3. TRANSISTOR - INTRODUCTION
• Transistor is a three terminal solid state device whose operation depends on the
flow of electric charge carriers within the solid
• It is capable of amplification also
• It is a current controlled device
5. Introduction
• A Bipolar Junction Transistor (also known as a BJT or BJT Transistor) is a three-
terminal semiconductor device
• It is a current controlled device.
• The three terminals of the BJT are the base, the collector and the emitter.
• Called bipolar as it uses both electrons and holes as charge carriers.
• It can be used as a switch in digital electronics or as an amplifier
in analog electronics.
• Nowadays, field-effect transistors are widely used in electronics applications but still,
BJTs are quite extensively used
• There are two types of BJTs –
– NPN transistors
– PNP transistors
6. Structure of BJT
• A PNP transistor has a layer of N type material sandwiched between two layers of P typ
e material.
• A NPN transistor has a layer of P type material sandwiched between two layers of N typ
e material.
• Both the types have two P-N Junctions
• The junction formed through the emitter and base is called emitter-base junction or
emitter junction
• The junction of base and collector makes the base-collector junction or collector jun
ction
7. • It appears as if two back to back diodes are connected in series.
Structure of BJT……
8. • Basically a transistor has three portions Emitter,Base and Collector
Emitter
• Left hand region of the transistor
• Main function is to supply majority carriers to the base
• It is heavily doped and always forward biased with respect to base so that
it can supply a large number of majority carriers
• Forward bias applied is very small as the junction resistance is small
• Moderate in size to avoid mesh formation
Structure of BJT……
9. Structure of BJT……
Collector
• Right hand region of the transistor
• Main function is to collect the majority carriers from the base
• It is moderately doped to avoid the formation of mesh
• Always reverse biased with respect to base so as to remove the charge carriers
from its junction with the base
• Applied reverse bias is large as the junction offers high resistance to collector
current
• Large in size to withstand the temperature generated at the collector
10. Structure of BJT……
Base
• Middle region of the transistor
• Lightly doped to reduce the recombination with the base as to increase
the collector current
• Very thin in size in comparison to emitter and collector so that it may
pass all the carriers from emitter to collector
• Large in size to withstand the temperature generated at the collector
11. WORKING OF BJT
• When no battery is connected between the transistor terminals, it is said to be in
unbiased state or open-circuit state
• The process of applying dc voltages across the different terminals of the transistor
is called Biasing
• For normal operation, emitter-base junction is always forward biased and
collector-base junction is reverse biased
• Forward bias at EB junction reduces the barrier
potential-narrows the depletion region
• CB junction produces a wide depletion region due to
reverse bias
• This produces a very narrow effective base width Wb
12. WORKING OF BJT……
• Electrons are injected into the emitter region by the emitter bias supply VEB
• This makes the electron concentration in emitter junction very large
• Some of these electrons combine with the holes in the P-type base (1-5%)
• Electron concentration in collector junction is small
• Since base width is less, the gradient of electron concentration is very large in base
• This causes the diffusion of electrons from emitter to collector
• For each electron combined in the base region,
an electron leaves the region via base terminal and
causes a small base current
13. • The emitter current is equal to the base and collector current.
IE =IC + IB
•The ratio between dc collector and dc emitter current is known as dc alpha or (𝛼DC).
αDC = IC/IE
• Value of αDC is from 0.95 to 0.99 or larger but it remains always less than one.
• The dc current gain of transistor is the ratio between dc collector and dc base current,
denoted as (βDC).
(βDC)= IC /IB
WORKING OF BJT……
14. • A transistor has three terminals but we need four, two for input and two for
output.
