Upload
rarewishbone
View
226
Download
0
Embed Size (px)
Citation preview
8/8/2019 0809 Part 1
1/18
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.
8/8/2019 0809 Part 1
2/18
EE1&ISE1 Analogue Electronics 2008/2009 - Intro/Synopsis ASH 2
EE1&ISE1 ANALOGUE ELECTRONICS 2008/2009
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---------------------------------------------------------------------------------
8/8/2019 0809 Part 1
3/18
EE1&ISE1 Analogue Electronics 2008/2009 - Intro/Synopsis ASH 3
EE1&ISE1 ANALOGUE ELECTRONICS 2008/2009
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.
8/8/2019 0809 Part 1
4/18
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)
8/8/2019 0809 Part 1
6/18
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
EE1&ISE1AnalogueElec
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
8/8/2019 0809 Part 1
7/18
EE1&ISE1Analo
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
EE1&ISE1AnalogueElec
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
8/8/2019 0809 Part 1
12/18
EE1&ISE1Analo
gueElectronics2008/2009-Part1
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
EE1&ISE1AnalogueElec
tronics2008/2009-Part1
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!)
8/8/2019 0809 Part 1
13/18
EE1&ISE1Analo
gueElectronics2008/2009-Part1
ASH
13
TheBJTasaSwitch
Nowintere
stedmainlyinCUT-OFFandSATURATIONmodes
Basiccircu
it:
VIN
RC
VOUT
VCC
RB
IB
IC
Inputside:
ForVIN
8/8/2019 0809 Part 1
14/18
EE1&ISE1Analo
gueElectronics2008/2009-Part1
ASH
15
TheBJTasaSwitch
Input-OutputRelation
ship-2
Threeregionsinlowergraphseparatedbydashedlines:
Left:
VIN>IB),solow-currentinput
signal
canswitchrelativelyheavyload
VOUTwhenVIN,socircuitperformsLogicalNOToperation;NOR
alsop
ossiblebyapplyingseveralinput
signals.Earlylogic(RTL)
usedthis.
EE1&ISE1AnalogueElec
tronics2008/2009-Part1
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)
8/8/2019 0809 Part 1
15/18
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)
8/8/2019 0809 Part 1
16/18
EE1&ISE1 Analogue Electronics 2008/2009 - Problems 1 ASH 2
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
8/8/2019 0809 Part 1
17/18
EE1&ISE1 Analogue Electronics 2008/2009 - Problems 1 ASH 3
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?
8/8/2019 0809 Part 1
18/18
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