Elements of Electronics Engineering 2019
Unit -2
TRANSISTOR
Introduction
A Transistor transfers a signal from a low resistance to high resistance. A Transistor consists
of two pn junctions formed by sandwiching either p-type or n type semiconductor between a pair of
opposite types. Accordingly, there are two types of transistors, namely;
(i) n-p-n transistor
(ii) p-n-p transistor
An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of
p-type as shown in Fig. 2.1 (i). However, a p-n-p transistor is formed by two p-sections separated by
a thin section of n-type as shown in Fig. 2.1 (ii).
Fig. 2.1: i. NPN Transistor ii. PNP Transistor
In each type of transistor, the following points may be noted:
(i) These are two pn junctions. Therefore, a transistor may be regarded as a combination of
two diodes connected back to back.
(ii) There are three terminals, one taken from each type of semiconductor.
(iii) The middle section is a very thin layer. This is the most important factor in the function
of a transistor.
Transistor Terminals
Figure 2.2 shows typical Transistor and the symbol used in circuits. The transistor has three
terminals namely Emitter, Base and Collector. NOTCH INDICATES EMITTER (very heavily
doped) < CENTER IS BASE (lightly doped)< THIRD ONE IS COLLECTOR(medium doping)
Fig. 2.2: i.Typical Transistor ii. Symbol used in circuit
(i) Emitter (E):-The section of transistor that supplies charge carriers (electrons or holes)
iscalled the emitter. The emitter is always forward biased w.r.t. base so that it can supply a
large number of majority carriers. In Fig. 2.3 (i), the emitter (p-type) of pnp transistor is
forward biased and supplies hole charges to its junction with the base. Similarly, in Fig. 2.3
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(ii), the emitter (n-type) of npn transistor has a forward bias and supplies free electrons to its
junction with the base in active mode.
(ii) Collector (C):-The section of transistor that collects the charge carriers is called the collector.
The collector is always reverse biased. Its function is to collect charges from its junction with
the base. In Fig. 2.3 (i), the collector (p-type) of pnp transistor has a reverse bias and receives
hole charges that flow in the output circuit. Similarly, in Fig. 2.3 (ii), the collector (n-type) of
npn transistor has reverse bias and receives electrons in active mode.
Fig. 2.3: i. Biasing of NPN ii. Biasing of PNP Transistor
(iii)Base (B):-The middle section which forms two pn-junctions between the emitter and collector
is called the base. The base-emitter junction is forward biased, allowing low resistance for the
emitter circuit. The base-collector junction is reverse biased and provides high resistance in
the collector circuit.
Construction of transistor
 The transistor has three regions, namely; emitter, base and collector.
 The emitter is heavily doped so that it can inject a large number of charge carriers
(electrons or holes) into the base. The base is lightly doped and very thin; it passes most
of the emitted charge carriers to the collector. The collector is moderately doped and
wider than emitter as shown in figure 2.4.

Fig. 2.4: i. Construction of NPN ii. Construction of NPN
The transistor has two PN junctions. The junction between emitter and base may be called
emitter-base diode. The junction between the base and collector may be called collector-base
diode.
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Working of npn transistor
Fig. 2.5 shows the npn transistor with forward bias to emitter-base junction and reverse bias to
collector-base junction. The forward bias causes the electrons in the n-type emitter to flow towards
the base. This constitutes the emitter current IE. As these electrons flow through the p-type base, they
tend to combine with holes. As the base is lightly doped and very thin, only a few electrons combine
with holes to constitute base current IB. The remainder cross over into the collector region to
constitute collector current IC. In this way, almost the entire emitter current flows in the collector
circuit. It is clear that emitter current is the sum of collector and base currents i.e.