• So one terminal of the transistor is made common tot the input and output
circuits
• There are three types of configurations for the operation of transistor
Common Base Configuration
Common Emitter Configuration
Common Collector Configuration
• Each configuration has its own merits and demerits
• Regardless of the configuration, emitter is always forward biased and collec
tor is always reverse biased
Transistor Circuit Configurations
16. • Input is connected between emitter and base and output is taken across collector and
base
• EB junction is forward biased and CB junction is reverse biased
• A change in the input emitter current produces a similar change in the collector current
• Since the impedances of the two circuits are different, some voltage and power gain can
be achieved
• The ratio of collector current IC and emitter current IE is called current amplification
factor α
• 𝜶𝒅𝒄 =
𝑰𝑪
𝑰𝑬
• This is dc alpha when there is no ac input
input signal
• 𝜶𝒂𝒄=
∆𝑰𝑪
∆𝑰𝑬
• This is ac alpha which refers to the ratio of
change in collector current to the change
in emitter current
• Both of them are treated as same practically
Common Base Configuration
17. Common Base Configuration…..
• 𝛼 value ranges from 0.95 to 0.99
• This can be increased by making base thin and lightly doped
• The collector current consists of two parts
• The collector current produced by normal transistor action (𝛼𝐼𝐸 )produced by majority carriers
• The leakage current due to minority carriers across the reverse biased CB junction (𝐼𝐶𝐵𝑂)
• Total collector current ,𝐼𝐶 = 𝛼𝐼𝐸 + 𝐼𝐶𝐵𝑂
• We also know 𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐸
𝐼𝐸
=
𝐼𝐶
𝐼𝐸
+
𝐼𝐵
𝐼𝐸
1 = 𝛼 +
𝐼𝐵
𝐼𝐸
α = 1 −
𝐼𝐵
𝐼𝐸
18. CHARACTERISTICS OF TRANSISTORS
• Performance of transistors can be determined from their characteristic curves
• These curves relate different DC currents and voltages and are known as Static
Characteristic Curves
• There are two important characteristics of a transistor
• Input Characteristics
• Output Characteristics
• Both can be found from the same circuit arrangement
18
19. • The curve between emitter current IE and emitter base
voltage VEB for a given value of VCB is called INPUT
CHARACTERISTICS
• For finding this, output voltage VCB is maintained
constant and the input voltage VEB is set at several
convenient levels.
• For each level of VEB, input current IE is noted
• The curve looks like that of a forward biased PN
Junction diode
• There is a cutin, offset or threshold voltage Vγ below
which emitter current is very small due do barrier
voltage of EB junction
COMMON BASE CONFIGURATION CHARACTERISTICS
Input Characteristics
20. • After cut in voltage the emitter current increases rapidly with small increase in
VEB
• The input resistance of the transistor is given by
𝑟𝑖𝑛 =
Δ𝑉𝐸𝐵
Δ𝐼𝐸
- VCB constant
• 𝑟𝑖𝑛 is the dynamic input resistance which is quite low
• The value of 𝑟𝑖𝑛
• Varies from point to point in the initial part of input characteristics
• Over the linear region, for lower values of VEB it is higher
• For higher values of VEB it is around 100Ω
• The dependence of 𝑟𝑖𝑛 on VEB causes distortion of the
signals handled by the transistors
Input Characteristics
21. OUTPUT CHARACTERISTICS
• The curve drawn between collector current IC and collector base voltage VCB for a given
value of emitter current IE is called output characteristics
• For determination, for each fixed level of IE, the output voltage VCB is adjusted in
convenient steps, and corresponding values of collector current IC are noted
• Output characteristics of a transistor in the common base configuration are divided in
to three regions
• Active region
• Saturation region
• Cutoff region
22. OUTPUT CHARACTERISTICS…..
Active region
• It is the region where EB junction is forward biased and CB junction is reverse biased
• In this region IC=IE and appears to remain constant when VCB is increased
• This is because of the widening of the depletion region at CB junction when VCB is
increased
• This shortens the distance between the two depletion regions
• Only very few electrons from emitter will be
able to recombine in the base region
or enter the base terminal
• IC depends only on the value of IE
• Transistor is usually operated in this region
23. • A very large change in VCB causes a very small change in IC
• This means the output resistance of Common Base Configuration is
very high (few kΩ)
• Dynamic output resistance (ro) is defined as the ratio of change in
output voltage or collector voltage (VCB) to the corresponding change
in output current or collector current (IC ), with the input current or
emitter current (IE) kept at constant.
