB

B

B

Csat

)(

forced

forced. called is

which,desiredat operatetoBJTforcecan one saturationin that noted be shouldIt

8


B

BEBB

EOSB

B

CsatEOSB

C

CEsatCCCsat

CEsatC

I

vVR

factordriveoverI

I

II

R

VVI

VVV

)(

min)(

2.0


BI

IC(sat) = 9.8 mAIB(EOS) = 0.196 mAIB = 1.96 mARB = 2.2 K



9


0

E

EE

BEBE

R

VI

VVV

1

1

EB

CCCCC

EC

II

RIVV

II



0

E

EE

BEBE

R

VI

VVV

1

1

EB

CCCCC

EC

E

II

RIVV

II

R


10



Exercises 5.22, 5.23 & 5.24Exercises 5.22, 5.23 & 5.24


11


0

EC

E

EE

BEBE

II

R

VI

VVV


1

1

EB

CCCCC

EC

II

RIVV

Exercises 5.25 & 5.26Exercises 5.25 & 5.26


12


BC

B

BEBB

II

R

VVI


CBE

CCCCC

BC

III

RIVV

Exercises 5.27Exercises 5.27

)(30 CCCC

B

CCCC

RiVVofRange

i

VVR


0.042 mA)(3.0 CCCC RiVVofRange

13




BCC

BB RRR

VV

2

EEBEB

BBE

BEBBE

EB

EEBEBBBBB

BBBB

BB

RIVV

RR

VVI

II

RIVRIV

RRR

RR

)]1/([

1

)||( 21

21


14


10028.1

28.1%

. of in terms write

0

C

BE

EEBEBBB

iChange

ii

RiVRiV

1.28 mA




15

Exercises 5.29 & 5.30Exercises 5.29 & 5.30




16



Biasing BJT Amplifier CircuitsBiasing BJT Amplifier Circuits


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.

17

Biasing BJT Amplifier CircuitsBiasing BJT Amplifier Circuits

)]1/([

1

BEBBE

EB

RR

VVI

II


Classical biasing for BJTs using a single power supply: (a) circuit; (b) circuit with the voltage dividersupplying the base replaced with its Thévenin equivalent. The small changes in the VBE are swept awayby large value of VBB. Also

1

)]1/([

BE

BE

RR

RR



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Biasing BJT Amplifier Circuit with 2 PSBiasing BJT Amplifier Circuit with 2 PS


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.

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Biasing BJT Amplifier Circuit with Feedback Biasing BJT Amplifier Circuit with Feedback ResistorResistor


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

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Biasing using Constant Current SourceBiasing using Constant Current SourceThis has advantage that emitter current is independent of beta and base resistance, so base resistance can be made large to havehigh input resistance with out affecting the bias stability. It is more simplified design.The circuit in (b) utilizes a matched transistors Q1 and Q2 (Q1 is connected as a diode), both transistors have high beta values, ifwe neglect both base current the current through Q1 is:

As both transistors have same VBE so their collector current will be equal; I = IREF.

By keeping V greater than ( V + V ) we can guarantee that Q2 remains in active region and the current remains constant

R

VVVI BEEECC

REF

)(

By keeping V greater than (-VEE + VBE), we can guarantee that Q2 remains in active region and the current remains constant.


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

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Small Signal OperationSmall Signal Operation

CB

CE

VvSC

RIVVV

II

II

eII TBE

So we first set vbe to zero, and following values of current and voltage are determined for DC.

beBEBE vVv

CCCCCEC RIVVV

beT

beCC

Tbe

VvCC

VvVVSC

VvVS

VvSC

mVvV

vIi

Vv

eIi

eeIi

eIeIi

Tbe

TbeTBE

TbeBETBE

10 if only valid is this1

suppose weif

)(

Collector Current & Transconductance


(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.

bemT

beCc

cCT

beCCC

T

vgV

vIi

iIV

vIIi

V

22

TransconductanceTransconductance


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.

Base, Emitter current and GainBase, Emitter current and Gain

bBB

beT

CCcCCB

iIi

vV

IIiIii

is;emitter and base into looking resistanceinput signal Small

1

CcCCcCCCCCcCCCCCCCC

eebee

E

Te

e

bee

eEcCcCC

E

B

T

b

be

RiVRiRIVRiIVRiVv

rrriirrr

I

Vr

i

vr

iIiIiIi

i

I

Vr

i

vr

)()(

1 & as & between iprelationsh findcan We

iprelationsh ealternativanother

is;emitter into lookingemitter & basebetween resistance signal Small

iprelationsh ealternativanother


Cmbe

Cbem

be

cv

CbemCcc

Rgv

Rvg

v

vA

RvgRiv

voltagesignal small The

23

Exercise 5.37 & 5.38Exercise 5.37 & 5.38


HybridHybrid--ππ ModelModel

The most widely used model.


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).

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The T ModelThe T Model


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 rfeatured in the hybrid- model.



(a) circuit; (b) dc analysis; (c) small-signal model.

25





(a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.

26


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


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

Small Signal Model to Account for Early EffectSmall Signal Model to Account for Early Effect

We have seen that the collector current not only depend on vBE but also on vCE, so the dependence on vCE can be modelled byassigning a finite output resistance to the controlled current source in the hybrid pi model.

)(

C

A

C

CEAo I

V

I

VVr


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

)||( oCbemo rRvgv

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