IE= IB+ IC
(2.1)
Fig. 2.5: Basic connection of NPN transistor
Working of pnp transistor
Fig. 2.6 shows the basic connection of a pnp transistor. The forward bias causes the holes in
the p-type emitter to flow towards the base. This constitutes the emitter current IE. As these holes
cross into n-type base, they tend to combine with the electrons. As the base is lightly doped and very
thin, only a few holes combine with the electrons. The remainder cross into the collector region to
constitute collector current IC. In this way, almost the entire emitter current flows in the collector
circuit. It may be noted that current conduction within pnp transistor is by holes. However, in the
external connecting wires, the current is still by electrons.
Fig. 2.6: Basic connection of PNP transistor
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Transistor Symbols
The symbols used for npn and pnp transistors are shown in Fig. 2.7.
Fig.
2.7: Symbols used for npn and pnp transistors
Note that emitter is shown by an arrow which indicates the direction of conventional current
flow with forward bias. For npn connection, it is clear that conventional current flows out of the
emitter as indicated by the outgoing arrow in Fig. 2.7 (i). Similarly, for pnp connection, the
conventional current flows into the emitter as indicated by inward arrow in Fig. 2.7 (ii).
Transistor Connections
A transistor can be connected in a circuit in the following three ways :
 common base connection
 common emitter connection
 common collector connection
Common
Input
OutputTerminal? Current Gain =
Name of
Terminal
Terminal?
Iout/Iin
Configuration
Base
Emitter
Collector
α = Ic / Ie
CB
Name of
Configuration
CE
Common
Terminal
Emitter
Name of
Configuration
CC
Common
Terminal
Collector
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Input
Terminal?
Base
Input
Terminal?
Base
Output
Terminal?
Collector
Current Gain
β = Ic / Ib
OutputTerminal? Current Gain
Emitter
γ = Ie / Ib
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Relations of α, β, γ
γ = β+1
α = β/ (1+β)
β= α / (1 – α)
Region
Comment
Cut Off Region
Active Region
SaturationRegion
Both Reverse
1stFwd, 2nd Rev
Both Forward
First junction
(Emitter to base)
Reverse Biased
Forward Biased
Forward Biased
Second Junction
(Base to Collector)
Reverse Biased
Reverse Biased
ForwardBiased
Common Base Connection
In this circuit arrangement, input is applied between emitter and base and output is taken from
collector and base. Here, base of the transistor is common to both input and output circuits and hence
the name common base connection. In Fig. 2.8 (i), a common base npn transistor circuit is shown
whereas Fig. 2.8 (ii) shows the common base pnp transistor circuit.
Fig. 2.8: (i) common base npn transistor circuit (ii) common base pnp transistor circuit
Characteristics of Common base Connection
The most important characteristics of common base connection are input characteristics and
output characteristics.
Input characteristic.
It is the curve of IE Vs VEB (base emitter voltage) at constant VCB (collector-base voltage)

The emitter current is generally taken along y-axis and emitter-base voltage along x-axis.
Fig. 2.10(i) shows the input characteristics of a typical transistor in CB arrangement. The
following points may be noted from these characteristics: The emitter current IE increases
rapidly with small increase in emitter-base voltage VEB. It means that input resistance is
very small.
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
Theemitter current is almost independent of collector-base voltage VCB. This leads to the
conclusion that emitter current (and hence collector current) is almost independent of
collector voltage.
Fig. 2.9: Common base npn transistor circuit
Fig. 2.10: i. Input characteristics
ii. Output characteristics
Output characteristic
It is the curve between collector current IC and collector-base voltage VCB at constant
emitter current IE. Fig. 2.10 (ii) shows the output characteristics of a typical transistor in CB
arrangement.
The following points may be noted from the characteristics:
 The collector current IC varies with VCB only at very low voltages ( < 1V). The
transistor is never operated in this region.
 When the value of VCB is raised (above 1−2 V), the collector current becomes constant
as indicated by straight horizontal curves. It means that now IC is independent of VCB
and depends upon IE only. This is consistent with the theory that the emitter current
flows almost entirely to the collector terminal. The transistor is always operated in
this region
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Current amplification factor (α).
