The document discusses semiconductors and PN junction diodes. It defines semiconductors as materials with conductivity between conductors and insulators. Semiconductors can be intrinsic, consisting of pure semiconductor material, or extrinsic, doped with impurities to create excess electrons (N-type) or holes (P-type). A PN junction forms when a P-type and N-type semiconductor are joined. It acts as a diode, allowing current in one direction (forward bias) but blocking it in the other (reverse bias) due to the potential barrier created at the junction.

2. Introduction
“ The material whose conductivity lies between conductivity of conductor
and that of insulator are called semiconductor.”
ex. Si, Ge
Conductivity and resistivity
On the basis of relative values of electrical conductivity (σ) or Resistivity
(ρ=1/ σ) is classified as-
i) Metals: Low resistivity ( or high conductivity)
ρ = 10-2 - 10-8 Ω m
σ = 102 - 108 S m-1
ii) Semiconductor : Resistivity or conductivity intermediate to metals and
insulator.
ρ = 10-5 - 106 Ω m
σ = 105 - 10-6 S m-1
iv) Insulators: High resistivity or Low conductivity
ρ = 1011 - 1019 Ω m
σ = 10-11 - 10-19 S m-1

3. Energy band theory
I) Valence Band (V.B):
The energy band formed by energy
levels of valence electrons of atoms in
solid is called valence band.
II) Conduction Band : A band of energy
levels which is occupied by the conduction
electrons of the solids is called conduction
band.
Energy band: The group of discrete but closely
spaced energy levels for the orbital electrons in a
particular orbit is called energy bands.

4. Band theory of a solid
• A solid is formed by bringing together isolated single atoms.
• Consider the combination of two atoms. If the atoms are far apart there is
no interaction between them and the energy levels are the same for each
atom. The numbers of levels at a particular energy is simply doubled
• If the atoms are close together the electron wave functions will overlap
and the energy levels are shifted with respect to each other.
n=1 n=1
n=2 n=2
n=3 n=3
Atom 1 Atom 2
n=1 n=1
n=2 n=2
n=3 n=3
Atom 1 Atom 2
n=1
n=2
n=3
Atom 1 + 2

5. • A solid will have millions of atoms close
together in a lattice so these energy levels
will creates bands each separated by a
gap.
• Band gap or forbidden Energy gap (Eg ):
The separation between conduction band
and valence band in energy band diagram
is called band gap or energy gap
• Conductors:
– If we have used up all the electrons
available and a band is still only half
filled, the solid is said to be a good
conductor. The half filled band is
known as the conduction band.
Eg =0
• Insulators:
– If, when we have used up all the
electrons the highest band is full and
the next one is empty with a large
gap between the two bands, the
material is said to be a good insulator.
The highest filled band is known as
the valence band while the empty
next band is known as the conduction
band.
Eg > 3ev
n=1
n=2
n=3
Conduction band,
half filled with
electrons
Valence band,
filled with
electrons
Empty conduction
band
Large energy gap
Valence band,
filled with
electrons

6. Semiconductors:
• Some materials have a filled valence band just like insulators but
a small gap to the conduction band.
• At zero Kelvin the material behave just like an insulator but at
room temperature, it is possible for some electrons to acquire
the energy to jump up to the conduction band. The electrons
move easily through this conduction band under the application
of an electric field. This is an intrinsic semiconductor.
• Eg < 3ev
Top valence
band now
missing some
electrons
Conduction band,
with some
electrons
At room temperature – some conduction
Valence bands,
filled with
electrons
Empty conduction
band
At zero Kelvin – no conduction
Small energy gap

8. Semiconductors are mainly two types
1. Intrinsic (Pure) Semiconductors
2. Extrinsic (Impure) Semiconductors

9. Intrinsic Semiconductor
 A Semiconductor which does not have any kind of
impurities, behaves as an Insulator at 0k and behaves as a
Conductor at higher temperature is known as Intrinsic
Semiconductor or Pure Semiconductors.
 Germanium and Silicon (4th group elements) are the best
examples of intrinsic semiconductors and they possess
diamond cubic crystalline structure.

