bipolar junction transistors (bjts)
DESCRIPTION
Bipolar Junction Transistors (BJTs). 1. Electronics Concept. THE PAST. 1904 Flemming invented a value-Diode Cathode & Anode Positive voltages at Anode; current flows Negative voltages at Anode; No current flows Acted as Detector 1906 De Forset - PowerPoint PPT PresentationTRANSCRIPT
1
Bipolar JunctionTransistors (BJTs)
Electronics Concept
THE PAST• 1904 Flemming
– invented a value-Diode– Cathode & Anode – Positive voltages at Anode; current flows– Negative voltages at Anode; No current flows– Acted as Detector
• 1906 De Forset– Put a third electrode in between and small change in voltage on
the grid resulted in a large plate voltage change.– Acted as an amplifier
• Vacuum Tubes not reliable
Solid State Components• December 1947
– Closely space two gold-wires probes were pressed into the surface of a germanium crystal and amplification of input voltages experienced.
– Low gain, low bandwidth and Noisy
• 1947 - 1950– Junction Transistor invented where the operation depended upon
diffusion instead of conduction current.– Had charged carrier of both polarities -- Electron and Holes
• 1951 – Solid state transistors were produced commercially.
• Transistor characteristics vary greatly with changes in temperature. Germanium had excessive variations above 750 C, thus Silicon transistor were invented as Silicon Transistors could be used up to 2000 C
Semi Conductor Concept
Solid State Physics
The Integrated Circuit• 1958
– Kilby conceived the monolithic idea – Building entire circuit out of Germanium or Silicon
– Phase-Shift Oscillator, Multivibrator were build from Germanium with thermally bonded gold connecting wires.
– Noyce manufactured multiple devices on a single piece of Silicon and was able to reduced the size, weight and cost per active element.
Technological Advances
• 1960 - Small Scale Integration (SSI) – less than100 components per chip
• 1966 - Medium Scale Integration (MSI) – More than 100 less than 1000 components per chip
• 1969 - Large Scale Integration (LSI) – More than 1000 less than 10,000 components per chip
• 1975 - Very Large Scale Integration (VLSI)– More than 10,000 components per chip
INTRODUCTION - BJT• Three terminal device
• Basic Principle– Voltage between two terminals controls current flowing in the third
terminal.
• Device is used in discrete and integrated circuits and can act as :– Amplifier
– Logic Gates
– Memory Circuits
– Switches
• Invented in 1948 at Bell Telephone Industries
• MOSFET has taken over BJT since 1970’s for designing of integrated circuits but still BJT performance under sever environment is much better than MOSFET e.g. Automotive Electronics
• BJT is used in – Very high frequency applications (Wireless Comm)– Very high speed digital logics circuit (Emitter Coupled
Logic)
• Innovative circuit combine MOSFET being high-input resistance and low power operating devices with BJT merits of being high current handling capacity and very high frequency operation – known as BiMOS or BiCMOS
INTRODUCTION
• Study would include
– Physical operation of BJT– Terminal Characteristics– Circuit Models– Analysis and design of transistor circuits
INTRODUCTION
A simplified structure of the npn transistor.
Device Structure & Physical Operation• npn & pnp Transistor
• Three terminal ---- Emitter, Base, Collector
• Consists of two pn junctions– np-pn -------- npn– pn-np -------- pnp
• Modes– Cutoff, Active, Saturation, Reverse Active
• Junctions – Emitter Base Junction (EBJ)– Collector-Base Junction (CBJ)
TWO EXAMPLES OF DIFFERENT SHAPES OF TRANSISTOR
A simplified structure of the npn transistor.
Current flow in an npn transistor biased to operate in the active mode.
Notation Summarized
Notation
Base (collector) Voltage with respect to Emitter
Base (collector) Current toward electrode from external circuit
Instantaneous Total Value (DC + AC) vB (vC) iB (iC)
Quiescent Value (DC) VB (VC) IB (IC)Instantaneous Value of varying component (AC) vb (vc) ib (ic)Effective Value of varying components Vb (Vc) Ib (Ic)
Supply Voltage (Magnitude) VBB (VCC)
npn Transistor
• Current in Forward biased junction in active mode:
– Emitter Current (IE) flows out of the emitter
– Base Current (IB) flows into the Base
– Collector Current (IC) flows into the Collector
• Majority carriers are electrons as emitter junction is heavily doped and is wider than the base junction, and base junction being lightly doped and has smaller area.
Current Flow• EBJ – Forward Biased, CBJ – Reversed Biased
• Only diffusion current is considered as drift current due to thermally generated minority carriers is very small.
