10.BJT(4).ppt [호환 모드]contents.kocw.net/KOCW/document/2015/korea_sejong/... ·...

전자회로1

여섯째주 1강

Chapter 3

Lecture Homepage : http://signal.korea.ac.kr

2016년 6월 27일2 pages

The BJT as a switchLarge-Signal Operation

The Transfer Characteristic

Chapter 3



V (5.50)O CE CC C CR i

/ V

/ V

BE T

I T

C S

S

i I e

I e

/ VV (5.51)I TO CC C SR I e

satsat

V V (5.52)CC CE

CC

IR

The BJT as a switchLarge-Signal Operation

The Transfer Characteristic

the active-mode expression for iC

We have neglected the Early effect

In the saturation region

Chapter 3



The BJT as an AmplifierAmplifier Gain

V V (5.54)CE CC C CR I

I V

A (5.55)BE

O

I

dd

(5.53) / TBE VVSC eII

To operate the BJT as a linear amplifier, it must be biased at point in the activeregion. Fig 5.26(b) shows such a bias point, labeled Q(for quiescent point), andcharacterized by a dc base-emitter voltage VBE and a dc collector-emitter voltage VCE.

then from the circuit in Fig 5.26(a) we can write

An expression for the small-signal gain Av can be found by differentiating theexpression in Eq. (5.51) and evaluating the derivative at point Q.

Chapter 3



The BJT as an AmplifierAmplifier Gain

V / V1AV

BE TS C

T

I e R

VA (5.56)

V VC C RC

T T

I R

V V V (5.57)RC CC CE

using Eq.(5.53) we can express Av in compact form

where VRC is the dc voltage drop across RC

satV VA (5.58)

VCC CE

T

it is useful to note that the theoretical maximum gain Av is obtained by biasing theBJT at the edge of saturation, which of course would not leave any room for negativesignal swing.

Chapter 3



The BJT as an AmplifierGraphical Analysis

VCE CC C Ci R

V 1CCC CE

C C

iR R

Chapter 3



The BJT as an Amplifier Graphical Analysis

Chapter 3




Effects of Bias-Point Location on Allowable Signal Swing

Chapter 3



The BJT as a switch Operation as a Switch

The base current will be

(5.60) B

BEIB R

Vvi

the collector current will be

(5.61) BC ii

(5.62) CCCCC iRVv

This edge-of-saturation (EOS) pointis defined by

(5.63) 3.0)(

C

CCEOSC R

VI

we have assumed that VBE isapproximately 0.7V

(5.64) )()(

EOSCEOSB

II

Signal Processing Lab., http://signal.korea.ac.krDept. of Elec. and Info. Engr., Korea Univ.


The BJT as an Amplifier and as a switch Operation as a Switch

The corresponding value of vI required todrive the transistor to the edge-of-saturationcan be found from

The ratio of the collector current ICsat to the basecurrent can be set at will and is therefore called the forced β

(5.65) )()( BEBEOSBEOSI VRIV

we shall usually assume that for a saturatedtransistor, VCEsat ≈ 0.2V

(5.66) C

CEsatCCCsat R

VVI

(5.67) ICsatforced

BI

Chapter 3



Biasing in BJT Amplifier CircuitsThe classical Discrete

Circuit Bias Arrangement

Chapter 3



Biasing in BJT Amplifier CircuitsThe classical Discrete

Circuit Bias Arrangement

2

1 2

V V (5.68)BB CCR

R R

1 2

1 2

(5.69)BR R

RR R

V V (5.70)

/( 1)BB BE

EE B

IR R

Fig 5.44(b) shows the same circuit with the voltage-divider network replaced by its Thevenin equivalent,

The current IE can be determined by writing a Kirchhoff loop equation for thebase-emitter-ground loop, labeled L, and substituting IB = IE / (β+1)

To make IE insensitive to temperature and β variation, we design the circuit tosatisfy the following two constraint

(5.72) 1

, (5.71)

BEBEBB

RRVV

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