Diode Applications & Transistor Basics

Basic Electronics Engineering (Diode Applications & Transistors)
Prepared By Mr. A. B. Shinde, Electronics Engg., PVPIT, Budhgaon 1
Unit- III: Diode Applications and Transistors
1. V-I Characteristics of p-n junction diode:
Circuit diagram
Volt-ampere or V-I
characteristic of a p-n junction
diode is the curve between
voltage across the junction and
the current. Usually, voltage is
taken along x-axis and current
along y-axis.
To plot the Voltage-
Ampere (V-I) characteristics
curve of p-n junction diode, the
circuit arrangement is made as
shown in figure. Volt meter (V)
is placed across and current
meter (mA) is placed in series
with p-n junction diode.
The characteristics can be
studied under three heads,
namely; zero external voltage,
forward bias and reverse bias.
V- I Characteristics Curve
(i) Zero external voltage: When the external voltage is zero, i.e. circuit is open at K, the
potential barrier at the junction does not permit current flow. Therefore, the circuit
current is zero as indicated by point O on the curve.
(ii) Forward bias: With forward bias to the p-n junction i.e. p-type connected to positive
terminal and n-type connected to negative terminal, the potential barrier is reduced. At
some forward voltage (0.7 V for Si and 0.3 V for Ge), the potential barrier is eliminated
and current starts flowing in the circuit. From the forward characteristic, it is clear that at
first (region OA), the current increases very slowly. However, once the external voltage
exceeds the potential barrier voltage, the p-n junction behaves like an ordinary
conductor. Therefore, the current rises very sharply with increase in external voltage
(region AB).

(iii) Reverse bias: With reverse bias to the p-n junction i.e. p-type connected to
negative terminal and n-type connected to positive terminal, potential barrier at the
junction is increased; hence junction resistance becomes very high hence, a very small
current (few μA) flows in the circuit. If reverse voltage is increased continuously, the
kinetic energy of electrons (minority carriers) may become high enough to knock out
electrons from the semiconductor atoms. At this stage breakdown of the junction
occurs, characterised by a sudden rise of reverse current.
Important Terms
(i) Breakdown voltage: It is the minimum reverse voltage at which p-n junction breaks
down with sudden rise in reverse current.
(ii) Knee voltage: It is the forward voltage at which the current through the junction
starts to increase rapidly.
It is clear from the above graph that knee voltage for silicon diode is 0.7 V and 0.3 V for
germanium diode. If the forward voltage exceeds the knee voltage, the current starts
increasing rapidly.
2. Rectifiers:
Rectifier is circuit which converts a.c. signal into d.c. signal.
The supply voltage is sinusoidal and has a frequency of 50 HZ. It is used for
lighting, heating and electric motors. But there are many applications (e.g. electronic
circuits) where d.c. supply is needed.
The following two rectifier circuits can be used:
i. Half-wave rectifier ii. Full-wave rectifier
2.1. Half-Wave Rectifier:
Here, the rectifier conducts current only during the positive half-cycles of input a.c.
supply. The negative half-cycles of a.c. supply are suppressed i.e. during negative half-
cycles, no current is conducted and hence no voltage appears across the load.
Therefore, current always flows in one direction (i.e. d.c.) through the load though after
every half-cycle.

Input ac signal Rectified output
Half wave rectifier circuit diagram
Circuit Arrangement: The circuit diagram of half
wave rectifier is as shown in figure. The a.c. supply is
applied in series with the diode and load resistance
RL. Input, a.c. supply is given through a transformer.
The use of transformer permits two advantages.
Firstly, it allows us to step up or step down the a.c.
input voltage as the situation demands. Secondly,
the transformer isolates the rectifier circuit from
power line and thus reduces the risk of electric
shock.
Operation: The a.c. voltage across the secondary
winding AB changes polarities after every half-cycle.
During the positive half cycle of input a.c. voltage,
end A becomes positive w.r.t. end B. This makes the
diode forward biased and hence it conducts current.
During the negative half cycle, end A is negative
w.r.t. end B. Under this condition, the diode is
reverse biased and it conducts no current.
Therefore, current flows through the diode during positive half-cycles of input a.c.
voltage only. In this way, current flows through load RL always in the same direction.
Hence d.c. output is obtained across RL. It may be noted that output across the load is
pulsating d.c.
Advantages:
1. Simple circuit arrangement.
2. Low cost
Disadvantages:
1. The pulsating current in the load contains alternating component whose basic
frequency is equal to the supply frequency.
2. The a.c. supply delivers power only half the time.

