MADE ERSY Bipolar Junction Transistors-Characteristics and Biasing 85

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Solution: ()
When twWO terminals of a transistor are shorted, it acts as a diode.
V

0.7

=10-13 le1x 26x1o9 = 49 mA

2.3 Early Effect

When BJT is biased in active region the emitter junction
(J)is forward-biased but the collector junction is reverse- V,-lVeal
biased, then in Fig.(2.3) the barrier width at J is negligible
as compared to space-charge width Wat J.
The transition region at junction is aregion of uncovered
charges on both sides of junction at positions ocCupied IVeal
by impurity atoms. As the voltage applied across the
junction increases, transition region penetrates deeper
into collector and base. As neutrality of charges must W
be maintained, so the number of uncovered charges on Jc
each side remains equal. Since the doping in base is Figure-2.3: Thepotential variation through ap
substantially smaller than that of collector, the penetration n-p transistor
of the transition region into the base is much larger than
neglected in Fig. (2.3), and allimmobile charges
that in collector. Hence thecollector depletion region is
are indicated in base region.
electrical base width is Wa=We -W. This modulation
If metallurgical base width is Wthen the effective modulation.
known as the Early effect or Base width
of effective base width by collector voltage is

2.3.1 Consequences of Early EffectANTS

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reverse collector voltage has three consequences.
The decrease in W½ with increase in
thereare less chances
base width of transistor decreases so
1 When Veis increased the effective
increases with increasing
recombination of charge carriers within
the base region. As a result a
of

|Vca increased within the

concentration gradient of minority carriers is
2. With decrease in basewidth the
base. As we have,

and is given by
where Ieis diffusion current
dP AP ..(2.9)
W
basewith.
where WA is effective
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With increase in VCB AP
increases due to decrease in
basewidth. As aresult increases
Wh
Consequently I also increases. Hence we see that I increases due to increase in gradient of
concentration of holes.
AIsO in BJT, voltage applied across one iunction has effect on current passing through otha.
Junction therefore junctions J- and J, are called interactive junctions.
3
erge value of |VoB depletion region can fully occupy the base region or in other words fv
exTemely large voltages, W% may be reduced to zero. This phenomenon is known as punch
through or reach through.
wnen punch-through occurs effective base width becomes zero and collector region
gets electricall
shorted to emitter. Due to this shorting,
the negative voltage applied at collector reaches emiter
also. This results in heavw current flowwhich can
damage the transistor.
2.4 BJT Configuration
Aconfiguration refers to the way three
terminals of BJT are used in amplitier.
Input node (1) + BJT
p+(2) Output node
V

(3) Reference or
common node

Figure-2.4
Based upon the reference or
common node a BJT can be used in
Table 2.2. three configurations as given in
Internal mechanism of current flow
remains S. No.
same in all three configurations. Configuration Input Node Output node
When BJT is in active mode carrier 1.
Common Base (CB)
flow OCcurs
from emitter (E) to collector (C and this 2
Common Emitter (CE)
flow
is controlled by base current 3
(2). This Common Collector (CC) B m
operation wil| remain same in all
configurations. Table-2.2
1.and I, are input and
output currents and Vand V, are
These four values are input and output voltages
interdependent and their interdependency
IVgraphs which are known as BJT characteristics.
respectively.
can be represented in the
We keep an assumption while formof
plotting BJT characteristics,
to be independent parameters input
whereas input voltage V, and current I, andloutput voltage V, as
dependent parameters. output current I, are Considered as
Thus V = f4, V,)
..(2. 10)
.(2.11)
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Inequation (2.10) if V, = constant, then
V, = {(1,)
Now if a graph is plotted between input current and input voltage keeping output voltage constant
then it is known as input characteristics.
In equation (2.11) ifl, = constant, then
4= 6(V,)
Now if agraph is plotted between output current and output voltage keeping input current con stant
then it is known as output characteristics.

2.5 The CommonBase Configuration

configuration. This circuit is also referred to
In Fig. 2.5 (b) ap-n-p transistor is shown in a grounded-base
common to the input and output circuits. For a p-n-p
9saCommon Base (CB) Configuration,since the base is down
holes.Since holes flow from emitter to collector and
transistor the largest current components are due to that
the polarity conventionsof Fig. (2.5). We have seen
towards ground out of the base terminal, then referring to for a
and Ig is negative for a forward-biased emitter junction, Ve is positive and
I- 0s positive, I, is negative polarities are
reverse-biased collector junction, Vce iS negative.
For an n-p-n transistor all current and voltage
transistor.
negative of those for a p-n-p
+ VoE
R
Emitter
Base
Emitter Collector +

n
+
VEB VoB Vcc
Je Je
Vc8
VEB

B (b)
(a)
Figure-2.5
relations, which give the
transistor of Fig. (2.5) by the following two
describe the Ve and input current I:
We may completely terms of the output voltage
in
output current I .(2.12)
input voltage Vea and ...(2.13)

Vca and I;").