24. • IC is independent of VCB over the active region of the transistor
• If VCB is kept on increasing, beyond a certain value IC eventually increase
rapidly due to avalanche or Zener (or both) effects
• This condition is known as punch through or reach through.
• Large currents flow, destroying the device
• It is very essential to maintain VCB below the maximum safe limit
OUTPUT CHARACTERISTICS…..
Breakdown
25. • In this region, both emitter-base junction and collector-base junction
are reverse biased.
• This region lies IE =0 and to the right side of VCB=0.
• A small collector current IC flows even when IE=0.
• This is the leakage current ICBO
OUTPUT CHARACTERISTICS…..
Cut off region
26. • Saturation region is the region below VCB =0
• When VCB≈0 IC still flows
• This is due to
• Even when the externally applied voltage is reduced to zero, there is still a barrier
potential existing at the CB junction
• This assists the flow of IC
• To stop the collector current CB junction has to be forward
biased
• This is called the saturation region and both EB junction and
CB junction is forward biased.
• Consequently IC is reduced to zero when the VCB forward
voltage is increased
OUTPUT CHARACTERISTICS…..
Saturation region
27. • A small signal voltage applied in the emitter circuit (low input resistance) will
cause a relatively large emitter current
• Almost same amount of current flows in the collector
• Due to high output resistance, the voltage at collector will be high
• Both output power and voltage is quite large compared to the tiny input voltage
and power at the emitter
• CB configuration is rarely used audio frequency circuits
as the current gain is less than unity
their input and output resistances are quite different
27
28. EARLY EFFECT AND BASE WIDTH MODULATION
• The Early effect is named after its discoverer James M. Early
• It is the variation in the effective width of the base in a bipolar junction transistor
(BJT) due to a variation in the applied base-to-collector voltage.
• A greater reverse bias across the collector–base junction, increases the collector–base
depletion width, thereby decreasing the width of the charge carrier portion of the base
• As the reverse bias at CB junction increases the transition region penetrates deeper
• As neutrality of the charge is to be maintained, the number of uncovered charges re
mains same on either side
• Since the doping of base region is very small, the penetration percentage of the transiti
on region into the base is much larger
than into the collector region
29. • The Early effect has three main consequences
• There is less chance of recombination with the base region
• The concentration gradient of minority carries is increased within the base,
and leakage current increases
• For extremely large VCB the base width may reduce to zero causing voltage
breakdown, excessive emitter current called PUNCH THROUGH
Consequences of Early Effect
31. • Input is connected between emitter and base and output is taken across collector
and emitter
• Bias voltages are applied between EB and CE
• EB junction is forward biased by making base is made more +ve than the emitter by
VBB
• CE junction is reverse biased by making collector more +ve than emitter by VCC
• VCC > VBB
• Base current IB flows in the input circuit and
collector current IC flows in the output circuit
• Current, power and voltage gains are high
• Input resistance < output resistance but
difference is not as much in the case of
Common Base Configuration
• This configuration is the commonly used one.
Common Emitter Configuration
32. Common Emitter Configuration…..
• The ratio of change in collector current IC (output) and base current IB (input) is
called Base Current Amplification Factor β
β =
∆𝑰𝑪
∆𝑰𝐵
• In almost all transistors IB< 5% of IE
• So β usually ranges from 20 to 100
• 𝛽 =
𝐼𝐶
𝐼𝐵
=
𝐼𝐶
𝐼𝐸−𝐼𝐶
=
𝐼𝐶
𝐼𝐸
1−
𝐼𝐶
𝐼𝐸
=
𝛼
1−𝛼
• A small collector current flows even when base
current is zero
• This is called collector cut off current (ICEO)
• ICEO > ICBO
33. • The circuit used for the determination of common emitter configuration is given
• Input voltage is applied between base and emitter terminals
• Output is taken from collector and emitter terminals
COMMON EMITTER CONFIGURATION CHARACTERISTICS
34. • The curve plotted between base current IB and the base-emitter voltage VEB, with c
ollector emitter voltage VCE kept constant is called Input characteristics curve.
