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EE1&ISE1 Analogue Electronics 2008/2009 - Intro/Synopsis ASH 1

EE1&ISE1 ANALOGUE ELECTRONICS 2008/2009

ABOUT THE COURSE

Introduction

In this course we shall look at a range of circuits built around bipolar junction transistors(BJTs) and field effect transistors (FETs). These devices form the backbone of all analogueand digital integrated circuits. The circuits we will be studying include single-transistoramplifiers and switching elements, together with two of the most important building blocksof analogue integrated circuits: the differential amplifier and the current mirror.

The emphasis throughout will be on how we, as electrical engineers, can use BJTs and FETsto make useful circuits. Our starting point in each case will be the terminal characteristics ofthe device in question. We will not spend too much time worrying about how thesecharacteristics come about - for those of you on the EE course, this aspect will be covered inDr Fobelets Semiconductor Devices course; it is also discussed in all of the textbookslisted below. We will draw heavily on material from Mr Brookes Analysis of Circuitscourse, so you should make every effort to become familiar with this.

Course Organisation

This year the Analogue Electronics course will comprise a series of 18 lectures, together withstudy groups and related experiments in the First Year Laboratory.

The lectures divide into five roughly equal blocks as shown in the synopsis overleaf. Initiallywe will concentrate on single-transistor circuits employing BJTs and FETs; we will then goon to look at some more complex circuits.

Lecture notes and a problem sheet will be distributed for each part of the course. You arestrongly advised to attempt the problems, preferably as we go along. They will help you tofollow the lectured material, and prepare you for the summer exams.

Sources of Information

The recommended textbook for this course is the one by Sedra and Smith listed below; youshould obtain your own copy of this text if possible. This book contains good coverage of allthe material in the course, and there are numerous worked examples and problems for eachtopic. You may also find the other two books useful.

Microelectronic Circuits (Recommended text)AS Sedra and KC Smith, OUP, 5th

Edition (3rd and 4th Editions also ok)

ElectronicsAR Hambley, Prentice Hall, 2nd Edition

Microelectronic Devices and CircuitsFonstad, McGraw Hill (out of print, but held in library)

If you have any problems relating to the course which cannot be sorted out in studygroups/tutorials, please e-mail me ([email protected]) to arrange a time for a meeting.My office is Room 701, E&EE Dept.

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COURSE SYNOPSIS

Part 1 - Bipolar Junction Transistors (4 lectures)---------------------------------------------------------------------------------Physical structure and modes of operationOperation in active mode; operating curvesCommon-emitter amplifier - a first lookBias stabilisationBJT as a switch---------------------------------------------------------------------------------

Part 2 - Small-Signal Analysis (4 lectures)---------------------------------------------------------------------------------

Introduction - basic principlesSmall-signal models for 2-terminal devicesSmall-signal BJT model; Early effectCommon-emitter amplifier revisitedMacro-modelsFrequency response; AC and DC coupling---------------------------------------------------------------------------------

Part 3 - Field Effect Transistors (4 lectures)---------------------------------------------------------------------------------Physical structure and operation; MOSFET types and symbolsOperating curves

Small-signal modelsCommon-source amplifierActive loads; Body effect---------------------------------------------------------------------------------

Part 4 - Some Important Analogue Building Blocks (3 lectures)---------------------------------------------------------------------------------Current mirror - as tail-current source, as active loadDifferential amplifierEmitter followerOutput stages

Operational amplifier - outline---------------------------------------------------------------------------------

Part 5 Transients and Oscillators (3 lectures)---------------------------------------------------------------------------------Transient behaviour of RC and RL networks - recapTransients in switched transistor circuitsTransistor oscillator circuits---------------------------------------------------------------------------------

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AIMS and OBJECTIVES

Aims

The aim of this course is to familiarise you with bipolar junction and field effect transistors,and to introduce a number of important circuit applications of these devices. The course alsoaims to develop basic skills in linear circuit analysis.

Objectives

By the end of the course, you should be able to:

Explain qualitatively the operation of all of the following:

Common-emitter (or common-source) amplifier

Active load

Current mirror

Differential amplifier

Emitter follower

Determine the operating modes and bias conditions of the transistors in the aboveconfigurations and in other simple circuits

Understand the small-signal models of BJTs and FETs at low frequencies, and use themto determine the small signal parameters for the above configurations and for other simpletransistor circuits

Determine the transient behaviour of circuits containing single transistor switches withreactive loads

Explain the operation of single-transistor oscillator circuits.