10. Si
Si
SiSiSi
Valence Cell
Covalent bonds
Intrinsic Semiconductor

11. Ef
Ev
Valence band
E
c
Conduction band
Ec
E
Electron
energy
Distance
KE of
Electron
= E - Ec
KE of Hole
=
Ev - E
Fermi energy level

12. Carrier Concentration in Intrinsic Semiconductor
When a suitable form of Energy is supplied to a
Semiconductor then electrons take transition from Valence
band to Conduction band.
Hence a free electron in Conduction band and
simultaneously free hole in Valence band is formed. This
phenomenon is known as Electron - Hole pair generation.
In Intrinsic Semiconductor the Number of Conduction
electrons will be equal to the Number of Vacant sites or
holes in the valence band.

13. Extrinsic Semiconductors
 The Extrinsic Semiconductors are those in which impurities
of large quantity are present. Usually, the impurities can be
either 3rd group elements or 5th group elements.
Based on the impurities present in the Extrinsic
Semiconductors, they are classified into two categories.
1. N-type semiconductors
2. P-type semiconductors

14. • When any pentavalent element such as Phosphorous,
• Arsenic or Antimony is added to the intrinsic
Semiconductor , four electrons are involved in covalent
bonding with four neighboring pure Semiconductor
atoms.
• The fifth electron is weakly bound to the parent atom.
And even for lesser thermal energy it is released Leaving
the parent atom positively ionized.
N Type semiconductor

15. N-type
Semiconductor
Si
SiPSi
Free electron
Si
Impure atom
(Donor)

16. The Intrinsic Semiconductors doped with pentavalent
impurities are called N-type Semiconductors.
The energy level of fifth electron is called donor level.
The donor level is close to the bottom of the conduction
band most of the donor level electrons are excited in to the
conduction band at room temperature and become the
Majority charge carriers.
Hence in N-type Semiconductors electrons are Majority
carriers and holes are Minority carriers.

17. Carrier Concentration in N-type Semiconductor
Consider Nd is the donor Concentration i.e., the number
of donor atoms per unit volume of the material and Ed is
the donor energylevel.
At very low temperatures all donor levels are filled with
electrons.
With increase of temperature more and more donor atoms
get ionized and the density of electrons in the conduction
band increases.

18. P-type semiconductors
 When a trivalent elements such as Al, Ga or Indium have
three electrons in their outer most orbits , added to the
intrinsic semiconductor all the three electrons of Indium are
engaged in covalent bonding with the three neighboring Si
atoms.
 Indium needs one more electron to complete its bond. this
electron maybe supplied by Silicon , there by creating a vacant
electron site or hole on the semiconductor atom.
 Indium accepts one extra electron, the energy level of this
impurity atom is called acceptor level and this acceptor level
lies just above the valence band.
 These type of trivalent impurities are called acceptor
impurities and the semiconductors doped the acceptor
impurities are called P-type semiconductors.

19. Si
Si
SiInSi
Hole
Co-Valent
bonds
Impure atom
(acceptor)
P-type Semiconductor

20. P-N junction Diode
• P-N junction:
When P-type semiconductor is suitably joined to N-
type semiconductor, the contact surface iscalled PN-
junction.
• Biased P-N Junction diode:
when an external source is connected to
the diode it is called biased diode.
•Unbiased P-N junction diode:
when no external source is connected
to the diode it is called unbiased diode

21. The P-N Junction Diode
Schematic diagram
p-type n-type
ID
+ VD –
Circuit symbol
Physical structure:
(an example)
p-type Si
n-type Si
SiO2SiO2
metal
metal
ID+
VD
–
net donor
concentration ND
net acceptor
concentration NA
cross-sectional area AD

22. Formation of P-N junction diode
1. Depletion Layer:
The region near the P-N junction which is depleted of free charges is
called depletion layer.
Width of depletion layer 0.5 micro to 1 micro meter
2. Barrier Potential:
The potential difference across the P-N junction which prevent continuous
diffusion of Electron and holes across the junction is called barrier potential.
Si= 0.7 volt Ge= 0.3 volt

23. BIASING APN-JUNCTION
In relation to a PN junction, there are two bias
condition:
Biasing a PN-junction
Forward biasing Reverse biasing

24. BATTERY
CONNECTION
 Forward Bias Mode: Positive terminal
connected to P-region and negative terminal
connected to N-region.
 Reverse bias mode: Negative terminal
connected to P-region and positive terminal
connected to N-region.