• Characteristics is similar to forward bias diode as the EB junction is indeed a forward
biased diode
• Compared to common base configuration, base
current (Input current) increases slowly with the
increase in VEB
• Indicates that ri in CE configuration > ri in CB
configuration
• Ri varies from point to point in the initial part
of the characteristic
• An increase in VCE causes IB to be lower for a
given value of VBE
• This is because higher VCE means higher CB
junction reverse bias
• Depletion region width decreases
• More electrons flow from emitter to collector
• Less electrons flow into base region
Input Characteristics
35. • The effect of CE junction does not cause large deviation on the curves, and hence the
effect of a change in VCE on the input characteristic is ignored.
• Input Resistance: The ratio of change in base-emitter voltage VBE to the change in b
ase current ∆IB at constant collector-emitter voltage VCE is known as input resistance
Input Characteristics
36. OUTPUT CHARACTERISTICS
• The curve drawn between collector current IC
and collector emitter voltage VCE for a given
value of base current IB is called output
characteristics
• For determination, for each fixed level of IB,
the output voltage VCE is adjusted in
convenient steps, and corresponding values
of collector current IC are noted
• Output characteristics of a transistor in the
common emitter configuration are divided
into three regions
• Active region
• Saturation region
• Cutoff region
37. • The features of common emitter configuration output characteristics are
– In active region, collector junction is reverse biased and emitter junction is forward biased
– This region lies above IB = 0 and to the right of the ordinate VCE= a few tenths of a volt.
– For a given value of IB the value of IC increases due to early effect as (VCE) increases.
– Transistor is operated in an active region if it is used as an amplifier.
– Collector current (output current) IC is larger than IB(input current) i.e. current gain is higher
than unity
• For very lower values of VCE the transistor is said to be in saturation region
– In this region, both emitter and collector junctions are forward biased equal to cut in voltage.
– Current IC varies with VCE but does not change with IB
• In cut off region, which lies below IB=0
– A small amount of collector current flows even when IB=0
– This is leakage current ICEO
– Since IC=0, it is called cutoff region
OUTPUT CHARACTERISTICS….
38. • Dynamic output resistance (ro) is defined as the ratio of change in output volta
ge or collector voltage (VCE) to the corresponding change in output current or col
lector current (IC), with the input current or base current (IB) kept at constant
• Moderate input to output resistance ratio makes Common Emitter configuration
ideal for amplifications and coupling between different stages
• Output resistance of common emitter configuration is less than that of common
base configuration (≈50kΩ)
• The dc and ac current gain is given by
OUTPUT CHARACTERISTICS….
40. • Input is connected between collector and base and output is taken across collector
and emitter
• Bias voltages are applied between CB and CE
• CB junction is forward biased by making base is made more +ve than the emitter by
VBB
• CE junction is reverse biased by making collector
more +ve than emitter by VCC
• VCC > VBB
• Base current IB flows in the input circuit and
emitter current IE flows in the output circuit
• This circuit is mainly used for amplification
because of this arrangement input resistance is
high, and output resistance is very low
Common Collector Configuration
41. Common Collector Configuration…..
• The ratio of change in emitter current to the change in base current is known as the
current amplification factor. It is expressed by the γ
γ =
∆𝑰𝐸
∆𝑰𝐵
Relation Between γ and α
We know,
Substituting the value of ΔIB in the first equation, we get,
42. • The circuit used for the determination of common collector configuration is given
• Input voltage is applied between base and collector terminals
• Output is taken from collector and emitter terminals
COMMON COLLECTOR CONFIGURATION CHARACTERISTICS
42
43. • The curve plotted between base current IB and the base-collector voltage VCB, with
collector emitter voltage VCE kept constant is called Input characteristics curve.