2.2. Full-Wave Rectifier:
In full-wave rectification, current flows through the load in the same direction for
both half-cycles of input a.c. voltage. For the positive half cycle of input voltage, one
diode supplies current to the load and for the negative half-cycle, the other diode does
so; current being always in the same direction through the load. Therefore, a full-wave
rectifier utilises both half-cycles of input a.c. voltage to produce the d.c. output.
The following two circuits are commonly used for full-wave rectification:
i. Centre-tap full-wave rectifier ii. Full-wave bridge rectifier
2.2.1. Centre-Tap Full-Wave Rectifier:
Circuit arrangement: The circuit has two diodes D1 and D2 as shown in figure. A centre
tapped secondary winding AB is used with two diodes connected so that each uses one
half-cycle of input a.c. voltage. The load resistance RL is connected between the
common point of cathode of both diodes and centre tap of transformer.
Operation:
During positive half cycle During negative half cycle
During the positive half-cycle, the point A becomes positive and point B becomes
negative. This makes the diode D1 forward biased and diode D2 reverse biased.
Therefore, diode D1 conducts while diode D2 does not. The conventional current flow is
through diode D1, load resistor RL and the upper half of secondary winding as shown by
the dotted arrows.
During the negative half-cycle, point A becomes negative and point B becomes
positive. Therefore, diode D2 conducts while diode D1 does not. The conventional
current flow is through diode D2, load RL and lower half winding as shown by solid
arrows. From the figure, it is clear that current in the load RL flows in the same direction
for both half-cycles of input a.c. voltage. Therefore, d.c. is obtained across the load RL.

Advantages:
1. Full wave rectification i.e. high efficiency
Disadvantages
1. It is difficult to locate the centre tap on the secondary winding.
2. The d.c. output is small as each diode utilises only one-half of the transformer
secondary voltage.
3. The diodes used must have high peak inverse voltage.
2.2.2. Full-Wave Bridge Rectifier:
Circuit arrangement: The need for a centre tapped
transformer is eliminated in the bridge rectifier. It
contains four diodes D1, D2, D3 and D4 connected to
form bridge as shown in figure. The a.c. input is
applied to the diagonally opposite ends of the bridge
through the transformer. Between other two ends of
the bridge, the load resistance RL is connected.
Operation: During the positive half-cycle, point P of
the secondary winding becomes positive and point Q
becomes negative. This makes diodes D1 and D3
forward biased while diodes D2 and D4 are reverse
biased. Therefore, only diodes D1 and D3 conduct.
The conventional current flow is shown by dotted
arrows.
During the negative half-cycle, point P
becomes negative and point Q becomes positive.
This makes diodes D2 and D4 forward biased
whereas diodes D1 and D3 are reverse biased.
Therefore, only diodes D2 and D4 conduct. The
current flow is shown by the solid arrows. It may be
seen that again current flows from A to B through the
load i.e. in the same direction as for the positive half-
cycle. Therefore, d.c. output is obtained across load
RL.
During positive half cycle
During negative half cycle

Advantages
i. The need for centre-tapped transformer is eliminated.
ii. The output is twice that of the centre-tap circuit for the same secondary voltage.
iii. The PIV is one-half that of the centre-tap circuit (for same d.c. output).
Disadvantages
i. It requires four diodes.
ii. As during each half-cycle of a.c., two diodes are conducting therefore, voltage
drop will be twice as compared to centre tap circuit.
2.2.3. Comparison of Rectifiers:
3. Zener Diode Regulator:
A properly doped crystal diode which has a sharp
breakdown voltage is known as a zener diode.
The following points may be noted about the
zener diode:
i. A zener diode is like an ordinary diode
except that it is properly doped so as to
have a sharp breakdown voltage.
ii. A zener diode is always connected in
reverse i.e. it is always reverse biased.
iii. A zener diode has sharp breakdown
voltage, called zener voltage VZ.
iv. When forward biased, its characteristics
are just like ordinary diode.

