3.

39
ANALOG ELECTRONICS

3.5.1 Introduction to PNP and NPN Transistors

layer, two junction and
Abipolar junction transistor is athreeoperation depends on the
three terminal semiconductor device. Its
interaction of majority and minority carriers. Therefore, it is named as bipolar
two word com bination,
device. The word transistor was derived from the signals are transferred
(TRANsfer + reSISTOR = TRANSISTORJ. Transistor means,
(output) circuit.
from low resistance circuit (input) into high resistance
together to
Transistor consists of two back to back PN junction joined
junctions gives three
form single piece of semiconductor device. The two types of transistors
two
region named as emitter, base and collector. There are
specifies whether the
such as PNP and NPN. The arrow on the emitter direction of
indicates the
transistor is NPN type or PNP type. This arrow also
biased. Figure 3.28
current flow, when the emitter base junction is forward
PNP transistor.
shows the circuit representation and symbols of NPN and
P N P NP N E
C E

"B B JB
B
a) PNP transistor and its symbol b) NPN transistor and its symbol
Figure 3.28
Emitter:
function
It is more heavily doped than any of other regions because, its main
base.
is to supply majority charge carriers (either electrons or holes) toas the Ip.
noted
The current through the emitter is emitter current. It is
Base:
emitter
Base is the middle section of the transistor. It separates the
and collector. It is very lightly doped. It is very thin as compared to either
emitter or collector. The current flows through the base section is base current.
It is denoted as Ig.
Collector:
It forms the right ha.d side section of the transistor. It is shown in
figure 3.28. The main function of the collector is to collect the majority
charge carriers coming from the emitter and passing through the base.
Generally, collector region is made physically larger than the emitter region,
because it has to dissipate much greater power. Collector is moderately doped.
The current flows through the collector section is collector current. It is
denoted as lc.
3.40 BASIC ELECTRICAL AND ELECTRONICS ENGINEERING

PNP and NPN transistors

To understand the basic operation of transistor, the following points are

need to be kept in mind:
1) Emitter section is meant to provide charge carriers, therefore, it is
always forward biased.
2) First letter of transistor type indicates the polarity of the emitter
voltage with respect to base.
3) The main function of collector is to collect or attract those carriers
through the base, hence it is always reverse biased.
4) Second letter of transistor type indicates the polarity of collector
voltage with respect to the base.

3.5.2 Operation of PNP Transistor

Base (B)
Emitter (E) Collector (C)
N

E C

Emitter o Collector
(E) (C)
Hole flow Hole flow
VEB Collector base
VcB
Emitter base IR junction
junction Base (B)
Figure 3.29
Figure 3.29 shows the connection diagram of PNP transistor. In this
circuit diagram, the emitter base junction is forward biased (i.e., positive
polarity of the battery is connected with P' type semiconductor and negative
polarity of the battery is connected with N' type semiconductor and collector
base junction is reverse biased.
The holes in the emiter are repelled by the positive battery terminal
towards the PN or emitter junction. Then, the potential barrier at emitter
junction is reduced. As a result of this, depletion region disappears
and hence holes cross the junction and enter into N-region (base).
This constitutes the emitter current Ig. Because, the base region is
thin and lightly doped, majority of the holes (about 97.5%) are able to
drift across the base without meeting electrons. Only 2.5% of the holes
recombine with the free electrons or N-region. This constitutes the base
current IB, which is very small. The holes which after crossing the N-P
collector junction, enter the collector region. They are swept out by the
negative collector voltage VcB. This constitutes the collector current Ic.
ANALOG ELECTRONICS 3.41

The following points about transistor circuits are:

1) In a PNP transistor, majority charge carriers are holes.
2) Emitter arrow shows the direction of flow of conventional current.
But electrons flows will be in the opposite direction.
3) Emitter base junction is always forward biased and collector base
junction is always reverse biased.
4) The collector current is always less than the emitter current because
same recombination of holes and electrons takes place.
lç = Ip - Ip
I = Ip + lc

3.5.3 0peration of NPN Transistor

Base (B)
Emitter (E) Collector (C)

Emitter o -o Collector
(E) (C)
B
VE8 Collector base
VcB
Emitter base junction
junction Base (B)

Figure 3.30

Figure 3.30 shows the connection diagram of NPN transistor. In this

circuit diagram, the emitter base junction is forward biased (i.e., negative
polarity of the battery is connected with Ntype semiconductor and positive
polarity of the battery is connected to with P type semiconductor) and
collector base junction is reverse biased.