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EE1&ISE1AnalogueElectronics2008/2009

-Intro/Synopsis

ASH

4

VOLTAGEAND

CUR

RENTSOURCENOTATIO

N

There

aresomedifferencesinnotatio

nbetweenMrBrookesAnalysisofCircuitsnotesandmyno

tesforthiscourse.

Thisshouldnotcauseyou

anyrealdifficulty,

butifyougetconfusedthetablebelowshouldbe

helpful.

Sourcetype

Thiscourse

AnalysisofCircuitscourse

Notes

DCorbias

voltagesource

1)V=V

1

V2whereV1andV2arethetermin

alvoltagesofthe

sourcere

ferredtoacommonground

2)Terminal1isidentifiedbylonglineorby+

sign

3)Ifsource

isunlabelled(i.e.

Visunspecified),youshouldassume

V1>V2

Small-signal

voltagesource

1)v=

v1

v2

wherev1

andv2

arethesmall-signalvoltageson

terminals

1and2

2)Terminal1isidentifiedbyarrowheadorby

+sign

3)DifferentshapesonRHSdenoteindependent(circular)and

controlled(square)sources

DCorbias

currentsource

1)CurrentflowisindirectionofarrowifI>0

andagainstarrow

ifI0andagainst

arrowwh

eni>p)

p-type(p>>n)

EE1&ISE1AnalogueElec

tronics2008/2009Part1Preamble

ASH

2

Thep-nJunctionatEquilibrium

Ap-njunctionisfo

rmedattheinterfacebetweenn-type

andp-typeregionsina

semiconductor.

Becausetheelectro

nconcentrationismuchhigherinthe

n-typeregionthanin

thep-type,electron

swilltendtoDIFFUSEfromthen-sidetothep-side.

However,thisleavesanettpositivechargeonthen-side

,resultinginanelectric

fieldwhichtendsto

produceanelectronflowintheoppo

sitedirection.At

equilibrium,theseeffectsexactlybalance,andthenettelectroncurrentiszero.

Asimilarargumentappliestotheholecurrent.

Wecanthinkofthebuilt-involtageasaPOTENTIALB

ARRIERwhich

opposesthediffusionofelectronsfromn-top-andholesfromp-ton-.

p-type

n-type

h+diff

e-diff

p>>n

n>>p

Electricfield

Atequilibrium:

Holediffusion

Holedrift

Electrondiffusio

n

Electrondrift

Nettcurrent=0

Electricpotential

x

Built-involtage

(

x)

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EE1&ISE1Analo

gueElectronics2008/2009Part1Preamble

ASH

3

p-nJunct

ioninFORWARDBIAS

p-nJunct

ioninREVERSEBIAS

p

h+

e-

n

x

(x)

Vd

p

h+

e-

n

x

(x)

Barrierheight

reduced

LargeFORWARD

CURRENT

Barrierheight

increased

SmallREVERSE

CURRENT


tronics2008/2009Part1Preamble

ASH

4

I-VCharacter

isticsofap-nJunction

BehaviourinbothforwardandreversebiasisdescribedbytheEbers-Moll

Equation:

1

VV

exp

I

I

T

S

whereISisthereve

rsesaturationcurrent,andVT=kT/eisthethermalvoltage

(25mVatroomtemp)

NB:Foramoredetailedexplanationofhowap-njunctionworks,seee.g.

MicroelectronicDevicesbyKDLeaver,ICPress,2ndE

d

T

S

VV

exp

II

(small)

I

I

S

p

n

V

I

V

I

0.7

REVERSE

FORWAR

D

T

S

VV

exp

II

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gueElectronics2008/2009-Part1

ASH

1

BipolarJunctionTran

sistors

PhysicalStructure&Symbols

NPN

(b)

(a)

B

C E

n-type

Collector

region

p-type

Base

region

n-type

Emitter

region

Emitter

(E)

Collector

(C

)

Base

(B)

Emitter-base

junction(EBJ)

Collector-base

junction

(CBJ)

PNP-similar,but:

N-andP-typeregionsinterchanged

Arrowonsymbolreversed

Operating

Modes

Cut-off

Active

Saturation

Reverse-active

Reverse

Forward

Forward

Reverse

Reverse

Reverse

Forward

Forward

Operatingmode

EBJ

CBJ

ActiveMode-voltagepolaritiesforNPN

B

C E

VCB>0

VBE>0

IB

IC IE


tronics2008/2009-Part1

ASH

2

B

JT-OperationinActiveM

ode

IEn

electrons

n

p

n

IEp

holes

E

{

C

B

IB

IE

IC

recombination

IEn,IEpbothproportionaltoexp(VBE/VT)

ICIEn

ICISexp(VBE/VT)

(1.1)

IBIEpVBE):

IC=

IB,regardlessofVCE

i.e.