The ratio of change in collector current to the change in emitter current at constant collectorbase voltage VCB is known as current amplification factor
α = ∆Ic/∆IE
|constant VCB
It is clear that current amplification factor is less than unity. This value can be increased (but
not more than unity) by decreasing the base current. This is achieved by making the base thin and
doping it lightly. Practical values of α in commercial transistors range from 0.9 to 0.99
Common Emitter Connection
In this circuit arrangement, input is applied between base and emitter and output is taken from
collector and emitter. Here, emitter of the transistor is common to both input and output circuits and
hence the name common emitter connection. In Fig. 2.11 (i), a common base npn transistor circuit is
shown whereas Fig. 2.11 (ii) shows the common base pnp transistor circuit.
Fig. 2.11: (i) common emitter npn transistor circuit (ii) common emitter pnp transistor circuit
Fig. 2.12: Common emitter npn transistor circuit
Input characteristic. It is the curve between base current IB and base-emitter voltage VBE at constant
collector-emitter voltage VCE.
The input characteristics of a CE connection can be determined by the circuit shown in Fig.
2.12. Keeping VCE constant, note the base current IB for various values of VBE. Then plot the readings
obtained on the graph, The following points may be noted from the characteristics :
(i)
(ii)
The characteristic resembles that of a forward biased diode curve as shown in fig.2.13 i. This
is expected since the base-emitter section of transistor is a diode and it is forward biased.
As compared to CB arrangement, IB increases less rapidly with VBE. Therefore, input
resistance of a CE circuit is higher than that of CB circuit.
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Output characteristic:
It is the curve between collector current IC and collector-emitter voltage VCE at constant
base current IB.
The output characteristics of a CE circuit can be drawn with the help of the circuit shown in
Fig. 2.12. Keeping the base current IB fixed at some value say, 5 µA, note the collector current IC for
various values of VCE. Then plot the readings on a graph. This gives the output characteristic at IB = 5
µA as shown in Fig. 2.13 ii. The test can be repeated for IB= 10 µA to obtain the new output
characteristic.
Fig. 2.13: i Input characteristics
ii. Output characteristics
The following points may be noted from the characteristics:
(i) The collector current IC varies with VCE for VCE between 0 and 1V only. After this, collector
current becomes almost constant and independent of VCE. This value of VCE upto which collector
current IC changes with VCE is called the knee voltage (Vknee). The transistors are always operated in
the region above knee voltage.
(ii) Above knee voltage, IC is almost constant. However, a small increase in IC with increasing
VCE is caused by the collector depletion layer getting wider and capturing a few more majority
carriers before electron-hole combinations occur in the base area.
(iii) For any value of VCE above knee voltage, the collector current IC is approximately equal to
β × IB.
Base current amplification factor ( β). In common emitter connection, input current is IB and
output current is IC.
The ratio of change in collector current ( IC) to the change in base current ( IB) is known as
base current amplification factor i.e.
∆I
β=
8
C
∆I B
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In almost any transistor, less than 5% of emitter current flows as the base current.
Therefore, the value of β is generally greater than 20. Usually, its value ranges from 20 to 500.
This type of connection is frequently used as it gives appreciable current gain as well as voltage
gain.
It is clear that as α approaches unity, β approaches infinity. In other words, the current
gain in common emitter connection is very high. It is due to this reason that this circuit
arrangement is used in about 90 to 95 percent of all transistor applications.


The total collector current consists of :
That part of emitter current which reaches the collector terminal i.e. αIE.
The leakage current Ileakage denoted by ICBO. This current is due to the movement of minority
carriers across base-collector junction on account of it being reverse biased. This is generally
much smaller than α IE.
Common Collector Connection
In this circuit arrangement, input is applied between base and collector and output is taken
from emitter and collector. Here, collector of the transistor is common to both input and output
circuits and hence the name common collector connection. In Fig. 2.14 (i), a common base npn
transistor circuit is shown whereas Fig. 2.14 (ii) shows the common base pnp transistor circuit.