3.1. Zener Diode regulator:
A zener diode can be used as a voltage regulator to provide a constant voltage
from a source whose voltage may vary. The circuit arrangement is as shown in figure.
The zener diode with voltage VZ is connected in reverse across the load RL. The series
resistance R absorbs the output voltage fluctuations so as to maintain constant voltage
across the load. It may be noted that the zener will maintain a constant voltage VZ (= E0)
across the load, provided that the input voltage should not fall below VZ.
Circuit arrangement for Zener regulator Equivalent Circuit
4. Voltage Multiplier:
Diodes and Capacitors can be used to build a circuit that will provide a d.c
output which is multiple of the peak input a.c. voltage. Such a circuit is called a voltage
multiplier. When a voltage multiplier increases the peak input voltage by a factor n, the
peak input current is decreased by approximately the same factor. Thus the actual
power output from a voltage multiplier will never be greater than the input power. In fact,
there are losses in the circuit (e.g. in diodes, capacitors etc.) so that the output power
will actually be less than the input power.
4.1. Half-Wave Voltage Doubler:
A half-wave voltage doubler consists of two diodes and two capacitors
connected as shown in figure. The basic idea in a voltage multiplier is to charge each
capacitor to the peak input a.c. voltage and to arrange the capacitors so that their stored
voltages will add.
Operation:
1. During the negative half-cycle of a.c.
input voltage, diode D1 is forward biased
and diode D2 is reverse biased.
Therefore, diode D1 can be represented
by a short and diode D2 as an open.
2. C1 will charge until voltage across it
becomes equal to peak value of source
voltage Vs (p).

3. During positive half-cycle, D1 is reverse biased and D2 is forward. Now C1 (charged
to source voltage (Vs (p)) act as series-aiding voltage sources. Thus C2 will be
charged to the sum of the series peak voltages i.e. 2 VS (p).
4. When VS returns to its original polarity (i.e. negative half-cycle), D2 is again turned
off (i.e. reverse biased). With D2 turned off, the only discharge path for C2 is through
the load resistance RL. The time required to discharge C2 (RL.C2) is too large, hence
C2 always appears like it is fully charged.
4.2. Full-Wave Voltage Doubler:
The circuit arrangement for full wave
voltage doubler is as shown in the figure. Here,
two diodes and two capacitors are used. Each
capacitor is charged during each half cycle of
input
Operation: When the sinusoidal input voltage
is positive, capacitor C1 charges up through
diode D1 and when the sinusoidal voltage is
negative, capacitor C2 charges up through
diode, D2. The output voltage is taken across
the series combination of two capacitors. As C1
is charged to the Vi and C2 is also charged to
Vi, total output becomes 2Vi.
5. Clipping Circuits:
The circuit with which the waveform is shaped by removing (or clipping) a
portion of the applied wave is known as a clipping circuit.
The clippers are used to remove signal voltages above or below a specified level. The
important diode clippers are
i. Positive clipper
ii. Biased clipper
iii. Combination clipper.
5.1. Positive clipper:
A positive clipper is the circuit which removes the positive half-cycles of the
input signal. Figure shows the typical circuit of a positive clipper.
During the positive half-cycle of the input voltage, the diode is forward biased
and conducts heavily. Therefore, the voltage across the diode and hence across the
load RL is zero. Hence output voltage during positive half-cycles is zero. During the
negative half-cycle of the input voltage, the diode is reverse biased and behaves as an
open circuit, hence input will appear as it is at the output.

Output is calculated with equation:
Generally, RL is much greater than R.
Output voltage = − Vm
5.2. Negative clipper:
A negative clipper is that which removes the negative half-cycles of the input
voltage. Figure shows the typical circuit of a negative clipper using a diode.
During the positive half-cycle of the input voltage, the diode is reverse biased
and acts as open circuit. Therefore, the voltage across load RL is same as that of input.
During the negative half-cycle of the input voltage, the diode is forward biased and
behaves as short circuit; hence the output across RL is zero.
Output is calculated with equation:
Generally, RL is much greater than R., hence
Output voltage = Vm

