CSE 477. VLSI Systems Design - GUC

ELCT 503: Semiconductors German University in Cairo (GUC)

ELCT503

Semiconductors Fall 2014

Lecture 09: BJT Circuit Analysis

Dr. Hassan Mostafa

حسن مصطفى. د

[email protected]


Introduction

npn transistor

pnp transistor


Introduction

Symbol


DC Models

npn

BEJ BCJ


DC Models

iC – vCB characteristics for common-base configuration


DC Models

iC – vCE characteristics for common-emitter configuration

(VA = Early voltage)


DC Models (npn)

Transistor OFF

(VBE < 0.7V) and (VBC < 0.4V)

IB = 0 IC = 0 IE = 0

Transistor in Active region

(VBE >= 0.7V) and (VBC < 0.4V)

VCE >=0.3V

IC = IS exp (VBE/VT)

IC = β IB

IE = IC + IB = (β+1) IB

IC = IE

β = / (1- ) and β >> 1

= β / (β+1) and <= 1


DC Models (npn)

Transistor in Saturation region

(VBE >= 0.7V) and (VBC >= 0.4V)

VCE =0.2V

ICsat = βforced IB

IE = IC + IB = (βforced+1) IB


DC Models (pnp)

Same equations as before but after changing letters order: VBE VEB

VBC VCB

VCE VEC


Example1

β = 100

VBE = 0.7V at IC = 1 mA

Design the circuit such that:

IC = 2 mA

VC = +5V


Example1

Solution

Transistor works in the active region

kmA

RC 52

515

VVVVV

V

CBBC

BE

4.05

7.0


Example1

Solution

kI

VR

mAmAII

V

VVVV

VmA

mAVV

E

EE

CE

E

EEBBE

TBE

07.7)15(

02.22*100

1100/

717.0

0717.0

717.0)1

2ln(*7.0


Example2

β = 100

Find IC, IB, IE, VB, VC, and VE

Assume Active region VBE = 0.7V


Example2

Check:

(VBE >= 0.7V)

(VBC = -1.3V < 0.4V)

Correct


Example3

β = 100

Find IC, IB, IE,

VB, VC, and VE

Assume Active

region

Check:

(VBE >= 0.7V)

(VBC = 3.52V > 0.4V)

Incorrect


Example3

Assume

Saturation

region

Check:

(VBE >= 0.7V)

(VBC = 0.5V > 0.4V)

Correct


BJT in Logic Gates RTL (Resistor Transistor Logic)

Increasing vBE decrease vo

C

T

BESCCCEo

T

BESC

BE

CCoC

BE

CCCCCEo

RV

vIVvv

V

vIi

ActiveAssume

ONVvWhen

Vvi

OFFVvWhen

RiVvv

*)exp(*

)exp(*

5.0

0

5.0

*


When vBE increases such that vBC becomes larger than 0.4V

Transistor enters Saturation Region

BJT in Logic Gates

Vvv CEo 2.0

Which Logic gate this Circuit implements?


Think…

Which Logic gate this Circuit implements?

BJT in Logic Gates


Small Signal Models

Derivation : BJT must be in active region to use the small signal model

TV

CI

bev

ci

mg

V

vIi

iIi

VvV

vIi

V

vI

V

v

V

VIi

V

vVIi

V

vIiIi

T

beCc

cCC

Tbe

T

beCC

T

beC

T

be

T

BESC

T

beBESC

T

BEScCC

)1(*

)exp()exp()exp(

)exp(

)exp(

B

T

m

C

T

b

be

T

beCcb

cCCbBB

I

V

gr

I

V

i

vr

V

vIii

iIiiIi


Small Signal Models

-model

C

Ao

mC

T

B

T

T

Cm

I

Vr

gI

V

I

Vr

V

Ig

||


Small Signal Models

T-model

C

Ao

mC

T

E

Te

T

Cm

I

Vr

gI

V

I

Vr

V

Ig

||


Common Emitter Amplifier

Why it is called common emitter?

Bypass capacitors

Open circuit at DC

Short circuit at small signal

How to draw the small signal equivalent circuit?



Amplifier parameters:

Voltage gain (AV) = vo/vi <Units V/V>

Input resistance (Rin) = vi/ii <Units >

Output resistance (Rout) = vo/io|vs=0 <Units >



Voltage gain (AV) = vo/vi = - gm*(ro//RC // RL) V/V

= - gm*(RC // RL) when ro >> RC and RL

= - gm*(RC ) when ro >> RC and RL =


Common Base Amplifier

VA = ro =

Voltage gain (AV) = vo/vi = (RC //RL)/ re = +gm (RC //RL)

= (RC )/ re = +gm (RC) when RL =


Common Collector Amplifier (Emitter follower)

VA = ro =

Voltage gain (AV) = vo/vi = RL / (RL + re)

= 1 when RL =

Why it is called Emitter follower?

CSE 477. VLSI Systems Design - GUC

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