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
– 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
INTRODUCTION
• 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
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.
npn Transistor : Collector Current
iC  I S e
vBE
VT
AE qDn ni
IS 
N AW
2
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 : 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
npn Transistor : Emitter Current
• 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
Active Mode Parameters
npn Transistor
Large-signal equivalent-circuit models of the npn BJT
operating in the forward active mode.
Common parameters
npn & pnp BJT

1
iB  iE 
iE

 1

i E  i B     1i B

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
Problem 5-20 (d)
Find VC & I E
Assume  is very high
 1
Solution Problem 5-20(d)
IC  I B  I E
I C  I E  I E  I B  0
IC  I E
10  VC
VC  0.7  10
IC 
IE 
15000
5000
10  VC VC  0.7  10

15000
5000
10  VC  3VC  0.7  10  VC  4.475v
 4.475 0.7  10
IE 
 0.965m A
5000
Solution Problem 5.21(c)
IE
IE
10  7

 3m A
1000
 I C  I B  3m A
I1k  I E  I C  I B  3m A
VC  3m A 1000  3V
 ?
6.3  VC
3 .3
IB 

mA
100k
100
IE
3  100
 1 

 90.9
IB
3.3
  89.9
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
Common Base Characteristics
• Intersects the vertical axis at a current
equal to
iC
I E  iC  i E  
Large signal
iE

• Small signal  or incremental  can be
determined by i due to i at constant v
C
E
iC

iE
CB
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
The Early Effect
•
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
 vCE 

iC  I S e 1 
vA 

Nonzero slopeindicatesthatoutput resist ance
is not infinite
v BE
vT
 i
and defined as ro   C
 vCE
v v
ro  A CE
IC


v BE Cons tan at 

1
I C & vCE are the valuesat operat ingpoint
vA
ro  '
IC
I 'C  I S e
v BE
vT
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
As vi is increased, iB will increase,
iC will correspondingly increaseand vC will decrease.
Eventually, vC will becom e lower han
t
vB by 0,.4 V
and BJT enters sarutation region.
This edgeof saturation (EOS)is defined as
I C(EOS)
VCC  0.3

RC
I B ( EOS ) 
I C ( EOS )

Vi ( EOS )  I B ( EOS ) RB  VBE
I Csat
VCC  VCEsat

RC
Forcingmorecurrentinto thebase has littleeffect
on I CEsat and VCESAT , in thisstate, T he switch is closed
 forced 
I Csat
IB
Operation as Switch
vi < 0.5
vC  VCC  RC iC
 Txr Cutoff
RC
iB  0 iC  0
vi  0.5
Current (iB ) flows
VC
Rb
vC  VCC Node ' c' is disconnect from ground
iB 
VCC
vi
Vi  VBE
RB
iC   iB
Active mode till CBJ is not forward biased
vC  v B - 0.4
vC  VCC  RC i
VBE  0.7V
Vi  v BE  RB i B
Operation as Switch
• vi increased, iB increased, ic will
corresponding increase, vc will
decreases till vc < vB – 0.4
• vc = vB + vCB
• The Edge of Saturation
I C ( EOS)
VCC  0.3

RC  v
I B ( EOS) 
I C ( EOS)

BE
 0.7V
Operation as Switch
Vi ( EOS)  I B( EOS) RB  VBE
Into Deep Saturation
vCE ( sat )  0.2V
I CSat 
VCC  VCE sat
RC
RCsat 
VCEsat
very small.
I Csat
Forced  
Finally, more increase
in iB current has very
little effect on I csat & Vcsat
Forced  <  F
I CSat
I Sat
BJT circuit @ DC
• |VBE| = 0.7 V
|VCE| = 0.2V
VCBsat =-0.4V
VCE = VCB + VBE
=-0.4+0.7=0.3V
Saturation Mode
iC  iB
Valid only in active mode
Figure P5.85
Solution
  ,
I CX  I EX ActiveModeVBEX  0.7V
10  0.7
V2  0.7, R1 
 4.65k  4.7k
2m
10
V3  0V , R2 
 5k  5.1k
2m
10  0.7
V4  0.7V , R3 
 4.65k  4.7k
2m
10  4
V5  4V , R4 
 3k
2m
V7  2V , R5 
V6  4.7V , R6 
10  2
 2k
4m
10  4.7
 1.325 k  1.3k
4m
R1  4.7k, R2  5.1k, R3  4.7k
R4  3k, R5  2k, R6  1.3k
I C1  I B 2 R2  VEB2    1I B 2 R3  0
1.96  I B 2 5.1  0.7  100 1I B 2  4.7  0
I B 2  0.0194m A, I E 2  1.96m A,
V 3  0.1V ,
V 4  0.8V
9.3
 1.98mA  I C1  I E1  1.96 mA, I B1  20 A
4.7k
I C1  I B 2 R2  VEB2    1I B 2 R3  0
V2  0.7V  I E1 
1.96  I B 2 5.1  0.7  100 1I B 2  4.7  0
I B 2  0.0194m A, I E 2  1.96m A,
V 3  0.1V ,
V 4  0.8V
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
 R2 

VBB  VCC 
 R1  R2 
 R1 R2 

RB  
 R1  R2 
Figure 5.44 Classical biasing for BJTs using a single
power supply:
Classical Discrete-circuit Bias arrangement
(a) I C  I S e
v BE
VT
Any variation in v BE , IC changes
(b) iC  i
i changes iC
Base Emitter Loop
VBB  I B RB  VBE  I E RE
IE
IB 
 1
RB 

I E  RE 
  VBB  VBE
  1

VBB  VBE
IE 
RB
RE 
 1
Classical Discrete-circuit Bias arrangement
• For stable Ic, IE must be
stable as IC =αIE
• To make IE insensitive to
VBE (temp.) & β variations
VBB  VBE
IE 
R
RE  B
 1
VBB >> VBE
RE>> RB/(β+1)
Classical Discrete-circuit Bias arrangement
VBB >> VBE
For higher VBB at given V CC
VRB2  VRB1
VRB1  VRC  VCB
For smallerVRB1
VRC smaller
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
1
VBB  VCC
3
1
VCE or VCB  VCC
3
1
I C RC  VCC
3
Classical Discrete-circuit Bias arrangement
RB
RE>> RB/(β+1)
 1
- RB be small, thuslower value
For RE 
of R 1 & R 2 resultsin large current
drain from thepower supply
- Results in lower input impedanceof
Amplifier- T hus LoadingEffect
T radeoff
Current through R c & R 2  I E - 0.1IE
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
For  independent biasing
R B can be eleminated, if signal
is not applied to theBase
& Base connectedto ground
Two Power Supplies Version
Base Emitter Loop
I B RB  VBE  I E RE  VEE 
IE
IB 
 1
I B RB  VBE  I E RE  VEE
VEE  VBE
IE 
RB
RE 
 1
VEE  VBE
RB
RE 
 1
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.
Biasing using collector- Base Feed Back Resistor
CommonEmitterconfiguration only
RB provide negative Feedback
A common-emitter transistor amplifier biased by
a feedback resistor RB.
I E  IC  I B
VCC  I E RC  I B RB  VBE
IB 
IE
 1
VCC  VBE
IE 
RB
RC 
 1
VCC  VBE
RB
RC 
 1
A common-emitter transistor amplifier biased by
a feedback resistor RB.
RB
RC 
 1
VCB
I E RB
 I B RB 
 1
 RB Det erminessignal swing at t hecollect or.
 RB small


Signal swing will be t hesmall.
Input resist ancewill be small Loading
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
Biasing using a constant current source
• 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
I = IREF=(VCC-VBE+VEE)/R