25. FORWARD BIASING
 When voltage is applied across a diode in such a way
that the diode allows current and the potential barrier
reduced, the diode is said to be forward-biased.

26. IN FORWARD BIAS
 The holes of P-side are repelled by the positive terminal of the battery, while
the electron of the N-side are repelled by the negative terminal of the battery.
 As a result some holes and free electrons enter the depletion region.
 The potential barrier as well as width of the depletion region are reduced.
 No current flows until the barrier voltage (0.3 for Ge) or (0.7 for Si) is
overcome.
 Then the curve has linear rise and the current increase with the
increase forward voltage.
 Above the 0.3 v or 0.7 v the majority carriers passing the junction gain
sufficient energyto knock out the electrons.
 Therefore, the forward current increase sharply

27. REVERSE BIASING
When voltage is applied across a diode in such a way
that the diode prohibits current and potential barrier
increase, the diode is said to be reverse-biased.

28. IN REVERSE BIAS
 The holes of the P-side are attracted towards the negative terminal of the battery,
the electrons of the N-side are attracted towards the positive terminal of the battery.
The majority charge carriers are pulled away from the junction thereby increasing
the depletion region and the potential barrier.
It becomes more difficult for the majority carrier to diffuse across the junction.
 As soon as a minority carrier is generated, it is swept across the junction by the
potential by the potential barrier.
At a given temperature, the rate of generation of minority carriers is constant,
whether the applied voltage is high or low. This current is called Reverse saturation
current.
The number of minority carrier is small so, current is small.
Breakdown Voltage (VBR): The reverse voltage, at which the diode breaks down is
called breakdown voltage.

29. V-I CHARACTERISTICS OF P-N JUNCTION DIODE
The curve drawn between voltage across the junction along
x axis and current through the y axis.

31. Advantages of semiconductor devices
Semiconductor devices are very small in size and light in weight.
They can operate at low voltage.
They are cheap.
They have high speed operation.
They have a complementary pair combination which is useful in
many circuits.
They can be integrated in small space.
Disadvantages of semiconductor devices
Noise level is higher in semiconductor devices.
Ordinary semiconductor device cannot handle as much power as
ordinary vacuum tubes can do.
In high frequency range, they have poor response.
The semiconductor devices are temperature sensitive.
A small over heating damages the semiconductor device.

32. Rectifiers
Definition:
”An electronic device which converts a.c voltage into d. c voltage is
called rectifier.”
The process of converting a.c voltage into d.c voltage is called rectification.
The junction diode offers a low resistance path when forward biased and high
resistance path when reverse biased. This feature of the junction diode enables it to
be used as a rectifier.
Rectifier produces unidirectional and pulsating voltage from a.c source.
The following two types of rectifier circuit can be used:
 Half wave rectifier
 Full wave rectifier

33. Half wave Rectifier
The process of removing one-half the input signal to
establish a dc level is called half-wave rectification.
In Half wave rectification, the rectifier conducts
current during positive half cycle of input ac signal only.
Negative half cycle is suppressed.
3
3

34. 3
4

35. Half wave Rectifier
 a.c voltage across
secondary terminals AB
changes its polarity after
each half cycle.
During negative half
cycle terminal A is
isnegative so diode
reversed biased and
Does not Conducts
current.
So current flows through diode during positive half cycle only.
V0= Id x RL
In this way current flows through load RL in one direction

36. Half wave Rectifier
Output frequency of HWR:
Output frequency of HWR
is equal to input frequency.
This means when input ac
completes one cycle, rectified
wave also completes one cycle.
fout = fin
3
6

37. Half wave Rectifier
Disadvantage of Half wave rectifier:
The pulsating current in output contains ac
components whose frequency is equal to supply
frequency so filtering is needed.
The ac supply delivers power during half cycle only
so output is low.
3
7

38. Full-Wave Rectifier
In Full wave rectification current flow through the load in same
direction for both half cycle of input ac.
This can be achieved with two diodes working alternatively.
For one half cycle one diode supplies current to load and for
next half cycle another diode works.

