This document provides an overview of the bipolar junction transistor (BJT). It discusses the structure of the BJT including the emitter, base, and collector regions. It describes the three modes of operation - cutoff, saturation, and active mode. The active mode is used for amplification as it forward biases the base-emitter junction and reverse biases the base-collector junction. The document also discusses the three transistor configurations - common base, common emitter, and common collector. It provides details on the input and output characteristics for both the common base and common emitter configurations.

1. ANALOG & DIGITAL
ELECTRONICS
by
Kumar Saliganti
Assistant Professor (C)
skjntum@gmail.com
Department of Electrical and Electronics Engineering
JNTUH University College of Engineering Manthani
UNIT-II
(Bipolar Junction Transistor)

2. Contents:
I. Diodes and Applications
II. Bipolar Junction Transistor
III. FETs and Digital Circuits
IV. Combinational Logic Circuits
V. Sequential Logic Circuits
Text Books:
1. Integrated Electronics: Analog and Digital Circuits and Systems,
2/e, Jaccob Millman, Christos Halkias and Chethan D. Parikh,
Tata McGraw-Hill Education, India, 2010.
2. Digital Design, 5/e, Morris Mano and Michael D. Cilette,
Pearson, 2011.

3. II. Bipolar Junction Transistor
Contents
 Transistor characteristics: The junction transistor,
transistor as an amplifier.
 CB, CE, CC configurations, comparison of transistor
configurations.
 Operating point.
 Self-bias or Emitter bias, bias compensation.
 Thermal runaway and stability.
 CE amplifier response, gain bandwidth product.
 Emitter follower, RC coupled amplifier.
 Two cascaded CE and multistage CE amplifiers.

4. Understanding of BJT

5. Understanding of BJT
force – voltage/current
water flow – current
- amplification

6. Transistor Structure
 The BJT (bipolar junction
transistor) is constructed
with three doped
semiconductor regions
separated by two pn
junctions.
 The three regions are
called emitter (E), base (B)
and collector (C).

7. The BJT have 2 types
1. The two n regions separated by a
p region - called ‘npn’
2. The two p regions separated by
an n region - called ‘pnp’.
Transistor Structure
(Cont. )

8.  The pn junction is joining the
base region and the emitter
region is called the base-
emitter junction or input
junction.
 The pn junction is joining the
base region and the collector
region is called the base-
collector junction or output
junction.
Transistor Structure
(Cont. )

9.  The base region is lightly doped
and very thin compared to the
heavily doped emitter and the
moderately doped collector
region.
Transistor Structure
(Cont. )

10. Transistor Currents
-The arrow is always
drawn
on the emitter
-The arrow always point
toward the n-type
-The arrow indicates the
direction of the emitter
current:
pnp:E B
npn: B E
IC= the collector current
IB= the base current
IE= the emitter current

11. 11
 Transistors can be
constructed as two
diodes that are
connected together.
Basic models of BJT
Diode
Diode
Diode
Diode
npn transistor
pnp transistor

12. Basic models of BJT

13. Modes of Operation
 Based on the bias voltages applied at the two p-n junctions,
transistors can operate in three modes:
1. Cut-off (both EB and CB junctions are reversed biased)
2. Saturation (both EB and CB junctions are forward biased)
3. Active mode (EBJ is forward biased and CBJ is reversed biased)
 Cut-off and Saturation modes are used in switching operation.
 Active mode is used in amplification purposes.

14. Active Mode Operation
EBJ:
Forward Biased
CBJ:
Reverse Biased
◦ Forward bias of EBJ injects electrons from emitter into base (Emitter current).
◦ Most electrons shoot through the base into the collector (Collector
current).
◦ Some emitted electrons recombine with holes in p-type base (Base Current)

15. • Both biasing potentials have been applied to a pnp transistor and resulting majority
and minority carrier flows indicated.
• Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type
material.
• A very small number of carriers (+) will through n-type material to the base terminal.
Resulting IB is typically in order of microamperes.
• The large number of majority carriers will diffuse across the reverse-biased junction
into the p-type material connected to the collector terminal.
Active Mode Operation

16. • Majority carriers can cross the reverse-biased
junction because the injected majority carriers will
appear as minority carriers in the n-type material.
• Applying KCL to the transistor :
IE = IC + IB
• The comprises of two components – the majority
and minority carriers
IC = ICmajority + ICOminority
• ICO – IC current with emitter terminal open and is
called leakage current.
Active Mode Operation (Cont.)