Fig. 2.14: (i) common emitter npn transistor circuit (ii) common emitter pnp transistor circuit
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Current amplification factor γ. In common collector circuit, input current is the base
current IB and output current is the emitter current IE. Therefore, current amplification in this
circuit arrangement can be defined as under :
The ratio of change in emitter current ( IE) to the change in base current ( IB) is known
as current amplification factor in common collector (CC) arrangement i.e.
γ=
∆IE
∆IB
This circuit provides about the same current gain as the common emitter circuit as However, its
voltage gain is always less than 1
Relation between γ and α
∆IE
∆IB
α= ∆IC
∆IE
γ=
10
----------- (i)
----------(ii)
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Rearranging above equation we can write α=β/(β+1)
Transistor Load Line Analysis
A DC load line is a line on output characteristics of a transistor. By constructing load line, it
is possible to determine the collector current for various values of collector emitter voltage in a
transistor. As this line is used for finding DC current & voltage components and also as resistor
Rc acts as load in the transistor circuits this line is known as DC Load line
Consider a common emitter npn transistor circuit shown in Fig.2.15 (i) The output
characteristics of this circuit are shown in Fig. 2.15 (ii).
The value of collector-emitter voltage VCE at any time is given by ;
VCE=VCC– IC RC
...(i)
Fig. 2.15 i. Common emitter transistor circuit ii. Load line on output characteristics
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the second point A (OA = VCC/RC) on the collector current axis as shown in Fig2.15ii. By joining
these two points, d.c. load line AB is constructed. Now we can observe in the fig 2.15ii that as VCE
varies, the collector current varies along the DC load line. And hence DC load line helps in the
analysis of transistor circuits.
Operating point
The zero signal values of IC and VCE are known as the operating point. It is called operating
point because the variations of IC and VCE take place about this point when signal is applied. It
is also called quiescent (silent) point or Q-point because it is the point on IC − VCE characteristic
when the transistor is silent i.e. in the absence of the signal.
Suppose in the absence of signal, the base current is 5µA. Then IC and VCE conditions in the
circuit must be represented by some point on IB = 5 µA characteristic. But IC and VCE conditions
in the circuit should also be represented by some point on the d.c. load line AB. The point Q
where the load line and the characteristic intersect is the only point which satisfies both these
conditions. Therefore, the point Q describes the actual state of affairs in the circuit in the zero
signal conditions and is called the operating point. Referring to Fig. 2.16 for IB = 5 µA, the zero
signal values are VCE=OC volts
IC=OD mA
It follows, therefore, that the zero signal values of IC and VCE (i.e. operating point) are determined by the point where d.c. load line intersects the proper base current curve.
Fig. 2.16 operating point
Cut off and Saturation Points
Fig. 2.17 shows the output characteristics along with the d.c. load line.
(i) Cut off :- The point where the load line intersects the IB = 0A curve is known as cut off.
At this point, IB = 0 and only small collector current (i.e. collector leakage current ICEO) exists.
At cut off, the base-emitter junction no longer remains forward biased and normal transistor
action is lost. The collector-emitter voltage is nearly equal to VCC i.e.VCE (cut off) =VCC
(ii) Saturation :- The point where the load line intersects the IB = IB(sat) curve is called
saturation. At this point, the base current is maximum and so is the collector current. At
saturation, collector-base junction no longer remains reverse biased and normal transistor action
is lost.Ic(sat) ≈ Vcc/Rc and VcE (sat) ≈ 0V
If base current is greater than IB(sat), then collector current cannot increase because
collector-base junction is no longer reverse-biased.
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Fig. 2.17 output characteristics along with the d.c. load line
(iii) Active region:-The region between cut off and saturation is known as active region. In
the active region, collector-base junction remains reverse biased while base-emitter junction
remains forward biased. Consequently, the transistor will function normally in this region.
Transistor as switch / Inverter
If the transistor is cut-off, there is no base current, so there is no collector or emitter current.