5.3. Biased positive clipper:
Sometimes it is desired to remove a small
portion of positive or negative half-cycle instead of
removing complete half-cycle. For this purpose,
biased clipper is used. Figure shows the circuit of a
biased clipper using battery of V volts. A portion of
each positive half-cycle will be clipped.
The diode will conduct heavily so long as
input voltage is greater than +V. When input voltage
is greater than +V, the diode behaves as a short and
the output equals +V. The output will stay at +V so
long as the input voltage is greater than +V. During
the period the input voltage is less than +V, the diode
is reverse biased and behaves as an open.
Therefore, most of the input voltage appears across
the output. In this way, the biased positive clipper
removes input voltage above +V. During the negative
half-cycle of the input voltage, the diode remains
reverse biased. Therefore, almost entire negative
half-cycle appears across the load.
5.4. Biased negative clipper:
5.5. Combination clipper:
It is a combination of biased
positive and negative clippers. With a
combination clipper, a portion of both
positive and negative half-cycles of
input voltage can be removed or
clipped as shown in figure.

When positive input voltage is greater than +
V1, diode D1 conducts heavily while diode D2 remains
reverse biased. Therefore, a voltage +V1 appear
across the load. This output stays at +V1, as long as
the input voltage exceeds +V1.
On the other hand, during the negative half-
cycle, the diode D2 will conduct heavily and the
output stays at − V2 as long as the input voltage is
greater than − V2. Between + V1 and − V2 neither
diode is on. Therefore, in this condition, most of the
input voltage appears across the load.
5.6. Applications of Clippers:
In general, clippers are used to perform one of the following two functions:
 Changing the shape of a waveform: Clippers can alter the shape of a waveform. For
example, a clipper can be used to convert a sine wave into a rectangular wave,
square wave etc.
 Circuit transient protection: Transients can cause considerable damage to many
types of circuits e.g., a digital circuit. In that case, a clipper diode can be used to
prevent the transient form reaching that circuit.
6. Clamping Circuits:
A circuit that places either the positive or negative peak of a signal at a desired
d.c. level is known as a clamping circuit.
6.1. Positive Clamper:
Figure shows the circuit of a positive clamper. The input signal is assumed to be
a square wave with time period T. The clamped output is obtained across RL. The
values of C and RL are so selected that time constant T = C.RL is very large. This
means that voltage across the capacitor will not discharge significantly during the
interval the diode is non-conducting.

Operation:
During the negative half-cycle of
the input signal, the diode is forward
biased i.e. acts as a short circuit. The
charging time constant (= C.Rf , where Rf
= forward resistance of the diode) is very
small so that the capacitor will charge to V
volts very quickly, therefore, Vout = 0 V.
When the input switches to +V state (i.e.
positive half-cycle), the diode is reverse biased and
behaves as an open circuit. Since the discharging
time constant (= C.RL) is much greater than the time
period of the input signal, the capacitor remains
almost fully charged to V volts during the off time of
the diode. By applying Kirchhoff’s voltage law to the
input loop, we have,
V + V - Vout = 0
Vout = 2V
The resulting waveform is as shown in figure.
It is clear that, it is a positively clamped output. That
is to say the input signal has been pushed upward by
V volts so that negative peaks fall on the zero level.
6.2. Negative Clamper
Figure shows the circuit of a negative
clamper. The clamped output is taken
across RL.
During the positive half-cycle of the input
signal, the diode is forward biased i.e. acts as a short
circuit. The charging time constant (= C.Rf) is very
small so that the capacitor will charge to V volts very
quickly, therefore, Vout = 0.
When the input switches to − V state (i.e.,
negative half-cycle), the diode is reverse biased and
behaves as an open circuit. Since the discharging
time constant (= C.RL) is much greater than the time
period of the input signal, the capacitor almost
remains fully charged to V volts during the off time of
the diode.

By applying Kirchhoff ’s voltage law to the input loop, we get,
- V - V - Vout = 0
Vout = - 2 V
The resulting waveform is as shown in figure. Note that total swing of the output signal
is equal to the total swing of the input signal.
7. Transistors:
When a third doped element is added to a crystal diode in such a way that two p-
n junctions are formed, the resulting device is known as a transistor.
7.1. Introduction
Transistors are far smaller than vacuum tubes, have no filament and hence need
no heating power and may be operated in any position.
A transistor consists of two p-n 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 left side figure.
However, a p-n-p transistor is formed by two p-
sections separated by a thin section of n-type as
shown in right side figure.
Symbols:
npn transistor pnp transistor
7.2. Transistor Terminals:
A transistor (pnp or npn) has three sections of doped semiconductors. The
section on one side is the emitter and the section on the opposite side is the collector.
The middle section is called the base and forms two junctions with the emitter and
collector.
Emitter: The section that supplies charge carriers (electrons or holes) is called the
emitter. The emitter is always forward biased w.r.t. base so that it can supply a large
number of majority carriers.