17. Operation of NPN transistor

18. Operation of PNP transistor

19. 1. Common base configuration (CB)
2. Common emitter configuration (CE)
3. Common collector configuration (CC)
Types of Transistor
Configurations
• When transistor is to be connected in a circuit, one terminal is
used as an input terminal, the other terminal is used as an output
terminal and third terminal is common to the input and output.
• Depending upon the input, output and common terminals, a
transistor can be connected in three configurations.
• They are

20. Common Base Configuration (CB)

21. Common Base Configuration (CB)

22. • The common-base configuration with pnp and npn
transistors are shown in the figures in the previousslide..
• The term common-base is derived from the fact that the
base is common to both the input and output sides of the
configuration.
• The arrow in the symbol defines the direction ofemitter
current through the device.
• The applied biasing are such as to establish currentin
the direction indicated for each branch.
• That is, direction of IE is the same as the polarity of VEE
and IC to VCC.
• Also, the equation IE = IC + IB still holds.
Common Base Configuration (CB)

23. Total collector current,
if IE = 0 (when the emitter circuit is open) then still a small current flow in the collector circuit
called leakage current. This leakage current is represented by as ICBO, i.e., collector-base current
with emitter circuit is open.
Collector Current:
The base current is because of the recombination of the electrons and holes in the base region. The whole
emitter current will not flow through the current. The collector current increase slightly because of the
leakage current flows due to the minority charge carrier. The total collector current consists;
1.The large percentage of emitter current that reaches the collector terminal, i.e., αIE.
2.The leakage current Ileakage. The minority charge carrier is because of the flow of minority charge carrier
across the collector-base junction as the junction is heavily reversed. Its value is much smaller than αIE.
The leakage current is also abbreviated as ICO i.e.,
the collector current with emitter circuit open.
Current gain (α)

24. • The driving point or input
parameters are shown in the
figure.
• An input current (IE) is a function of
an input voltage (VBE) for various
of output voltage(VCB ).
• This closely resembles the
characteristics of a diode.
• In the dc mode, the levels of
IC and IE at the operationpoint
are related by:
αdc = IC /IE
•Normally, α 1.
•For practical devices, α is
typically from 0.9 to0.998.
Input
characteristics

25. Output
characteristics

26. Output characteristics
• The output set relates an output current (IC ) to an output voltage (VCB) for
various of level of input current(IE ).
There are three regions ofinterest:
Active region
• In the active region, the b-e junction is forward-biased, whereas the c-b junction
is reverse-biased.
• The active region is the region normally employed for linear amplifier. Also, in
this region, I C IE
Cutoff region:
• The cutoff region is defined as that region where the collector current is 0A.
• In the cutoff region, the B-E and C-B junctions of a transistor are both reverse-
biased.
Saturation region:
• It is defined as that region of the characteristics to the left of VCB= 0 V.
• In saturation region, the B-E and C-B junctions of a transistor are both forward
biased.

27. Base Width Modulation: “Early”
Effect
• When bias voltages change, depletion widths change and the
effective base width will be a function of the bias voltages
• Most of the effect comes from the C-B junction since the bias on
the collector is usually larger than that on the E- B junction
Base width gets smaller as applied voltages get larger

28. Early Effect
Range is -100V to -200 V
Converge ~ at single point called "Early Voltage".
Large "Early Voltage" = Absence of "Base WidthModulation"

29. Common-Emitter Configuration
(CE)
– Most common
configuration of transistor
is as shown
– emitter terminal is
common to input and
output circuits this is a
common-emitter
configuration
– we will look at the
characteristics of the
device in this
configuration
– The current relations are
still applicable, i.e.,
– IE = IC + IB
and
IC =α IE
Figure: Common-emitter configuration ofpnp
transistor
Figure: Common-emitter
configuration of npn
transistor

30. Common-Emitter Configuration
(CE)

31. Collector Current:
In CE configuration, the input current IB and the output current IC are
related by the equation shown below.

32. If the base current is open (i.e., IB = 0). The collector current
is current to the emitter, and this current is abbreviated as
ICEO that means collector- emitter current with the base
open.
Substitute the value IB in equations (1), we get,

33. Input
characteristics
– the input takes the form of a
forward- biased pn junction
– the input characteristics are
therefore similar to those of a
semiconductor diode
An input current (IB) is a
function of an input voltage
(VBE) for various of output
voltage (VCE ).

34. Output
characteristics

35. Output
characteristics
–The magnitude of IB is in μA and not as horizontal as IE in common-base
circuit.
– The output set relates an output current (IC) to an output voltage (VCE) for
various of level of input current (IB ).
• There are three portions asshown:
Active region
 The active region, located at upper-right quadrant, has
the greatest linearity.
 The curve for IB are nearly straight and equally spaced.
 In active region, the B-E junction is forward-biased,
whereas the C-B junction isreverse-biased.
 The active region can be employed for voltage, current or power amplification.

36. •The region below IB = 0μA is defined as cutoff
region.
•For linear amplification, cutoff region should be
avoided.
• The small portion near the ordinate, is the saturation
region, which should be avoided for linear application.
• In the dc mode, the levels of IC and IB at the operation point
are related by: Normally,  ranges from 50 to 400.
dc = IC /IB
For ac situations,  is
defined as
B
IC
 a c 
I
CE
V con stan t
Cutoff region:
Saturation region:

37. Common - Collector Configuration
(CC)
• The common-collector configuration with npn
and pnp transistors are shown in thefigures.
Figure: Common-collector
configuration of npn
transistor
Figure: Common-
collector
configuration of pnp
transistor

38. Common - Collector Configuration
(CC)

39. • It is used primarily for impedance-matching
purpose since it has a high input impedanceand
low output impedance.
• The load resistor can be connected from emitterto
ground.
• The collector is tied to ground and thecircuit
resembles common-emitter circuit.
• The output set relates an output current (IE) to an
output voltage (VCE) for various of level of input
current (IB ).
Common - Collector

40. Emitter Current:
We know that,

41. Input characteristics
• It is a curve whichshows therelationshipbetween
• the base current,IB and the collector base voltage VCB at constant VCE This
method of determining the characteristic is as follows.
• First, a suitable voltage is applied between the emitter and
• collector.
• Next the input voltageVCB is increased in a number
of steps and corresponding values of IE are noted.
•The base current is taken on the y-axis, and the input voltage
is taken on the x-axis. Fig. shows the family of the input
characteristic at different collector- emitter voltages.