• Current components across the EBJ are due to:
– Electrons injected from the emitter into the base• This current component is at higher level due to heavily doped emitter• High density of electrons in emitter
– Holes injected from the base into the emitter• This current component is small due to lightly doped base• Low density of holes in base
Profiles of minority-carrier concentrations in
the base and in the emitter of an npn transistor
operating in the active mode: vBE > 0 and vCB ³ 0.
IC is independent of VCE, as long as the CBJ is reversed biased – collector is positive w.r.t base
• IS (Saturation Current) is– Inversely proportional to the base width– Directly proportional to the area of the EBJ– Typical range 10 -12 ---- 10 -18 A – Varies with changes in temperature (Doubles @
every 50 C rise in temperature)
npn Transistor : Collector Current
T
BE
V
v
SC eIi WN
nqDAI
A
inES
2
npn Transistor : Base Current
iB=iC/β• β (Beta) Common emitter current gain
– Ranges from 50 –200– Constant for a particular transistor– Influenced by :
• Width of the base junction (W)• Relative doping of base region w.r.t. emitter region
• α (Common – Base Current Gain) is – constant for a particular transistor– Less or close to unity
• Small change in α corresponds to a very large change in β
npn Transistor : Emitter Current
Active Mode Parameters npn Transistor
Large-signal equivalent-circuit models of the npn BJT
operating in the forward active mode.
Common parametersnpn & pnp BJT
EEB iii1
1
BBE iii 1
Cross-section of an npn BJT.
Model for the npn transistor when operated in the reverse active mode (i.e., with the CBJ forward biased and the EBJ reverse biased).
Equivalent Circuit model
•vBE – forward biased EBJ causing an exponentially related current iC to flow
• iC is independent of value of the collector voltage as long as CBJ is reversed biased.
vCB ≥ 0 V
• Collector terminal behaves as an ideal constant current
source and its value is determined by vBE
iC = αiE
The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor enters the saturation mode of operation for vCB –0.4 V, and the collector current diminishes.
Current flow in a pnp transistor biased
to operate in the active mode.
Large-signal model for the pnp transistor
operating in the active mode.
Current flow in a pnp & npn transistor biased to operate in the active mode.
Large-signal model for the pnp & npn transistor
operating in the active mode.
Circuit symbols for BJTs.
Active Mode Parameters pnp Transistor
Comparison BJTs
EC IVFind &
1
high very is
Assume
Problem 5-20 (d)
Solution Problem 5-20(d)
mAI
vVVV
VV
VI
VI
II
IIII
III
E
CCC
CC
CE
CC
EC
BEEC
EBC
965.05000
107.0475.4
475.4107.03105000
107.0
15000
105000
107.0
15000
10
0
Solution Problem 5.21(c)
9.89
9.903.3
10031
100
3.3
100
3.6
310003
3
3
31000
710
1
B
E
CB
C
BCEk
BCE
E
I
I
mAk
VI
VmAV
mAIIII
mAIII
mAI
?
Figure 5.16 The iC –vBE characteristic for an npn transistor.
Figure 5.17 Effect of temperature on the iC–vBE characteristic. At a constant emitter current (broken line), vBE changes by –2 mV/C.
The iC–vCB characteristics of an npn transistor.
Common Base Characteristics • iC – vCB for various iE
• Base at constant voltage – grounded thus acts a common terminal potential
• Curve is not horizontal straight line but has a small positive slop.
iC depends slightly on vCB
• At relatively large vCB, iC shows rapid increases – Breakdown phenomena curve
• Intersects the vertical axis at a current equal to
• Small signal or incremental can be determined by due to at constant
signal LargeE
CECE i
iiiI
E
C
i
i
EiCi CBv
Common Base Characteristics
The Early Effect
• In real world
– (a) Collector current does show some dependence on collector voltage
– (b) Characteristics are not perfectly horizontal line iC - vCE
Figure 5.19 (a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT. (b) The iC –vCE characteristics of a practical BJT.