CONTROLLEDCURRENTSOUR

CE

SATURATIONREGION(VCE0

VCB0

VBE

>VBE

IErelativelyinsensitivetoexactvalueofVBE

GetICfrom

IC=IE

where=/(1+)1

istheCOM

MON-BASECURRENTGAIN

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ASH

11

BiasStabilisation

-2

REprovide

sNEGATIVEFEEDBACK

i.e.iftheemittercurrentstartstoriseasaresultofsomechangein

thetr

ansistorscharacteristics,thenthevoltageacrossRE

rises

accord

ingly.Thisinturnlowersthebase-emittervoltageofthe

transistor,tendingtobringtheemittercurrentbackdowntowards

itsoriginalvalue.

STABILISATION

BUTREalso

:

Reducessm

all-signalvoltagegain:

Av

=

-RCgm/(1+IERE/V

T)

(1.12)

-RC/RE

Reducesou

tputswing



ASH

12

BiasStabilisation-3

RecoveryofSmall-SignalVoltageGain

WecanrecovertheoriginalvalueofAvforA

Csignalsbyusinga

BYPASSCAPA

CITOR:

(b)

VBIAS

vin

RC

RE

VCC

CEV

OUT+vout

Nowwehav

e:

Av

=-RCgm/(1+IEZE/VT)

(1.12b)

whereZEisthecombinedimpedanceofREand

CE:

ZE

=RE/(1+jRECE)

BymakingCElargeenough,wecanmaketheparallelcombinationappear

likeashortcircuit(i.e.|ZE|0)atallACfrequenciesofinterest,sothat

Equation1.12bre

ducestoAv-RCgmasforourori

ginalcommon-emitter

amplifier.Ontheotherhand,thecapacitorhas

noeffectonbiasing,

becauseitpassesn

oDCcurrent.

NB

Techniqueo

nlyreallyrelevanttodiscretecircuits(nobigcapacitors

insideICs!)

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ASH

13

TheBJTasaSwitch

Nowintere

stedmainlyinCUT-OFFandSATURATIONmodes

Basiccircu

it:

VIN

RC

VOUT

VCC

RB

IB

IC

Inputside:

ForVIN

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ASH

15

TheBJTasaSwitch

Input-OutputRelation

ship-2

Threeregionsinlowergraphseparatedbydashedlines:

Left:

VIN>IB),solow-currentinput

signal

canswitchrelativelyheavyload

VOUTwhenVIN,socircuitperformsLogicalNOToperation;NOR

alsop

ossiblebyapplyingseveralinput

signals.Earlylogic(RTL)

usedthis.



ASH

16

TheBJTasaSwitch

ChoosingIB

Weneedtoens

urethetransistorwillnotdriftout

ofsaturationandinto

theactiveregio

nwhentheswitchisON

Onsetofsatura

tionoccurswhentheoutputvoltagedropstoaround

0.7

V.Ifwedenote

thecollectorcurrentatthispointa

sC,then:

C(VCC

-0.7)/RC

(1.15)

andthecorresp

ondingbasecurrentisBC/.

Toensurethe

transistorisdrivenwellintosa

turation,weactually

apply:

IB

=nB=nC/

(1.16)

wheren=OVE

RDRIVEFACTOR(typ2to10)

Asthetransistormovesfurtherintosaturation,VCEdropstoaround0.2

V,sothecollectorcurrentwellintosaturationisgivenby:

ICsat

(VCC

-0.2)/RC

(1.17)

Thisisthemax

imumcurrentthecollectorcircuitcansupport

Theeffectivecurrentgaininsaturation(i.e.ICsat/IB)isreducedtoapprox

/n.ThisquantityisreferredtoastheFORCED,becauseitsvaluecan

besetarbitrarily(byalteringparametersinEquations1.13band1.17)

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EE1&ISE1 Analogue Electronics 2008/2009 - Problems 1 ASH 1

ANALOGUE ELECTRONICS

PROBLEMS 1

1. (a) Using the relation IC = ISexp(VBE/VT) calculate the base-emitter voltage required to

give a collector current of 1 mA in an active BJT for which IS = 10-14 A. (Assume VT = 25

mV)

(b) By considering the ratio of the collector currents for two different base-emitter

voltages, or otherwise, show that the collector current in an active BJT increases tenfold

for a VBE change of about 60 mV.