Collector: The section that collects the charges carriers is called the collector. The
collector is always reverse biased. Its function is to remove charges from its junction
with the base.
Base: The middle section which forms two p-n 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.
7.3. Origin of the name “Transistor” (Why transistor s called as “transistor”?)
A transistor has two p-n junctions. One junction is forward biased and the other
is reverse biased. The forward biased junction has a low resistance path whereas a
reverse biased junction has a high resistance path.
The weak signal is introduced in the low resistance circuit and output is taken
from the high resistance circuit. Therefore, a transistor transfers a signal from a low
resistance to high resistance. The prefix ‘trans’ means the signal transfer property of the
device while ‘istor’ classifies it as a solid element.
Transfer + resistor = Transistor.
7.4. Working of npn transistor:
Figure 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, therefore, only a few electrons (less
than 5%) combine with holes to constitute base current IB.
The remaining electrons (more than 95%) crosses the junction and reaches 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
7.5. Working of pnp transistor:
Figure 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, therefore, only
a few holes (less than 5%) combine with the electrons. The remaining holes (more than
95%) crosses the junction and reaches collector region to constitute collector current IC.
In this way, almost the entire emitter current flows in the collector circuit.
Current conduction within pnp transistor is by holes. However, in the external
connecting wires, the current is still by electrons.
8. Transistor Configurations:
There are three leads in a transistor viz., emitter, base and collector. However,
when a transistor is to be connected in a circuit, we require four terminals; two for the
input and two for the output. This difficulty is overcome by making one terminal of the
transistor common to both input and output terminals.
Accordingly; a transistor can be connected in a circuit in the following three ways:
i. Common Base Configuration
ii. Common Emitter Configuration
iii. Common Collector Configuration
8.1. Common Base Connection (CB):
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 (CB) connection. Figure shows common
base npn transistor configuration.
Current amplification factor (α): It is the ratio of output current to input current. In a
common base connection, the input current is the emitter current IE and output current
is the collector current IC.
The ratio of change in collector current (∆IC) to the change in emitter current (∆IE)
at constant collector base voltage VCB is known as current amplification factor i.e.
8.2. Common Emitter Connection (CE):
applied between base and emitter and output is
taken from the collector and emitter. Here,
emitter of the transistor is common to both input
and output circuits and hence the name
common emitter (CE) connection. Figure shows
common emitter npn transistor circuit.

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.
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.
Base current = IB
Collector current = β.IB + ICEO
Emitter current = Collector current + Base current
= (β.IB + ICEO) + IB
= (β+ 1) IB + ICEO
Relation between β and α: A simple relation exists between β and α. This can be
derived as follows:
It is clear that as α approaches unity, β approaches infinity. In other words, the current
gain in common emitter (CE) connection is very high. Hence, this circuit arrangement
(CE) is used in about 90 to 95 percent of all transistor applications.
Example 1: Find the value of β if (i) α = 0.9 (ii) α = 0.98 (iii) α = 0.99.
Example 2: Calculate IE in a transistor for which β = 50 and IB = 20 μA.

8.3. Common Collector Connection:
applied between base and collector while
output is taken between the emitter and
collector. Here, collector of the transistor is
common to both input and output circuits and
hence the name common collector (CC)
connection. Figure shows common collector
npn transistor.
Current amplification factor (γ).
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.
8.4. Comparison of Transistor Connections:
9. Transistor as an amplifier:
Figure shows the common emitter (CE) npn amplifier circuit. 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.