The electrons in the em'tter region are repelled by the negative

battery terminal towards the emitter junction. The electrons crossover
into the P-type base region because potential barrier is reduced due to
forward bias and base region is very thin and lightly doped, most of the
electrons (about 97.5%) cross over to the collector junction and enter the
collector region where they are readily swept up by the positive collector
voltage VcB. Only 2.5% of the emitter electrons combine with the holes
in the base and are lost as charge carriers.
The following points about transistor circuits are:
3.42 BASIC ELECTRICAL AND ELECTRONICS ENGINEERING

1) In a NPN transistor, majority charge carriers are electrons.

2) Emitter arrow shows the direction of flow of conventional current.
3) Collector current lç is less than emitter current Ig.
The choice of NPN transistor is made more often because majority
charge carriers are electrons whose mobility is much more than that of
holes.

3.5.4 Transistor Circuit Configurations

There are three terminals in a transistor such as emitter, base
and collector. However, when a transistor is to be connected in a circuit,
we require four terminals.
Two terminals are used for input connection and other two terminals
are used for output connection. This difficulty can be overcome by making
one terminalcommon to both input and output circuits. Accordingly,
there are three types of circuit configurations.
i) Common Base (CB) configuration
ii) Common Emitter (CE) configuration
iii)Common Collector (CC) configuration

Input Output Output

Input
E

(a) CB Configuration (b) CE Configuration

Output
Input

(c) CC Configuration
Figure 3.31
ANALOG ELECTRONICS 3.43

The term 'common' is used to denote the electrode that is common

to the input and output circuits, because the common electrode is generally
grounded. Figure 3.31 shows the different configurations of NPN transistor.
Each circuit configuration has specific advantages and disadvantages.
It may be noted here that, regardless of circuit configuration, the emitter
base junction is always forward biased and collector base junction is always
reverse biased.

3.5.4.1 Common Base Configuration

In this configuration, input is applied between emitter and base while
transistor is
output is taken from collector and base. Here, base of the
common to both input and output circuits and hence, named as common
base configuration. It is shown in figure 3.32.

-NPN
Vca

Figure 3.32 CB configuration

The two important characteristics of common base connection are:

i) Input characteristics ii) Output characteristics
Input Characteristics
characteristics
Figure 3.33 shows the circuit diagramn for obtaining
with input
of CB configuration. It shows houw, the input current I varies determine
voltage VEB when output voltage VcB is held constant. To
set at zero,
the input characteristics initially, the output voltage VcB is
then the input voltage VER 0s increased.

VEE R. VEB (v)Vce Vcc

configuration
Figure 3.33 Circuit for obtaining the characteristics of CB
 characteristics
is
Figure voltage measured. is
of voltage is Output 3.44
(VcB). he the conclusion
i.e., tindependent current
indicatesemitter-base IgY-axis
input
Figure taken steps
andthe kept increases
The To resulting
transistor. This The
2 4 6
3.35 VcB Characteristics
determine
characteristics and
3.34 collector-base constant, VEg is
‘ along Input characteristics the input
I(mA) at almost
that emitter
Input 0.642 shows and of
constant is change rapidly voltage
input
Vcay Y-axis resistance collector-base
It emitter characteristics
corresponding
collector
drawn varying
stics the atthe is independent basc
=
1 in the resistance with of
VcB10V
B Voutput emitter outputa a VEg
andbetween voltage suitable emitter ratio current voltage
0.8 R2 common
AVEB (volts)
lVea smnall The
collector-base and Rin may
characteristics current (VcR)
characteristics, ofvoltage of emitter is BASIC
= current change be Ip is
collector e
thvalue collector-baseincrease base Vra drawn
very
used and
igure isoutput along ELECTRICAL
VcB current
Saturation
region
I,increased constantVCB at henceconfiguration.
small.
current by (AlE) in to between
3.35 (mA) Generally, current emitter-base find in
X-axis.
voltage adjusting
Cut-off
v(V)
region
of current at collectorAnother emitter (l) AND
utput CB voltage.
I,=0constantI= Ic
from the constant the
regionActive Figureemitter ELECTRONICS
is
configuration.(VcB) lç is generally
collector and (collectoremitter input
tics noted. zero base The
the voltage current This one,
Breakdown emitter 3.34current
collector-base
along emitter-base collector-base resistance voltage.
This in leads the ENGINEERING
current a current showstaken
(AVEB) is
number current) almost emitter current
X-axis. output to Ig
This along
the the and
lc Is to of
ANALOG ELECTRONICS
3.45

The output characteristics gives us the following tmportant points.