Common Emitter Configuration
• Emitter serves as a common terminal between input and output terminal
• Common Emitter Characteristics (ic-vCE)
can be obtained at different value of vBE
and varying vCE (dc), Collector current can be measured
• vCE < - 0.4 V CBJ become forward biased & BJT leaves active mode & enters saturation mode
• Characteristics is still a straight line but with a finite slope
• when extra-polated, the characteristics lines meet at a
point on the negative vCE axis @ vCE = -VA
• Typical value of VA ranges 50-100v & called early voltage, after the name of english scientist JM Early
The Early Effect
• At given vBE , increasing vCE increases reverse biased voltage on CBJ & thus depletion region increases, Resulting in a decrease in the effective base width W
– Is is inversely proportional to the base width
– Is increases , and Ic also increases proportionally
– called Early Effect
The Early Effect
T
BE
BE
T
BE
v
v
SCC
A
CE
C
CEA
atConsvCE
Co
A
CEv
v
SC
eIII
vr
v
I
vvr
v
ir
v
veIi
''o
C
o
1
tan
point operatingat values theare &I
as defined and
infinitenot is
resistanceoutput that indicates slope Nonzero
1
The Early Effect
Figure 5.20 Large-signal equivalent-circuit models of an npn BJT operating in the active mode in the common-emitter configuration.
Table 5.3 Symbols & Large Signal Model
Table 5.3 Symbols & Large Signal Model
npn Transistor
pnp Transistor
Common Emitter Configuration
Figure 5.27 Circuit whose operation is to be analyzed graphically.
Little Practical Value
Figure 5.28 Graphical construction for the determination of the dc base current
Figure 5.29 Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE
Figure 5.30 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB
Figure 5.30 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB
Figure 5.32 A simple circuit used to illustrate the different modes of operation of the BJT.
Operation as a Switch
• Cutoff and saturation modes of operation
• vi less than 0.5 V the transistor is in cutoff mode,
iB =o, iC=o, vC= VCC
• vi greater than 0.5 V (≈ 0.7V), the transistor conducts
iB=(vi-VBE)/RB
iC= βiB
• vC>vB-0.4 V …. vC=VCC – RCIC
Operation as a Switch
B
Csat
I
Iforced
CESATCEsat closed isswitch The,state in this,V and Ion
effect little has base theintocurrent more Forcing
ease. will decr and vy increaseespondingl will corri
ease, will incrsed, i is increaAs v
CC
Bi
as is definedion (EOS) of saturatThis edge
on.ation regiters sarutand BJT en
V,. by han vme lower t will beco, vEventually BC 40
C
CEsatCCCsat
BEBEOSBEOSi
EOSCEOSB
C
CCC(EOS)
R
VVI
VRIV
II
R
.VI
)()(
)()(
30
Operation as Switch
ground from disconnect is c'' Node
0 0
Cutoff
5.0
CCC
CB
xr
CCCCCi
Vv
ii
T
iRVvv
CCV
CR
CVbR
iv
iRVv
vv
ii
R
VV
i
v
CCCC
BC
BC
B
BEi
B
i
0.4-
biased forwardnot is CBJ tillmode Active
i
flows )(Current
5.0
B
BBBEi
BE
iRvV
VV
7.0
• vi increased, iB increased, ic will corresponding increase, vc will decreases till vc < vB – 0.4
• vc = vB + vCB
• The Edge of Saturation
)(
)(
7.0
)(
3.0
EOSCEOSB
VvC
CCEOSC
II
R
VI
BE
Operation as Switch
Sat
CSat
Csat
CEsatCsat
C
satCECCCSat
satCE
I
I
I
VR
R
VVI
Vv
Into
Forced
small. very
2.0
Saturation Deep
)(
BEBEOSBEOSi VRIV )()(
F
csatcsat
V & Ion effect little
very hascurrent in
increase more ,
Forced
i
Finally
B
Operation as Switch
BJT circuit @ DC
• |VBE| = 0.7 V |VCE| = 0.2V VCBsat =-0.4V
VCE = VCB + VBE
=-0.4+0.7=0.3VSaturation Mode
Valid only in active mode
BC ii
Figure P5.85
SolutionVVActiveModeII BEXEXCX 7.0 ,
kkm
RV 7.465.42
7.010,7.0 12
kkm
RVV 1.552
10,0 23
km
RVV 32
410,4 45
kkm
RVV 7.465.42
7.010,7.0 34
km
RVV 24
210,2 57
kkm
RVV 3.1325.14
7.410,7.4 66
3.1 ,2 ,3
7.4 ,1.5 ,7.4
654
321
kRkRkR
kRkRkR
AImAIImAk
IVV BECE 20,96.198.17.4
3.97.0 11112
VVVV
mAImAI
II
RIVRII
EB
BB
BEBBC
8.04 ,1.03
,96.1 ,0194.0
07.411007.01.596.1
01
22
22
322221
VVVV
mAImAI
II
RIVRII
EB
BB
BEBBC
8.04 ,1.03
,96.1 ,0194.0
07.411007.01.596.1
01
22
22
322221
Biasing of BJT Amplifier circuit
• Biasing to establish constant DC Collector current Ic & should be
• Calculatable• Predictable• Insensitive to temp. variations• Insensitive to large variations in β
– To allow max. output signal swing with no distortion
Figure 5.43 Two obvious schemes for biasing the BJT: (a) by fixing VBE; (b) by fixing IB.