Operation: During the positive half-cycle of the input signal, the forward bias across the
emitter-base junction is increased. Therefore, more electrons flow from the emitter to
the collector via the base. This causes an increase in collector current. The increased
collector current produces a greater voltage drop across the collector load resistance
RC. However, during the negative half-cycle of the input signal, the forward bias across
emitter-base junction is decreased. Therefore, collector current also decreases. This
results in the decreased output voltage. Hence, an amplified output is obtained across
the load.
9.1. Transistor Testing:
An ohmmeter can be used to check (test) the transistor. We know that base-
emitter junction of a transistor is forward biased while collector-base junction is reverse
biased. Therefore, forward biased base-emitter junction should have low resistance and
reverse biased collector-base junction should register a much higher resistance.
Figure shows the process of testing an npn transistor with an ohmmeter.
i. The forward biased base-emitter junction should show low resistance approx. 100
Ω to 1 kΩ.
ii. The reverse biased collector-base junction should higher resistance approx. 100 kΩ
or higher.
10. Transistor Load Line Analysis:
In the transistor circuit analysis, it is generally required to determine the collector
current for various collector-emitter voltages. A load line method is most commonly
used. This method is quite easy and is frequently used in the analysis of transistor
applications.
Consider a common emitter npn transistor circuit shown in figure, where no
signal is applied. The output characteristics of this circuit are shown in graph.
The value of collector-emitter voltage VCE at any time is given by;
VCE = VCC – IC RC
As VCC and RC are fixed values, therefore, it is a first degree equation and can be
represented by a straight line on the output characteristics. This is known as d.c. load
line. To add load line, we need two end points of the straight line. These two points can
be located as under :

i. When the collector current IC = 0, then collector-emitter voltage is maximum and is
equal to VCC i.e. Max. VCE = VCC – IC.RC
= VCC (as IC = 0)
This gives the first point B (OB = VCC) on the collector-emitter voltage axis as
shown in graph.
ii. When collector-emitter voltage VCE = 0, the collector current is maximum and is
equal to VCC /RC i.e. VCE = VCC - IC.RC
or 0 = VCC - IC RC
Max. IC = VCC / RC
Cut off: The point where the load line intersects the IB = 0 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
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.
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.
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 and VCE take place about this point
when signal is applied. It is also called quiescent
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.

The point Q describes the actual state of affairs in the circuit in the zero signal
conditions and is called the operating point.
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.
11. Transistor Biasing :
11.1. Faithful Amplification:
The process of raising the strength of a weak signal without any change in its
general shape is known as faithful amplification. The transistor will function properly if
its input circuit (i.e. base-emitter junction) remains forward biased and output circuit (i.e.
collector-base junction) remains reverse biased at all times. This is then the key factor
for achieving faithful amplification.
To ensure faithful amplification, the following basic conditions must be satisfied:
a. Proper zero signal collector current
b. Minimum proper base-emitter voltage (VBE) at any instant
c. Minimum proper collector-emitter voltage (VCE) at any instant
11.2. Transistor Biasing: Introduction
The fulfilment of conditions for achieving faithful amplification is called as
transistor biasing.
The basic purpose of transistor biasing is to keep the base-emitter junction
properly forward biased and collector-base junction properly reverse biased during the
application of signal. The circuit which provides proper biasing to the transistor is known
as biasing circuit. It may be noted that transistor biasing is very essential for the proper
operation of transistor in any circuit.
11.3. Methods of Transistor Biasing:
Normally, transistor circuit should have a single source of supply (VCC). The
following are the most commonly used methods of obtaining transistor biasing from one
source of supply (i.e. VCC):
i. Base resistor method
ii. Biasing with collector-feedback resistor
iii. Voltage-divider bias
In all these methods, the same basic principle is employed i.e. required value of base
current (and hence IC) is obtained from VCC in the zero signal conditions. The value of
collector load RC is selected keeping in view that VCE should not fall below 0.5 V for
germanium transistors and 1 V for silicon transistors.

11.3.1. Base Resistor Method:
In this method, a high resistance RB (several
hundred kΩ) is connected between the base and +ve end
of supply for npn transistor (See figure) and between base
and negative end of supply for pnp transistor.
Here, the required zero signal base current is
provided by VCC and it flows through RB. It is because now
base is positive w.r.t. emitter i.e. base-emitter junction is
forward biased. The required value of zero signal base
current IB (and hence IC = β.IB) can be made to flow by
selecting the proper value of base resistor RB.
Advantages:
1. This biasing circuit is very simple as only one resistance RB is required.
2. Biasing conditions can easily be set and the calculations are simple.
3. There is no loading of the source by the biasing circuit since no resistor is employed
across base-emitter junction.
Disadvantages:
1. This method provides poor stabilisation. It is because there is no means to stop a
self increase in collector current due to temperature rise and individual variations.
2. The stability factor is very high. Therefore, there are strong chances of thermal
runaway.
11.3.2. Biasing with Collector Feedback Resistor:
In this method, one end of RB is connected to the
base and the other end to the collector as shown in figure.
Here, the required zero signal base current is
determined not by VCC but by the collector-base voltage
VCB. It is clear that VCB forward biases the base-emitter
junction and hence base current IB flows through RB. This
causes the zero signal collector current to flow in the
circuit.
Advantages:
1. It is a simple method as it requires only one resistance RB.
2. This circuit provides some stabilisation of the operating point as discussed below :
VCE = VBE + VCB
Disadvantages:
1. The circuit does not provide good stabilisation because stability factor is fairly high,
therefore, the operating point does change.
2. This circuit provides a negative feedback which reduces the gain of the amplifier.
During the positive half-cycle of the signal, the collector current increases. The