39. Centre Tap Full Wave Rectifier
Circuit has two diodes D1 , D2 and a centre tap transformer.
During positive half cycle Diode D1 conducts and during
negative half cycle Diode D2 conducts.
It can be seen that current through load RL is in the
same direction for both cycle.
V0= Id x RL10

40. Full wave Rectifier
Output frequency of FWR:
Output frequency of FWR is
equal to double of input
frequency.
This means when input ac
completes one cycle, rectified
Wave completes two cycle
fout = 2 fin

42. Zener Effect and Zener Diode
 The applied reverse biased voltage cannot increase without limit since at
some point breakdown occurs causing current to increase rapidly.
 The voltage at that point is known as the breakdown voltage, VZ
 Diodes are fabricated with a specifically design breakdown voltage and are
designed to operate in the breakdown region are called Zener diodes.
Circuit symbol of the Zener diode:
 Such a diode can be used as a constant-voltage
reference in a circuit.
 Diodes can be operated in the breakdown region
by limiting the current to a value within the
capacities of the device.
 Zener voltage: when a reverse bias reaches a
particular value, the current increases suddenly.
This voltage is called zener voltage
NOTE: When a Zener diode is reverse-
biased, it acts at the breakdown region,
when it is forward biased, it acts like a
normal PN junction diode

43. A voltage regulator supplies constant voltage to a load.
Voltage Regulator - Zener Diode

44.  Construction:
 The circuit diagram of zener diode as a voltage regulator.
 The input voltage Vi (Vps ) is connected across zener diode through a series
resistance Rs
 The load resistance RL is connected in parallel with zener diode ( VL = Vz )
 The output voltage is taken across the load resistance RL.
 The zener diode is reverse biased i.e P-side of diode is connected to negative
terminal and N-side of diode is connected to positive terminal.
=Vz
Zener diode as voltage regulator

45.  Working :
 When voltage is applied to the circuit, current I flows through it. I is
divided into IZ and IL .
 From fig. I= IZ + IL
Vi = Vs + Vz
‘,’ Vi = I Rs + Vz
But, I= IZ + IL
Vi = (IZ + IL ) Rs + Vz
Vi = (IZ + IL ) Rs + VL ‘,’ VL = Vz
 If input voltage Vi is increased beyond zener voltage, I increases such
that current IZ through zener diode increases but current IL remains same.
Therefore output voltage VL across load resistance same.
 Whether the input voltage increases or decreases, the output voltage
remains constant. So, zener diode acts as a voltage regulator.
Zener diode as voltage regulator

46.  This type of breakdown occurs for a reverse bias voltage between 2 to
8V.
 Even at low voltage, the electric field intensity is strong enough toexert a
force.
 The valence electrons of the atom such that they are separated fromthe
nuclei.
 This type of break down occurs normally for highly doped diode with
low breakdown voltage and larger electric field.
 As temperature increases, the valence electrons gain more energy to
disrupt from the covalent bond and the less amount of externalvoltage is
required.
 Thus zener breakdown voltage decreases with temperature.
Zener breakdown

47. AvalancheBreakdown
 This type of breakdown occurs at the reverse bias voltage
above 8V and higher.
 It occurs for lightly doped diode with large breakdown voltage.
 As minority charge carriers (electrons) flow across the device.
 They tend to collide with the electrons in the covalent bond and
cause the covalent bond to disrupt.

48. AvalancheBreakdown
 As voltage increases, the kinetic energy (velocity) of the
electrons also increases.
 The covalent bonds are more easily disrupted, causing an
increase in electron hole pairs.
 The avalanche breakdown voltage increases with temperature.

49. Special Diodes (Optocouplers instrument)
Mainly three types:
1) Photodiode
2) Light emitting diode (L.E.D)
3) Solar Cell
1) Photo diode: What is photo diode?
A junction diode made from “ light or photo sensitive semiconductor” is called “Photo
diode”
Symbol:
Construction:
A photodiode is a PN junction diode that consumes light energy to produce electric
current. Sometimes it is also called as photo-detector.
A light detector, and photo-sensor. These diodes are particularly designed to work in
reverse bias condition.
It means that the P-side of the photodiode is associated with the negative terminal of the
battery and n-side is connected to the positive terminal of the battery.