That is collector emitter pathway is open as shown in fig 12.18 ii. And hence Ic=0A
In saturation, the collector and emitter are, in effect, shorted together as shown in fig
12.18iii. and collector current Ic ≈ IE. That is, the transistor behaves as though a switch has been
closed between the collector & emitter and hence, a transistor acts as switch when it is operated
in cutoff and saturation region.
CASE 1: When V input = 0 V
B-E junction is reverse biased (because minimum 0.7 V required for BE diode to cut-in)
 Both junctions reverse
 Transistor is in Cut – off
 Acts as OFF switch
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



Hence Vout is connected to Vcc but not to ground
no current flows, no loss of voltage
Hence Vout = Vcc
FINALLY _ Vin = 0V(low input) has given Vout = Vcc= 5V(high output)
Now second case:
CASE 2: When V input = 5 V
B-E junction is heavily forward biased (because minimum 0.7 V required for BE diode
to cut-in and we have supplied 5V – much higher than required)




IB is very high
So Ic is very very high (because Ic = βIB, the value is greater than saturation limit)
Transistor is in saturation
Acts as ON switch

 Hence Vout is connected to ground through On switch – i.e shorted to ground (=0V)
 heavy current flows, full loss of voltage in Rc itself
 Hence Vout = Ground voltage = 0V
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 FINALLY _ Vin = 5V(high input) has given Vout = 0V (low output)
CONCLUSION
Input = 5 V = High  On switch Vout = 0 V= Low
Input = 0 V = Low Off switch Vout = 5 V= High
Thus transistor acts as a switch / inverter
Transistor as an Amplifier in CE configuration
When the transistor is in the active state,. Fig 2.19 shows the common emitter npn amplifier
circuit. Note that a battery VBB is connected in the input circuit in addition to the signal voltage.
This d.c. voltage is known as bias voltage and its magnitude is such that it always keeps the
emitter-base junction forward biased regardless of the polarity of the signal source.
Fig.2.19 Transistor as amplifier ii. Collector Current waveforms
When no signal is applied, the input circuit is forward biased by the battery VBB. Therefore,
a d.c. collector current IC flows in the collector circuit. This is called zero signal collector
current. When the signal voltage is applied, the forward bias on the emitter-base junction
increases or decreases depending upon whether the signal is positive or negative. During the
positive half-cycle of the signal, the forward bias on emitter-base junction is increased, causing
total collector current iC to increase. Reverse will happen for the negative half-cycle of the
signal. Hence the amplification is achieved.
Fig. 2.19ii shows the graph of total collector current iC versus time. From the graph, it is
clear that total collector current consists of two components, namely;
(i) The d.c. collector current IC (zero signal collector current) due to bias battery VBB. This is
the current that flows in the collector in the absence of signal.
(ii) The a.c. collector current ic due to signal.
∴
Total collector current, iC =ic + IC
The useful output is the voltage drop across collector load RC due to the a.c. component ic.
The total voltage gain of typical transistor amplifier in CE configuration is given by
Av= Vo/Vin = - β Rc/Rin
Where Rin input resistance of a transistor
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Fixed Bias circuit
Transistor Biasing is the process of setting a transistors DC operating voltage or current
conditions to the active region, so that any AC input signal can be amplified by the transistor
faithfully. Fig 2.20 shows a means of providing biasing to the transistor with only one voltage
supply. And this is known as fixed biasing circuit
Here, the required DC base current is provided by Vcc and it flows through RB and hence
Base-Emitter junction is forward biased.
Fig 2.20 fixed bias circuit
Circuit analysis:Appliying KVL to input loop we get
Vcc=IBRB+VBE

IB = (Vcc-VBE) / RB
Hence required DC base current cab adjusted by selecting proper values of RB.
W.K.T
Ic = β IB
Applying KVL to output Loop we get
Vcc=IcRc+VCE
Or
VCE = Vcc – IcRc
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