increased collector current would result in greater voltage drop across RC. This will
reduce the base current and hence collector current.
11.3.3. Voltage Divider Bias Method:
This is the most widely used method of
providing biasing and stabilisation to a transistor. In
this method, two resistances R1 and R2 are
connected across the supply voltage VCC (refer the
figure) and provide biasing.
The emitter resistance RE provides
stabilisation. The name ‘‘voltage divider’’ comes from
the voltage divider formed by R1 and R2. The voltage
drop across R2 forward biases the base-emitter
junction. This causes the base current and hence
collector current flows in the zero signal conditions.
12. Bias Compensation Methods :
The stabilization of biasing techniques occurs due to negative feedback action.
The negative feedback, although improves the stability of operating point, it reduces the
gain of the amplifier. The gain of the amplifier is a very important consideration while
designing but in some cases the gain should not be reduced. In such cases, we need to
use compensation techniques to stabilize the Q-point instead of using d.c. biasing
circuits. In compensation techniques, temperature sensitive devices like diodes and
thermistors are used. Sometimes a combination of biasing circuit and temperature
sensitive devices are used.
12.1. Bias Compensation using diode:
The figure shows the circuit diagram of a
transistor amplifier with diode D used for
compensation of variation in ICO. Here, diode is
reverse biased by base emitter junction voltage
(VBE). It allows reverse saturation current to flow.
When temperature increases, the reverse
saturation current (ICO) through the transistor also
increases. This in turn also increases the leakage
current through the diode, which will reduce base
current (IB). Therefore the collector current is
dependant only on the temperature.

Now the base current is, IB = I − IO
I = VCC−VBE / R
= VCC / R
= Constant
Substituting the above value in the expression for collector current,
IC=β (I − IO) + (1 + β) ICO
If β = 1,
IC = βI − βIO + βICO
I is almost constant and if IO of diode and ICO of transistor track each other over the
operating temperature range, then IC remains constant.
12.2. Thermistor Compensation
Figure shows the thermistor compensation
technique. The thermistor has negative
temperature coefficient i.e. its resistance
decreases with increase in temperature.
Thermistor is placed in parallel with R2 or in place
of R2. The values of R1 and R2 are selected in
such a way that it will provide correct bias at
normal temperature. As temperature increases,
the voltage drop across RT decreases. This in turn
reduces the forward bias across base-emitter
junction, which will further reduce the base current
and hence collector current.
IC= β.IB + (β+ 1) ICEO
The equation shows if there is increase in ICO and decrease in IB keeps IC almost
constant.
13. Transistor as switch :
Very often, bipolar
junction transistors are used
as electronic switches. With
the help of such a switch, a
given load can be turned ON
or OFF by a small control
signal. This control signal
might be the one appearing
at the output of a digital logic
or a microprocessor.

The power level of the control signal is usually very small and, hence, it is
incapable of switching the load directly. However, such a control signal is certainly
capable of providing enough base drive to switch a transistor ON or OFF and, hence,
the transistor is made to switch the load. When using BJT as a switch, usually two
levels of control signal are employed. With one level, the transistor operates in the cut-
off region (open) whereas with the other level, it operates in the saturation region and
acts as a short circuit. Figure (b) shows the condition when control signal vi = 0. In this
case, the BE junction is reverse-biased and the transistor is open and, hence acts as an
open switch. However, as shown in figure (c) if vi equals a positive voltage of sufficient
magnitude to produce saturation i.e. if vi = vi the transistor acts as a closed switch.

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