(This equation is read, "I, is some function of , of
consider the
2.5.1 Input Characteristics characteristics isnot difficult if we
understanding of form of input and output "back to back" (with
two cathodes
A qualitative placed in series
transistors consists
of two diodes
fact that the characteristics
connected together). forward direction. The input
(emitter to base) is biased in diode for various
collector
reaion the input diode of emitter to base
" In the active characteristics
forward
represent simply the
Of Fig. (2.6)
voltages.

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Hrom input characteristics of Fig. (2.6) We 0.6
Vce Open
Conclude that there exists acut-in, offse V

or threshold voltage V, below which

the VaE
emitter current is very small. In
IS approximately 0.1Vfor
general, Vy voltage 0.4

germanium and
0.6V for Si transistors.
Ifwe increase the magnitude of
Emiter 0.2
VoB=0V

10
by Early effect, it causes
colector voltage, -20
increase in collector
Current when Ve iS kept constant.
Thus
Curve 20 30 40
shifts downward as Voal 10
50

The curve with collector

increases.
Emitter current I, MA
open represents the
characteristics of forward-biased
emitter Figure-2.6 Common-base input characteristics ofa typicalp-a
diode. When the collector is germaniumjunction transistor
shorted to base,
the emitter current
increases for a given VEa
and nence base can attract more holes Since the collector removes minority carriers from the base.
from emitter. This means that the curve
downwards from the collector with VCB=0IS shifted
characteristicsmarked"Vo Open.
2.5.2 Output Characteristics
Ihe relation of Equation (2.13) is given
collector current Iversus collector to in Fig. (2.7) for atypical p-n-p germanium transistor and a plot of
Curves of Fig. (2.7) are known as base voltage drop Vop, With emitter current Ie as a
output, or collector, static parameter. The
Saturation
region
characteristics.
Active region
-50

40 I=40 mA

MA
-30 30
Io,
curent
Colector -20 20

-10 10

Iço
Cut-off region
-2
-6
-8
Collector-to-base voltage drop Vca, V
Fiqure-2.7 Typicalcommon-base
Cut-off, active and saturation
regions
output characteristics of a
areindicated. Note the e p-n-p transistor
eexpanded voltage scale in the
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The collector-to-basediode is normally biased in reverse direction. If I, =0, the collector current is
I=loo For other values ofI, the output diode reverse current is augmented by a
fraction ofinput diode
forward currentt which reaches the collector. Note that I,oo iS negative for p-n-p transistor and
positivefor
npn transistor.

Active Region
In this region the Collector junction is biased in reverse direction and the emitter junction in forward
direction.

f emitter current is zero then collector current is small and equals the reverse saturation current loo Of
collector junctionConsidered as a diode.
Now if forward emitter current Iis caused to flow inemitter circuit then a fraction -ol_of this current will
reach the collector and i will be given by following equation:
Io=-d +lco |
depends
In the active region, the collector current is essentiallv independent of collector voltage and
(perhaps 0.5 percent) increase in
only upon emitter Current. However, due to Early effect there is small
lic with | Vol.
Because a is less than, but almost equal to unity, the magnitude of the collector Current
is (siightly) less
than that of emitter current.

Saturation Region
in which both emitter
The region to the left of the ordinate, Vp=0, and above theI=0 characteristics
"bottoming" has
and collector junctions are forward-biased, is called the saturation region.W½ say that
Voe0. Actually,
taken place because the voltage has fallen near bottom of the characteristics where
biasing of the collector
Va is slightly positive (for p-n-p transistor) in this region, and this forward
accounts for large change in collector current with small changes in collector voltage.

Cut-off Region
other characteristics. This
The characteristic forI=0passes through the origin, but is otherwise similar to
lgis
characteristics is not coincident with voltage axis, though the separation is difficult to show because
characteristics,
only a few nanoampheres ormicroamperes. The region belowand to the right of theIe= 0 region.
cut-off
for which the emitter and collector junctions are both reverse-biased, is referred to as
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2.6 The Common-Emitter Configuration
Most transistor circuits have the emitter terminal common
to both input andoutput. Sucha Common-Emitter(CE) R
or grounded-emitter, configuration is indicated in B
Fig. (2.8). VCE
Incommon-emitter configuration, the input current and
Output voltage are taken as independent variables, VBE E
Vcc
whereas the input voltage and output current are the
+

dependent variables. We may write

Vag = f(Vcpl) ..(2.14)
...(2.15)
Figure-2.8 Atransistor common-emitter confiquration.
Equation (2.14) describes the family of input
characteristics and equation (2.15) describes the family
The symbolV ispositive numberrepresenting the
magnitude of the supply voltage
of output characteristics.
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2.6.1 -0.6
The Input Characteristics T=25°C
Typicallinput characteristic curves for ap-n-pjunction
germanium
-0.5 -VcE =-1.0V
transistor are given in Fig. (2.9). voltage
V
VaE
-0.4 02
win Collector shorted to the emitter and forward-biased emitter,
the input characteristics are essentiallytthat of aforward biased- -0.3
diode.
VBE Decomes zero, then I, will be zero. since under these S Base -0.2