42. Input characteristics

43. Output
characteristics
• This is almost the same as the
output characteristics of common-
emitter circuit, which are the
relations between IC and VCE for
various of level of input current IB.
Since that: IE  IC.

44. Comparison between CB, CE & CC
Configurations

45. BIASING
Biasing is the process of providing DC voltage which helps
in the functioning of the circuit.
A transistor is biased in order to make the emitter base
junction forward biased and collector base junction
reverse biased, so that it maintains in active region, to
work as an amplifier.

46. NEED FOR BIASING
If a signal of very small voltage is given to the input of BJT,
it cannot be amplified. Because, for a BJT, to amplify a
signal, two conditions have to be met.
•The input voltage should exceed cut-in voltage for the
transistor to be ON.
•The BJT should be in the active region, to be operated as
an amplifier.

47. TRANSISTOR BIASING
The proper flow of zero signal collector current and
the maintenance of proper collector emitter voltage
during the passage of signal is known as Transistor
Biasing.
The circuit which provides transistor biasing is called
as Biasing Circuit.

48. TRANSISTOR BIASING
If appropriate DC voltages and currents are given
through BJT by external sources, so that BJT
operates in active region and superimpose the AC
signals to be amplified,
The given DC voltage and currents are so chosen
that the transistor remains in active region for
entire input AC cycle. Hence DC biasing is needed.

49. Factors affecting the operating point
 The main factor that affect the operating point is the
temperature. The operating point shifts due to change
in temperature.
 As temperature increases, the values of ICE, β, VBE gets
affected.
 So, the main problem which affects the operating
point is temperature.
 Hence operating point should be made independent
of the temperature so as to achieve stability. To
achieve this, biasing circuits are introduced.
• ICBO gets doubled (for every 10o rise)
• VBE decreases by 2.5mv (for every 1o rise)

50. Stabilization:
Need for Stabilization:
 The process of making the operating point independent of
temperature changes or variations in transistor parameters is
known as Stabilization.
 Once the stabilization is achieved, the values of IC and VCE become
independent of temperature variations or replacement of
transistor.
 A good biasing circuit helps in the stabilization of operating point.
Stabilization of the operating point has to be achieved due to the following reasons.
• Temperature dependence of IC
• Individual variations
• Thermal runaway

51. Thermal Runaway:
 As the expression for collector current IC is
IC = βIB + (1+β)ICBO
IC = βIB + ICEO
 The flow of collector current and also the collector leakage current causes heat dissipation.
If the operating point is not stabilized, there occurs a cumulative effect which increases this
heat dissipation.
 The self-destruction of such an unstabilized transistor is known as Thermal run away.
 In order to avoid thermal runaway and the destruction of transistor, it is necessary to
stabilize the operating point, i.e., to keep IC constant.

52. Stability Factor:
 It is understood that IC should be kept constant in spite of variations of ICBO or ICO.
 The extent to which a biasing circuit is successful in maintaining this is measured
by Stability factor.
 It is denoted by S.
 The rate of change of collector current IC with respect to the collector leakage current
ICO at constant β and IB is called Stability factor.
S
dIC
dICBO
=
When VBE &  = Constant
The stability factor should be as minimum as possible

53. Differentiating above expression with respect to IC, we get
Hence the stability factor S depends on β, IB and IC.
1 =
βdIB
dIC
+ (1+β)
dICBO
dIC
dIC
dIC
=
d(βIB)
dIC
+ (1+β)
dICBO
dIC
IC = βIB + (1+β)ICBO
The general expression of stability factor for a CE configuration
1 =
βdIB
dIC
+ (1+β)
1
S
(1+β)
1
S
= 1 −
βdIB
dIC
(1+β)
1 −
βdIB
dIC
S =

54. iii) S’
dIC
dVBE
=
When ICBO &  = Constant
iv) S”
dIC
d
=
When ICBO & VBE = Constant
Note:
i) If the Stability factor is low then transistor is thermally more stable.
ii) If the Stability factor is high then corresponding transistor is thermally less
stable.

55. Stabilization Techniques:
Biasing Methods Compensation Techniques
Fixed Bias or Base Resistor
Bias
Collector to Base Bias or
Collector Feed Bias
Emitter Resistor Bias
Voltage Divider Bias or Self Bias
Diode Compensation
Thermistor Compensation
Sensistor Compensation

56. THANK YOU