Hence estimate the range of VBE values for which IC varies from 100 A to 100 mA in

the transistor of Q1(a).

2. For each of the configurations below, determine the operating mode of the transistor or, if

the mode is indeterminate, state all the possibilities.

5V

(a) (b)

5V

(c)

5V

(d)

3. Figure Q3a shows the IC-VBE characteristic of a given BJT at temperatures of 15, 25 and

35 C. The transistor is to be biased using one of the two configurations shown in Figs

Q3b and Q3c.

(a) With the aid of Figure Q3a, determine the values of VBIAS in Fig Q3b and RE in Fig

Q3c to give IC = 1 mA at 25 C.

(b) Using the method of load-lines estimate the maximum and minimum values of I C

occurring in each configuration as the temperature is varied from 15 C to 35 C. In the

light of your answer, comment on the usefulness of the configuration in Fig Q3b as a

practical biasing arrangement.

NOTE: You may assume IE IC (i.e. >> 1)

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1.00.90.80.70.60.50.4

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

VBE (V)

15 oC

35 oC VBIAS

(c)

IC

IC

1V

RE

(b)

(a)

25 oC

IC

(mA)

Figure Q3

4. (a) For the circuit below, calculate the values of IE and IC when VB = 5 V, assuming the

transistor is in active mode. Hence determine the value of RC required to give VO = 12.5

V when VB = 5 V. (Assume VBE 0.7 V)

(b) With RC as in part (a), what is the maximum value of VB for which the transistor willremain in the active region?

VB

RC

VO

4.3 k

= 100

+ 20 V

Figure Q4

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5. (a) Figure Q5 shows a common-emitter amplifier biased for operation at (almost)

constant base current. Choose the value of RB such that the quiescent output voltage lies

mid-way between the power rails for a BJT with = 100. What is the small-signal

voltage gain of the amplifier in this case? (hint: use Equn 1.10 in the lecture notes)

(b) Assuming RB has the above value, calculate the quiescent output voltages and small-

signal voltage gains for transistors with = 50 and = 150.

VOUT

+ 6 V

RB

10 k

VIN

Figure Q5

6. (a) Figure Q6 shows an alternative bias arrangement for the common-emitter amplifier.Show that for this circuit the quiescent emitter current and output voltage are given by the

equations to the right of the diagram. Hence determine VO when VS = 6 V, RC = 10 k,

RB = 774 k and = 100. (Assume VBE 0.7)

VORB

RC

VS

VO = VS - RCIE

IE =VS - VBE_______________

RC + RB/(1 + )

IE

Figure Q6

(b) Repeat the above calculation to determine the quiescent output voltages for transistors

with values of 50 and 150. Why is this circuit more tolerant to variations in than theone in Question 5 i.e. why do variations have a smaller effect on the operating point?

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EE1&ISE1 A l El t i 2008/2009 P bl 1 ASH 4

7. The circuit below is to be used to buffer a 5 V logic signal so that it can switch a 1 k

load requiring 12 V. You are given a transistor with a of 100. What value of RB will

ensure that the transistor is driven into saturation with an overdrive factor of 5? (You may

assume that the logic gate can source up to 1 mA without any drop in output voltage)

RB

1 k load

+12 V

1

5 V logic

Figure Q7

Answers

1 (a) 633 mV; (b) 573 mV to 753 mV

2 (a) active; (b) cut-off; (c) active/saturated; (d) active and PNP!3 (a) 650 mV, 350 ; (b) 0.6 to 1.8 for Q4b and 0.95 to 1.05 for Q4c

4 (a) IE = 1 mA, IC = 0.99 mA, RC = 7.58 k; (b) 7.73 V

5 (a) 1.77 M, AV = -120; (b) VOUT = 4.5 V, 1.5 V and Av = -60, -180 for = 50, 150

6 (a) 3.0 V; (b) 3.9 V, 2.5 V for = 50, 150

7 7.6 k

0809 Part 1

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