Conditions both enmitter and collector junctions will be short -0.1

circuited.
For any other valueof Vs base current for Var=0 is not actually -1 -2 -3 4
zero but is too small to be observed in Fig. (2.9). Base current / mA
" In general increase in V with constant Vacauses adecrease
Fiqure-2.9 Typical common-emie
In base width W
(the Early effect) and results in decreased
input characteristics of the p-n
recombination base current.
germanium junction transistor offa
These considerations account for the shape of input
characteristics shown in Fig. (2.9).
2.6.2 The Output Characteristics
lypical output characteristics curves for a p-n-p junction germanium transistor are given in Fig.
(2.10
-50
1 ; = - 0 . 3 5mA

T= 25°C
mA -40 0.30

voltage
Colectr-mi
Ic,
-0.25

-Pe= 150 mw
-30

-0.15

-20

-0.10
Load line
-10
0.05

0
-2 4
-6 -8 -10

Collector-emitter voltage Vo V
Figure-2.10 Typical common-emitter output
characteristics ofa p-n-pgermaniumjunction
Vec=10 Vand R, =500 2is superimposed transistor. Aload line corresponding to
The family of curves may be
divided into three regions. These are
saturation region. active reaion, cut-off regiono
Active Region
In Fia. (2.10), active region is
the area to the right of the
Ig=0. ordinate Voe=a few tenths of a volt and
adu
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" In this
regionthe transistor output current responds
most sensitively to an input signal.
If transistor is to beused as an amplifying device
region.
without appreciable distortion, it must be restricted to
operate in this
bo ld also be clear that a slight change in
a has large effect on the
an that common-emiter characteristics are common-emitter curves, and
normally subjected to wide variation even among transistor
of a given type. 1hiS variability is caused by the fact that I,
is the difference between large and nearly
equal currents, I_and
Cut-off Region
We might be inclined to think that cut-off in Fig. (2.10) occurs at the intersection of the
urrent I,=0; however we now find that appreciable collector current may exists
load line with
under these conditions.
We have
lo=-al+ Ico ..2.16)
From Fig. (2.8), it lg =0, then I =-ç;hence we have
Ico_= lcEO ..2.17)
. The actual collector current with collector junction reverse-biased and base open-circuited is designated
by the symbol IcEo:
" Since, even in the neighbourhood of Cut-off, a may be as large as 0.9 for germanium, then I, ~ 101co at
zero base current.
Accordingly, in order to cut-off the transistor, it is not enough to reduce I, to zero. Instead, it is necessary
to reverse-bias the emitter junction slightly.
" In summary, cut-off means that I =0, I,=Icolg==oo and Vpg is areverse voltage whose
magnitude is of the order of 0.1 Vfor germanium and0 Vfor a silicontransistor.
Saturation Region
The saturation region may be defined as the one where collector junction (as well as the emitter junction)
is forward-biased.
" In this region bottoming occurs, VEL drops to few tenths of a volt, and the collector current is
approximately independent of base current, for given values of Voc andR,.
Hence we may consider that onset of saturation takes place at knee of the transistor curves in
Fig. (2.10).

Remember Test for Saturation Region:

It is often important to know whether or not a transistor is in saturation region. There are
two methods for making such a determination which are summarized below.
IfI, and , can be determined independently from the circuit under consideration,

the transistor is in saturation ifl¡

hE (or )
If Vogis determined from the circuit configuration and if this quantity is positive for
ap-n-p transistor (or negative for an n-p-n), transistor is in saturation of course, the
emitter junction must be simultaneously forwarcd-biased, but then wNe should not be
testing for saturation if this condition were not satisfied,

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2.7 The Common-Collector Configuration

Another transistor configuration, shown in Fig. (2.11), is
known as the Common-Collector (C) configuration. The
circuit is basically the same as the circuit of Fig. (2.8), with
the excèption that the load resistor is in emitter circuit rather
than in collector circuit. If we continue to specify the lg
operation of circuit in terms of the currents which flow, the
+ Vcc
operation for Common-collector is much same as for
common-emitter configuration. When base current is oo I|Re Vo
the emitter current will be zero, and noCurrent will flow in
load. As the transistor is brouaht out of this back-biased
condition by increasing the magnitude of the base current,
the transistor will pass through the active region and Figure-2.11: The transistor common
eventually reach saturation. In this condition whole supply collector configuration
transistor,
voltages,except for a very small drop across the
Will appear across the load.

(a) collector follower

(b) emitter follower
(c) base follower
Student's none of these