Figure 5.44 Classical biasing for BJTs using a single power supply:
• Typical Biasing – Single power supply– Voltage Divider Network
– RE in Emitter Circuit
Typical Biasing
21
21
21
2
RR
RRR
RR
RVV
B
CCBB
Figure 5.44 Classical biasing for BJTs using a single power supply:
Classical Discrete-circuit Bias arrangement
C
C
CBE
V
v
SC
ii
ii
, Iv
eIIa T
BE
changes
(b)
changes in ion Any variat
)(
Base Emitter Loop
1
1
BE
BEBBE
BEBBB
EE
RR
VVI
VVR
RI
1
E
B
EEBEBBBB
II
RIVRIV
Classical Discrete-circuit Bias arrangement
• For stable Ic, IE must be stable as IC =αIE
• To make IE insensitive to VBE (temp.) & β variations
1
B
E
BEBBE R
R
VVI
VBB >> VBE
RE>> RB/(β+1)
Classical Discrete-circuit Bias arrangement
VBB >> VBE
smaller smaller For
V
Vgiven at Vhigher For
1
1
1RB2
CCBB
RCRB
CBRCRB
RB
VV
VVV
V
But for higher gain VRC should be more Larger signal Swing (before cutoff)
Av=-VRC / VT
VCB be large VCE is large for larger signed swing (before saturation)
Compromise Role of thumb CCBB VV
3
1
CCCC
CCCBCE
VRI
VVorV
3
13
1
EE2c
21
0.1I- I R & Rrough Current th
off Trade
Effect Loading Thus -Amplifier
of impedanceinput lower in Results -
supplypower thefromdrain
current largein results R & R of
elower valu thussmall, be -
1
B
BE
R
RRFor
Classical Discrete-circuit Bias arrangementRE>> RB/(β+1)
For Stable IE - Negative Feed Back through RE
If IE increases somehow, VRE increases,hence VE increases correspondingly, VBB = VBE + VE ; VBE decreases for maintaining constant VBB
Reduces collector (Emitter) current. Stable IE
Figure 5.45 Biasing the BJT using two power supplies.
Two Power Supplies Version
ground toconnected Base &
Base the toappliednot is
signal if ,eleminated becan R
biasingt independen For
B
1
LoopEmitter Base
E
B
EEEEBEBB
II
VRIVRI
1
VEE
B
E
BE
RR
V
Two Power Supplies Version
1
B
E
BEEEE
EEEEBEBB
RR
VVI
VRIVRI
Figure 5.46 (a) A common-emitter transistor amplifier biased by a feedback resistor RB.
A common-emitter transistor amplifier biased by
a feedback resistor RB.
ResistorBack Feed Base-collector using ingBias
onlyionconfiguratEmitterCommon
RB provide negative Feedback
1
E
BBEBBCECC
BCE
IIVRIRIV
III
1
B
C
BECC
RR
VV
A common-emitter transistor amplifier biased by
a feedback resistor RB.
1
B
C
BECCE R
R
VVI
Loading small be willresistanceInput
small. thebe willswing Signal
small
collector. at the swing signal Determines
B
B
R
R
A common-emitter transistor amplifier biased by
a feedback resistor RB.
1
BEBBCB
RIRIV
1
B
C
RR
A BJT biased using a constant-current source I.
Biasing using a constant current source
• Current in Emitter means
– Constant IC IC =α IE
– Independent of RB & β value thus RB can be made large to
• Increase Input resistance• Large signal swing at collector
•Q1 acts as Diode CBJ is short circuits
Biasing using a constant current source
Q1 acts as Diode CBJ is short circuits
VCC-IREFR-VBE+VEE=0
I = IREF=(VCC-VBE+VEE)/R
Since Q1 & Q2 have VBE is same
I constant till Q2 in Active Mode (Region) & can be guaranteed by
–Voltage at collector V > (-VEE+VBE)
Current Mirror
• IE is independent of β & RB
• RB can be made large thus increasing input resistance
• Simple Design
• Q1 & Q2 are matched pair
• Q1 is Diode collector- Base connected
• β high IB can be neglected α = 1 IC = IE
Biasing using a constant current source
I = IREF=(VCC-VBE+VEE)/R