Edc unit 3

EDC UNIT-3 Question&answer
GRIET-ECE G.Surekha Page 1
UNIT-III Bipolar Junction Transistor
1.Explain the construction and working of BJT.
A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of
doped semiconductor material and may be used in amplifying or switching applications. Bipolar
transistors are so named because their operation involves both electrons and holes. Charge flow
in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions
of different charge concentrations. An NPN transistor can be considered as two diodes with a
shared anode. In typical operation, the base-emitter junction is forward biased and the base–
collector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is
applied to the base–emitter junction, the equilibrium between thermally generated carriers and
the repelling electric field of the depletion region becomes unbalanced, allowing thermally
excited electrons to inject into the base region. These electrons wander (or "diffuse") through the
base from the region of high concentration near the emitter towards the region of low
concentration near the collector. The electrons in the base are called minority carriers because
the base is doped p-type which would make holes the majority carrier in the base.
To minimize the percentage of carriers that recombine before reaching the collector–base
junction, the transistor's base region must be thin enough that carriers can diffuse across it in
much less time than the semiconductor's minority carrier lifetime. In particular, the thickness of
the base must be much less than the diffusion length of the electrons. The collector–base junction
is reverse-biased, and so little electron injection occurs from the collector to the base, but
electrons that diffuse through the base towards the collector are swept into the collector by the
electric field in the depletion region of the collector–base junction. The thin shared base and
asymmetric collector–emitter doping is what differentiates a bipolar transistor from two separate
and oppositely biased diodes connected in series.
Transistor 'alpha' and 'beta'
The proportion of electrons able to cross the base and reach the collector is a measure of the BJT
efficiency. The heavy doping of the emitter region and light doping of the base region cause
many more electrons to be injected from the emitter into the base than holes to be injected from
the base into the emitter. The common-emitter current gain is represented by βF or hfe; it is
approximately the ratio of the DC collector current to the DC base current in forward-active
region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors
designed for high-power applications. Another important parameter is the common-base current
gain, αF. The common-base current gain is approximately the gain of current from emitter to
collector in the forward-active region. This ratio usually has a value close to unity; between 0.98
and 0.998. Alpha and beta are more precisely related by the following identities (NPN
transistor):
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Fig Simplified cross section of a planar NPN bipolar junction transistor
A BJT consists of three differently doped semiconductor regions, the emitter region, the base
region and the collector region. These regions are, respectively, p type, n type and p type in a
PNP, and n type, p type and n type in a NPN transistor. Each semiconductor region is connected
to a terminal, appropriately labeled: emitter (E), base (B) and collector (C).
The base is physically located between the emitter and the collector and is made from lightly
doped, high resistivity material. The collector surrounds the emitter region, making it almost
impossible for the electrons injected into the base region to escape being collected, thus making
the resulting value of α very close to unity, and so, giving the transistor a large β. A cross section
view of a BJT indicates that the collector–base junction has a much larger area than the emitter–
base junction.
Small changes in the voltage applied across the base–emitter terminals causes the current that
flows between the emitter and the collector to change significantly. This effect can be used to
amplify the input voltage or current. BJTs can be thought of as voltage-controlled current source,
but are more simply characterized as current-controlled current sources, or current amplifiers,
due to the low impedance at the base.
NPN
The symbol of an NPN Bipolar Junction Transistor.
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NPN is one of the two types of bipolar transistors, in which the letters "N" (negative) and "P"
(positive) refer to the majority charge carriers inside the different regions of the transistor. Most
bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in
semiconductors, allowing greater currents and faster operation. NPN transistors consist of a layer
of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the
base in common-emitter mode is amplified in the collector output. In other terms, an NPN
transistor is "on" when its base is pulled high relative to the emitter. The arrow in the NPN
transistor symbol is on the emitter leg and points in the direction of the conventional current flow
when the device is in forward active mode.
PNP
The other type of BJT is the PNP with the letters "P" and "N" referring to the majority charge
carriers inside the different regions of the transistor.
The symbol of a PNP Bipolar Junction Transistor.
PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped
material. A small current leaving the base in common-emitter mode is amplified in the collector
output. In other terms, a PNP transistor is "on" when its base is pulled low relative to the emitter.
The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the
conventional current flow when the device is in forward active mode.
Regions of operation
Applied voltages Mode
E < B < C Forward active
E < B > C Saturation
E > B < C Cut-off
Bipolar transistors have five distinct regions of operation, defined by BJT junction biases.
The modes of operation can be described in terms of the applied voltages (this description
applies to NPN tranistors; polarities are reversed for PNP transistors):
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Forward active: base higher than emitter, collector higher than base (in this mode the
collector current is proportional to base current by βF).