• An increase in VCB causes IB to be lower for a given value of VCE
INPUT CHARACTERISTICS
44. OUTPUT CHARACTERISTICS
• The curve drawn between emitter current IE
and collector emitter voltage VCE for a given
value of base current IB is called output
characteristics
• For determination, for each fixed level of IB,
the output voltage VCE is adjusted in
convenient steps, and corresponding values
of collector current IE are noted
• Output characteristics of a transistor in the
common emitter configuration are divided
into three regions
• Active region
• Saturation region
• Cutoff region
45. • Common collector configuration has high input impedance and very low output
impedance
• Voltage gain is always less than unity
• This configuration is seldom used for amplification
• Primarily used for impedance matching
• Also called emitter follower
OUTPUT CHARACTERISTICS….
46. Characteristics CB CE CC
Common Terminal for
Input and Output
Base Terminal Emitter Terminal Collector Terminal
Input voltage applied
between
Emitter and Base terminal Base and Emitter Terminal
Base and Collector
Terminal
Output Voltage taken
across
Collector and Base Terminal Collector and Emitter Terminal
Emitter and Collector
Terminal
Input Impedance
Very Low(only 50 to 500 ohm
)
Medium(500 to 5000 ohm)
Very high(200 to 750
kilo ohm)
Output Impedance
Very High(1 to 10 Mega Ohm
)
Medium(50 to 500 kilo ohm)
Very Low( up to 50
ohm)
Input Current Emitter Current or IE Base Current or IB Base Current or IB
Output Current Collector Current or IC Collector Current or IC Emitter Current or IE
Output Signal Phase Same phase with input 180 degree out of phase
Same phase with
input
Current Gain
Always less than Unity
α = IC/IE
Between 35 to 500
β = IC/IB
Very High
γ = IE/IB
Voltage Gain About 150 About 500 Less Than Unity
Leakage Current Very Small Very Large Very Large
Power Gain Medium High Medium
Application High Frequency Circuits RF Signal Processing Switching Circuits
COMPARISON
OF
BJT
CONFIGURATIONS
48. TRANSISTOR MODELLING
• Transistor are most commonly used as amplifiers in analog signals/ac signals
• To mathematically analyse and model a transistor, sinusoidal ac analysis is
important
• One of the main concerns in this analysis is the magnitude of the input signal
The transistor may use small signal operation or large signal operation
• Study of small signal operation is done either
Graphically
by using small signal equivalent circuit - preferred
• Large signal operation is best done graphically
• We study the small signal operation of transistor
49. Small signal Analysis
• For study of small signal operation
• The transistor is replaced by its equivalent circuit or model
• Network analysis methods are used to obtain the operating characteristics like
input impedance, output impedance, voltage gain, current gain etc
• Different network analysis techniques are available to obtain the parameters
• Here we study HYBRID(h) parameter equivalent circuit (or model) for the transistor
• Low frequency small signal common emitter amplifier is analysed
50. HYBRID PARAMETERS
• Hybrid parameters, or h-parameters are much more convenient for circuit analysis.
• These are used only for ac circuit analysis, although dc current gain factors are also ex
pressed as its parameters.
• Transistor h-parameter models simplify transistor circuit analysis
• It separates the input and output stages of a circuit to be analyzed.
• For this analysis a network has to be considered as a two port network
54. H Parameters of a Transistor
H parameters are useful in describing the input-output characteristics of circuits wher
e it is hard to measure Z or Y parameters (such as in a transistor)
The relationship between voltages and current in h parameters can be represented as:
55. Calculation of h parameters
• By assuming that the given network has no reactive element and by applying open
circuit (I1=0) or short circuit (V2=0) the h parameters can be defined.
• For V2=0 , applying in the first equation,
• Ratio of input voltage to input current, at short circuited output port is referred to as
the short circuit input impedance
• For V2=0 , applying in second equation,
• the ratio of the output current to input current at the short-circuited output port is
called short-circuit current gain of the network
56. Calculation of h parameters
• For I2=0 , applying in the first equation,
• Ratio of input voltage to input current, at short circuited output port is referred to as
the short circuit input impedance
• For V2=0 , applying in second equation,
• the ratio of the output current to input current at the short-circuited output port is
called short-circuit current gain of the network