50. Working of Photo diode:
The working principle of a photodiode is, when a photon of energy strikes the diode, it
makes a couple of an electron-hole. This mechanism is also called as the inner photoelectric
effect.
 If the absorption arises in the depletion region junction, then the carriers are removed
from the junction by the inbuilt electric field of the depletion region. Therefore, holes in the
region move toward the anode, and electrons move toward the cathode, and a photocurrent
will be generated.
The entire current through the diode is the sum of the absence of light and the
photocurrent.

51. Light emitting diode (L.E.D):
What is LED ?
LED stands for Light Emitting Diode.
It is heavily doped P-N junction diode which emits visible light of particular color when
energized.
Symbol :
Construction:
One of the methods used to construct LED is to deposit
three semiconductor layers on the substrate.
 The three semiconductor layers deposited on the substrate
are n-type semiconductor, p-type semiconductor and active
region.
When LED is forward biased, free electrons from n-type semiconductor
and holes from p-type semiconductor are pushed towards the active region.
When free electrons from n-side and holes from p-side recombine with the opposite charge
carriers in active region, an invisible or visible light is emitted.

52. Working:
Light Emitting Diode (LED) works only in forward bias condition. When Light Emitting Diode
(LED) is forward biased, the free electrons from n-side and the holes from p-side are pushed
towards the junction.
When free electrons reach the junction or depletion region, some of the free electrons
recombine with the holes in the positive ions.
Holes from p-side recombine with electrons in the depletion region.
Because of the recombination of free electrons and holes in the depletion region,
the Width of depletion region decreases. As a result, more charge carriers will cross the P-N
junction and visible light is emitts.

53. Solar cell :
What is solar cell :
A semiconductor device that converts solar energy into electrical energy
is called solar cell of photo voltaic cell.
Symbol :
Construction:
The semiconductor materials like arsenide, indium, silicon,
selenium and gallium are used for making the PV cells.
Consider the figure shows the constructions of the
silicon photovoltaic cell.
The upper surface of the cell is made of the thin layer of the p-type material so that
the light can easily enter into the material.
The metal rings are placed around p-type and n-type material which acts as their
positive and negative output terminals respectively.

54. Working:
The PV cell consists the P and N-type layer of semiconductor material. These layers
are joined together to form the PN junction.
When the semiconductor material absorbs light, the electrons of the material starts
emitting. This happens because the light consists small energies particles called photons.
When the electrons absorb the photons, they become energized and starts moving into
the material. Because of the effect of an electric field, the particles move only in the one
direction and develops current.
The semiconductor materials have the metallic electrodes through which the current goes
out of it.
The junction is the interface between the P-type and N-type material.
 When the light fall on the junction the electrons starts moving from one region to another.

55. Application:
1) Photo diode:
These diodes are used in consumer electronics devices like smoke detector, compact disc
players, and televisions and remote controls in VCRs.
In other consumer devices like clock radios, camera light meters, and street lights, photo
conductors are more frequently used rather than photodiodes.
Photodiodes are frequently used for exact measurement of the intensity of light in science
& industry.
Photodiodes are also widely used in numerous medical application like instruments to
analyze samples, detectors for computed tomography and also used in blood gas monitors.
2) LED:
LED is used as a bulb in the homes and industries.
 The light emitting diodes are used in the motorcycles and cars.
 These are used in the mobile phones to display the message .
At the traffic light signals led’s are used

56. • Toys, watches, calculators
• Electric fences
• Remote lighting systems
• Water pumping
• Water treatment
• Emergency power
• Portable power supplies
• Satellites
3) Solar cell its application:

58. Introduction
 A transistor is a 3 terminal electronic device made of semiconductor
material.
 It is consists of two p-n junctions formed by sandwiching either p-
type or n-type semiconductor between a pair of opposite types.
 The word “transistor” is a combination of the terms “transfer” and
“variable resistor”. Actually it means transfer current across resistors.
Figure: Variety of shapes and sizes
of Transistor

59. Transistor has three terminal device
1. Emitter
2. Base
3. Collector
1. Emitter:
i) It is the left most part of the transistor.
ii) It emit the majority carrier towards base.
iii)It is highly doped and medium in size.
2. Base:
i) It is the middle part of transistor which is sand witched by emitter(E)
and collector (C).
ii) It is highly doped and very thin in size.
3. Collector:
i) It is right part of the transistor which collect majority carrier emitted by
emitter.
ii) It has large size and moderate doping.