Saturation: base higher than emitter, but collector is not higher than base.
Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor
is not letting conventional current to go through collector to emitter.
In terms of junction biasing: ('reverse biased base–collector junction' means Vbc < 0 for NPN,
opposite for PNP)
Forward-active (or simply, active): The base–emitter junction is forward biased and the
base–collector junction is reverse biased. Most bipolar transistors are designed to afford
the greatest common-emitter current gain, βF, in forward-active mode. If this is the case,
the collector–emitter current is approximately proportional to the base current, but many
times larger, for small base current variations.
Saturation: With both junctions forward-biased, a BJT is in saturation mode and
facilitates high current conduction from the emitter to the collector. This mode
corresponds to a logical "on", or a closed switch.
Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse
biased) are present. There is very little current, which corresponds to a logical "off", or an
open switch.
Bipolar Transistor Construction
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The construction and circuit symbols for both the NPN and PNP bipolar transistor are shown
above with the arrow in the circuit symbol always showing the direction of conventional current
flow between the base terminal and its emitter terminal, with the direction of the arrow pointing
from the positive P-type region to the negative N-type region, exactly the same as for the
standard diode symbol.
2. Explain CB configuration with the help of input and output characteristics.
There are basically three possible ways to connect a Bipolar Transistor within an electronic
circuit with each method of connection responding differently to its input signal as the static
characteristics of the transistor vary with each circuit arrangement.
1. Common Base Configuration - has Voltage Gain but no Current Gain.
2. Common Emitter Configuration - has both Current and Voltage Gain.
3. Common Collector Configuration - has Current Gain but no Voltage Gain.
The Common Base Configuration.
As its name suggests, in the Common Base or Grounded Base configuration, the BASE
connection is common to both the input signal and the output signal with the input signal being
applied between the base and the emitter terminals. The corresponding output signal is taken
from between the base and the collector terminals as shown with the base terminal grounded or
connected to a fixed reference voltage point. The input current flowing into the emitter is quite
large as its the sum of both the base current and collector current respectively therefore, the
collector current output is less than the emitter current input resulting in a Current Gain for this
type of circuit of less than "1", or in other words it "Attenuates" the signal.
The Common Base Amplifier Circuit
This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal
voltages Vin and Vout are In-Phase. This type of arrangement is not very common due to its
unusually high voltage gain characteristics. Its Output characteristics represent that of a forward
biased diode while the Input characteristics represent that of an illuminated photo-diode. Also
this type of configuration has a high ratio of Output to Input resistance or more importantly
"Load" resistance (RL) to "Input" resistance (Rin) giving it a value of "Resistance Gain". Then the
Voltage Gain for a common base can therefore be given as:
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Common Base Voltage Gain
The Common Base circuit is generally only used in single stage amplifier circuits such as
microphone pre-amplifier or RF radio amplifiers due to its very good high frequency response.
Input/ Output Characteristics
Common-Base:
o Input characteristics: The EB junction is essentially the same as a forward biased
diode, therefore the current-voltage characteristics is essentially the same as that
of a diode:
Also the collector-base voltage V CB > 0 helps
enhance the current IE to some extent.
Output characteristics:
As the CB junction is reverse biased, the current IC depends totally on IE. When IE=0,IC=ICB0 is
the current caused by the minority carriers crossing the pn-junction. This is similar to the diode
current-voltage characteristics seen before, except both axes are reversed (rotated 180 degrees),
as both voltage ICB and current IC are defined in the opposite directions. When I E is increased
IC=αIE+ICBO is increased correspondingly. Higher VCB can slightly increase α and there by IC.
AsIC<IE CB configuration does not have current-amplification effect.
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3. Explain CE configuration with the help of input and output characteristics.
The Common Emitter Configuration.
In the Common Emitter or Grounded Emitter configuration, the input signal is applied between
the base, while the output is taken from between the collector and the emitter as shown. This
type of configuration is the most commonly used circuit for transistor based amplifiers and
which represents the "normal" method of connection. The common emitter amplifier
configuration produces the highest current and power gain of all the three bipolar transistor
configurations. This is mainly because the input impedance is LOW as it is connected to a
forward-biased junction, while the output impedance is HIGH as it is taken from a reverse-biased
junction.
The Common Emitter Amplifier Circuit
In this type of configuration, the current flowing out of the transistor must be equal to the
currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load
resistance (RL) is connected in series with the collector, the Current gain of the Common Emitter
Transistor Amplifier is quite large as it is the ratio of Ic/Ib and is given the symbol of Beta, (β).
Since the relationship between these three currents is determined by the transistor itself, any
small change in the base current will result in a large change in the collector current. Then, small
changes in base current will thus control the current in the Emitter/Collector circuit.