i) The collector current le varies with VcB only at very low
(< 1V). voltages
ii) When the value of VcR 0s raised abve 1-2 V, the
collector current
becomes constant as indicated by straight horizontal curves. It means
that, now lç is independent of collector base voltage Vce and depends
upon emitter current only. This is consistent with the theory that
emitter current flows almost entirely to the collector terminal.
iii)A very large change in collector-base voltage
produces only a tiny
change in collector current. Due to this, the output
very high. resistance is
iv) Output resistance is the ratio of
change in collector-base voltage
(AVCB) to the resulting change in collector current (Alc) at constant
emitter current (lE) i.e.,

Output resistance Rout = AVcB at constant I,

Al
v) This characteristic may be used to find current amplification factor
(a). It is defined as the ratio of
change in output current (Alc) to
the change in input curent (Alp) at constant collector-base voltage.
Al, at constant VCR

vi) The output characteristics curve mnay be

divided into three regions.
They are saturation region, active region and
cut-off region.
Saturation Region: It is the region left to the vertical
this region, collector-base voltage VoB is line. In
base junction is also forward biased and a negative, i.e. the collector
results in larger variation in collector current. small change in VcB
Active Region: It is the region between the veritical line to
axis. In this region, the collector current is horizontal
equal to the emitter current. In this region, almost constant and is
is forward biased and the emitter-base junction
collector-base junction is reversed biased.
Cut-off Region: It is the region along the
in figure 4.19. It corresponds to the horizontal axis shown
curve marked IE =0. In this region,
both junctions are reverse biased. Due
to this, there is no current
flow in collector terminal due to
due to minority carriers will flow.majority carriers, but the current
This current is known as
saturation current. reverse
3.46 BASIC ELECTRICAL AND ELECTRONICS ENGINEERING

Characteristics of CB Configuration
1) The input impedance of this configuration is low (about 100 2).
2) The output impedance is very high (about 1M2).
3) The voltage gain is medium (about 150).
4) The current gain is less than unity.
5) This configuration is mainly used for high frequency applications.
3.5.4.2 Common Emitter Configuration
In CE configuration, input is applied between base and emitter and
output is taken from the collector and emitter. Here, the emitter terminal
is common to both input and output circuits and hence named as common
emitter configuration. The most important characteristics of this circuit
configuration are input characteristics and output characteristics.
Input Characteristics
Figure 3.36 shows the circuit diagram for common-emitter configuration.
It shows how the input current Ig varies with change in input voltage
VBg while output voltage Vcg is held constant at a particular value.
To begin with, collector-emitter voltage VcE is maintained constant at a
convenient value and then VRE is increased in steps. The corresponding
values of base current IR are noted at each step. This procedure is repeated
for different constant values of VcE.

R VRE VcE R2 Vcc

Figure 3.36 Circuit for CE configuration

This input characteristics is drawn between base current IR and
base-emitter voltage VBE at constant collector-emitter voltage Veg.
Here, the base current Ig is taken along Y-axis and base-emitter voltage
VBE along X-axis. Figure 3.37 shows the input characteristics of CE
configuration.
 ANALOG ELECTRONICS 3.47

Active Breakdown
I, (mA) 4
lc (mA region region
VcE=1 V VCE2=2V
8 B IB

2 A A

0.2 0.4 0.6 0.8 Cut off VCEV)

region
Figure 3.37 Input characteristic Figure 3.38 Output characterisics
The input characteristics gives us the following points.
i) If the input voltage (Vpg) is less than threshold or knee voltage, below
which the base current is very small. The value of knee voltage is
0.7 V for silicon and 0.3 Vfor germanium transistor. The knee
voltage means the voltage at which conduction starts. i.e., input
current (base current) increases. This characteristics is similar to
the forward biased diode curve.

ii) As compared to CB arrangement, Ig increases less rapidly with Va..

Therefore input resistance of a CE configuration is higher than that
of CB configuration.
iii)The input resistance of this configuration is the ratio of change in
base-emitter voltage (AVa) to the resulting change in base current
(Alg) at constant VcE

Rin = at constant Vcp

The value of input resistance for this configuration is of the order

of a few hunderd ohms.
Output Characteristics
It is the curve between collector current Ic and collector-emitter
voltage VcE at constant base current Ig, For obtaining this curve, first
base current IR is set to a convenient value and maintained constant.
Then, collector-emitter voltage VcE is increased from zero in steps, collector
current lc being noted at each step. Next, VcE is reduced to zero, and I is
increased to another convenient value and maintained constant and then