By combining the expressions for both Alpha, α and Beta, β the mathematical relationship
between these parameters and therefore the current gain of the amplifier can be given as:
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Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the
base terminal and "Ie" is the current flowing out of the emitter terminal.
Then to summarise, this type of bipolar transistor configuration has a greater input impedance,
Current and Power gain than that of the common Base configuration but its Voltage gain is much
lower. The common emitter is an inverting amplifier circuit resulting in the output signal being
180o
out of phase with the input voltage signal.
Common-Emitter:
Input characteristics:
Same as in the case of common-base configuration, the EB junction of the common-
emitter configuration can also be considered as a forward biased diode, the current-
voltage characteristics is similar to that of a diode:
The collector-emitter voltage VCE has little effect on IB.
Output characteristics:
The CB junction is reverse biased, the current
depends on the current IB. When IB=0,IC=ICEO the current caused by the minority carriers crossing
the pn-junctions. When IB is increased IC is correspondingly increased by β.
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Various parameters of a transistor change as functions of temperature. For example, β
increases along with temperature.
4. Explain CC configuration with the help of input and output characteristics.
The Common Collector Configuration.
In the Common Collector or Grounded Collector configuration, the collector is now common
and the input signal is connected to the Base, while the output is taken from the Emitter load as
shown. This type of configuration is commonly known as a Voltage Follower or Emitter
Follower circuit. The Emitter follower configuration is very useful for impedance matching
applications because of the very high input impedance, in the region of hundreds of thousands of
Ohms, and it has relatively low output impedance.
The Common Collector Amplifier Circuit
The Common Emitter configuration has a current gain equal to the β value of the transistor itself.
In the common collector configuration the load resistance is situated in series with the emitter so
its current is equal to that of the emitter current. As the emitter current is the combination of the
collector and base currents combined, the load resistance in this type of amplifier configuration
also has both the collector current and the input current of the base flowing through it. Then the
current gain of the circuit is given as:
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This type of bipolar transistor configuration is a non-inverting amplifier circuit in that the signal
voltages of Vin and Vout are "In-Phase". It has a voltage gain that is always less than "1" (unity).
The load resistance of the common collector amplifier configuration receives both the base and
collector currents giving a large current gain (as with the Common Emitter configuration)
therefore, providing good current amplification with very little voltage gain.
5.Give the Comparisions between CB,CE,CC configurations.
with the characteristics of the different transistor configurations given in the following table:
Characteristic
Common
Base
Common
Emitter
Common
Collector
Input impedance Low Medium High
Output impedance Very High High Low
Phase Angle 0o
180o
0o
Voltage Gain High Medium Low
Current Gain Low Medium High
Power Gain Low Very High Medium
6. What is the Relation between , & ?
RELATION BETWEEN , &
a) We know IE = IC + IB, But IC = IE.
IE = & IE + IB
IE - IE = IB or IB = IE (1 - )
Dividing both sides by , B E
c
C C
I I
I
I I
(1 - )
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Or
1 1
1
1
or
(b) CE B
B B B
II I
I I I
OR 1E
B
I
I
Or 1CE
C B
II
I I
OR 1CE
C B
II
I I

 
Or 1
. 1 or =
1
(c) E
B
I
I
, Substituting IB = IE - IC.
,
E
E C
I
I I
by diving Numerator & Denominator on R.H.S, by IE.
/ 1
/ / 1
E E
E E C E
I I
I I I I
Putting the value of = / + 1.
1
1
1
1 1
( ) 1
1 1
1
or
1
( ) 1
1
or
7. Explain early effect or base width modulation in CB configuration.
EARLY EFFECT OR BASE WIDTH MODULATION.
As VCC made to increase the reverse bias, the space charge width between collector and
base tends to increase. This results in decrease of effective width of the base. This dependence of
base width on collector voltage is known as „Early Effect‟. This decrease of effective base width
has three consequences.
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(i) There is less chance of recombination in base region and Ic increases causing to
increase with increase in VCB.
(ii) The charge gradient is increased within the base and current of minority carries
injected across emitter junction increases.
(iii) For extremely large VCB, the effective base width becomes zero causing voltage
breaks down in the transistor. This phenomenon is called the “Punch through” effect.
Problems
1. In common base connection IE = 1mA, IC = 0.95 mA calculate value of IB.
IB = IE – IC = 1 – 0.95 = 0.05mA.
2. In a CB configuration current amplification factor is 0.90 and emitter current is 1mA.
Determine base current.
= 0.9, IE = 1mA
= C
E
I
I
; IC = .IE = 0.9 x 1 = 0.9mA
IB = IE – IC = 1 – 0.9 = 0.1mA.