VcE is increased from zero in steps, lc being noted at each step. In this
way a family of curves is obtained.
 and
output. 3.5.4.3 Characteristics 3.48

output In 6) 5) 4) 3) 2) From
this1) The
This 5) 4) 3) 2) 1)
Here, this The The The The Output to
defined
Output input as As
This intoFurther theregion is When which
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almost current
Common resulting may zero, isfollowing
the a
nfiguration, current
signal configuration voltageoutput input characteristics transistor
breakdownresult shown
input current
(AlB). ratio in be I
collector
resistance resistanceas increase saturationthe constant
= lç
of CB
noted curve,VcE
isCllector impedance CE the 0, rapidly
signal gain
gainimpedance change of junction transistor in points
taken of this, a
Configuration ratio in that, small figure
is
change region. in
active
current and
collector Rout canenormous VcE current increases.
Configuration
out is
is ishigh
in of is B if amount increases may
applied mainly high. is also is independent 3.38.
collector
= change = VcE is BASIC
from (about high
is low in be Itcausesincreased, region. said le be
AVcE found output used is is (Icro). changes After
terminal used AL Al This noted ELECTRICAL
between
lector (about shown Iç increased of from
avalanche to
500).(about constant at
Igcurrent in to It collector this,
for wi l so be
Since, region from
collector-emitter from current find is of zero
is audio 750 inflow shown that in is
base 45 current figure collector
VcE. AND
common this breakdown continuously cut-off the called is to the
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frequency k2). collector
This called voltage,
contain ELECTRONICS
and (Alc) increases
in main
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at 3.38. the thcharacteristics:
e current
rminal. gain figure region.
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then
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(AVCE) enter called
and It upto
is in
 ANALOG ELECTRONICS 3,49

Characteristics of cc Configuration
A
R
A

(VeC VEE
VgB R
V) Vac

Figure 3.39 Circuit for CC configuration

Figure 3.39 shows the circuit arrangement for common collector
configuration. To determine the output characteristics, the base current Ip
is kept constant, at a suitable value by adjusting the base collector voltage
and varying R2 and the output current (emitter current (Ig)) is measured.
We know that the CE output characteristics are plotted as Ic Vs VCE. Since
lc is apprOximately equal to Ig,common collector characteristics is identical to CE
configuration. Figure 3.41 shows the output characteristics of CC configuration.
From this characteristics, we can find the output resistance (Rout)
Ig (mA) +

100

80 |VEC =2 VVEC= 4 V
60
2
40 Ig Constant
20

0
1 2 3 4 5 Vac
Vec(M)
Figure 3.40 Input characteristics Figure 3.41 Output characteristics
It is defined as the ratio of change in collector-enmitter voltage (4VEc) to
the resulting change in emitter current (AlE) at constant base current (R).

AVEC
Rout at constant IB

This characteristics may be used to find current amplification factor

(r). It is defined as the ratio of change in output current (Al) to the change
in input current (Alp) at constant VcE.

at constant VcE
3.50 BASIC ELECTRICAL AND ELECTRONICS ENGINEERING

Input Characteristics
The common collector input characteristics are different from CB
and CE configuration. The difference is due to the fact the input voltage
VBc is largely determined by the level of emitter-collector voltage Vgc.
This is because when the transistor is biased on, VBE remains around
0.7 V, for Silicon transistor (and 0.3 V for Ge transistor) and VBC may be
much larger than 0.7 V. From the circuit diagram 3.39,
Vpc = Vpc + VBE
VBE Vgc - VBc
Suppose VEc = 2 Vat IR = 100 uA and VRE = 0.7 V, then Vac 1.3 V.
Then VEc is maintained constant at 2 volts while the input voltage VBc is
increased to 1.5 V then VRE reduced to 0.5 V. Since VBE is reduced, IB is
also reduced from 100 uA to zero.
Figure 3.40 shows the input characteristics of CC configuration. In this
curve, VEc is mnaintained constant. At particular value of Vac, base current
value is noted. Then increasing VBC, the base current Ip reduces out and
reaches to zero. From this curve, we can find the input resistance R;n

Rin = at constant Vpc

Characteristics of CC Configuration
1) The input impedance is high (about 750 k2).
2) The output impedance is low (about 50 ).
3) The voltage gain is less than 1.
4) The current gain is high.

5) This configuration is mainly used for impedance matching.

3.5.4.4 Comparison of Transistor Configurations

Characteristics CB CE CC

Highlabout 750 k)
|Input Impedance Low (about 100 2) Low (about 750 2)
Low (about 50 )
|Output Impedance Very high (about 1 M2)| High (about 45 kQ)
|Voltage Gain About 150 (Medium) About 500 (Medium) Less than 1
Current Gain Less than unity High High

|Application For high frequency For audio frequency For impedance

application application matching