3. A BJT has IB = 10 A, = .99 and ICBO = 1 A what is collector current.
IC = IB + (1+ ) I CBO
B = 0.99 0.99
1 1 0.99 0.01 99
IC = 99 x 10 + (1 + 99)1
= 990 + 100 = 1090 A = 1.09 mA.
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4. A transistor operating in CB configuration has IC = 2.98mA, IE = 3.0mA and
Ico = 0.01mA. What current will flow in collector circuit of that transistor when
connected in CE configuration and base current is 30 A.
Given : In CB IC = 2.98mA, IE = 3.0mA = 2.98
3.0 0.99 ICO = 0.01mA.
In CE IB = 30 A IC = ? IC = IB
0.99 .99
99
1 1 0.99 0.01
IC = IB + ( + 1) Ico.
= 99 x 30 x 10-6
+ (100)0.01 x 10-3
= 2.97 x 10-3
+ 1 x 10-3
= 3.97mA
5. Given an NPN transistor for which = 0.98, Ico = 2 A IEO = 1.6 A. A CE configuration
is used and VCC = 12V and RC = 4.0K. What is the min. base current required in order
that transistor enter in to saturation region.
Given = 0.98, ICO = ICB = 2 A,
IEO = ICEO = 1.6 A.
VCC= 12 V, VCE = VCC = 12V,
RL = 4.0K IB = ? (In saturation)
Solution:-
Where Transistor is in saturation VCE = 0.2 (Assumed)
VRL = 12 – 0.2 = 11.8 Volts.
3
3
11.8
2.95 10 2.95
4 10
RL
c
L
V
I mA
R
We know
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Ic = IB + ( +1) ICBO
0.98 .98
49
1 1 0.98 0.02
2.95 x 10-3 = 49 IB + (49+1) 2 x 10-6
2950 A = 49IB +100 A
49IB = (2950 – 100) A = 2850 A
IB = 2850
49 58.16 A
6. Calculate the values of IE, dc and dc for a transistor with Ic = 13 A, IB=200mA, ICBO =
6 A. Also determine the new level of Ic which will result in reducing IB = 100mA.
Given a. Ic = 13 A IB = 200mA ICBO = 6 A IE = ? dc = ? dc = ?
b. find Ic, when IB = 100 mA
a. When IB = 200mA Ic cannot be 13 A.
as Ic = x IB.
Assume Ic = 13 Amperes
Then dc = C
B
I
I
= 13/200 x 103
= 65
IE = IC + IB or IE = 13+0.2 Amperes.= 13.2 Amperes
We can also use the formulae
1 1
,
1 1
E CBOI I IB
Which will also result Ic 13.2 Amperes.
13
0.985
13.2
C
dc
E
I
I
b. IC = dc. IB = 65 x 200 x 10-3
= 6.5 Amperes
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7. A transistor operating in CB configuration has Ic = 2.98 mA ,IE- =3.00mA and Ico = 0.01mA.
What current will flow in the collector circuit of the transistor when connected in CE configurat-
-ion with base current of 30 A.
Given Ic = 2.98 mA If IB = 30 A, Ic = ? IE = 3 mA Ico = 0.01 mA
Ic = IB + ( +1) ICO and ,
1
C
E
I
I
= 2.98
3.0 0.99 0.99
1 0.99 99
1
Ic = 99 x 30 x 10-6
+ (99+1) x 0.01 x 10-3
= 3970 A = 3.97 mA.
8. The reverse saturation current in a transistor is 8 A. If the transistor common base current
gain is 0.979, calculate the collector and emitter current for 40 A base current.
Given ICO = ICBO = 8 A IC & IE for IB = 40 A = 0.979
1 1 1 1
; 47.62
1 1 1 1 0.979
E CBO BI I I
d
6 6 6
47.62 8 10 47.62 40 10 2285 10 2285EI A
Ic = IE – IB = 2285 – 40 = 2245 A.
9. Given an NPN transistor for which = 0.98, ICO = 2 A and IEO = 1.6 A. A common
emitter connection is used and VCC = 12V and RL = 4.0K. what is the minimum base
current required in order that transistor enter into saturation region.
Given = 0.98 ICO = 2 A IB for Icsat=? IEO = 1.6 A VCC = 12V RL = 4.0 K.
When the transistor is in saturation Ic = Icsat and VCE of ideal transistor = 0 volts.
12
3
4000
cc
csat
L
V
I mA
R
0.98
49
1 1 0.98
B
Ic
I and
3
3 10
0.061
49
BI mA Or 61 A.
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10.The current gain of a transistor in CE circuit is 49. Calculate CE gain and find base current
where the emitter current is 3mA.
Given = 49 To find = ? IB for IE = 3mA.
IE = ( +1) IB or
3
3 10
60
1 1 49
E
B
I
I A
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