60. Bipolarjunction transistor (BJT)
 It is called bipolar because conduction channel uses both majority
and minority carriers for main electric current.
Transistor are two types:
1. N-P-N Transistor: If a thin layer of P-type semiconductor is sandwiched between two
thick layers of N-type semiconductor is known as N-P-N Transistor.
2. P-N-P Transistor : If a thin layer of N-type semiconductor is sandwiched between two
thick layers of P-type semiconductor is known as P-N-P Transistor.

61. Transistor Operation
1) Working of NPN transistor:
 Forward bias Is
applied to emitter-
base junction and
reverse bias is
applied to collector-
base junction.
The forward bias in the emitter-base junction
causes electrons to move toward base. This
constitute emitter current, IE

62. Transistor Operation
1) Working of NPN transistor:
 As this electrons flow toward p-type base,
they try to recombine with holes. As base is
lightly doped only few electrons recombine
with holes within the base.
These recombined electrons constitute small
base current.
The remainder electrons crosses base and
constitute collector current.

63. Transistor Operation
2) Working of PNP transistor:
 Forward bias is
applied to emitter-
base junction and
reverse bias is
applied to collector-
base junction.
The forward bias in the emitter-base junction
causes holes to move toward base. This
constitute emitter current, IE

64. Transistor Operation
2) Working of PNP transistor:
 As this holes flow toward n-type base, they
try to recombine with electrons. As base is
lightly doped only few holes recombine with
electrons within the base.
These recombined holes constitute small base
current.
The remainder holes crosses base and
constitute collector current.

65. Transistor Operating Modes
• Active Mode
 Base- Emitter junction is forward and Base-
Collector junction is reverse biased.
• Saturation Mode
 Base- Emitter junction is forward and Base-
Collector junction is forward biased.
• Cut-off Mode
 Both junctions are reverse biased.

66. Transistor Connection
• Transistor can be connected in a circuit in
following three ways-
1) Common Base
2) Common Emitter
3) Common Collector

67. Common Emitter Connection
• The common-emitter terminology is derived from
the fact that the emitter is common to both the
input and output sides of the configuration.
• First Figure shows common emitter npn configuration and second
figure shows common emitter pnp configuration.

68. Common Emitter Connection
• Base Current amplification factor β:
• In common emitter connection input current is base
current and output current is collector current.
• The ratio of change in collector current to the
change in base current is known as base current
amplification factor,
β =
Ic
Ib

69. Relation between α and β:
Ie = Ib + Ic
Dividing the equation by Ic, we get
Ie Ib
= + 1
Ic Ic
Ic
β =
Ic
Ib
But α = and
=
1
α
+ 1
Ie
1
β
or β = α
1 – α
and α =
β
1 + β
α= common base current gain

70. Characteristics of common emitter
configuration
• Input Characteristics: VBE vs IB characteristics is
called input
characteristics.
 IB increases rapidly with
VBE . It means input
resistance is very small.
 IE almost independent
of VCE.
IB is of the range of micro
amps.

71. Characteristics of common emitter
configuration
• Output Characteristics:
VCE vs Ic
characteristics is called
output characteristics.
 IC varies linearly
with VCE ,only when VCE
is very small.
 As, VCE increases, IC
becomes constant.

72. Input and Output Resistance of
common emitter conf.
• Input Resistance: The ratio of change in
emitter-base voltage to the change in base
current is called Input Resistance.
• Output Resistance: The ratio of change in
collector-emitter voltage to the change in
collector current is called Output Resistance.

73. NPN Transistor as a Switch:
IE
VCC
RC
E
C
N
N
P
●●
●
VBB
●
●
ICRC
BIB
IC
Vi
RB
+
● o
+
V
Vi
Vo
Cutoffregion
Activeregion
Saturation
region
Based on the voltage applied at the base terminal of a transistor switching operation
is performed.
When a sufficient voltage (Vin > 0.7 V) is applied between the base and emitter,
collector to emitter voltage is approximately equal to 0. Therefore, the transistor acts
as a short circuit.
 Similarly, when no voltage or zero voltage is applied at the input, transistor operates
in cutoff region and acts as an open circuit.
In this type of switching connection, load (here LED lamp) is connected to the
switching output with a reference point. Thus, when the transistor is switched ON,
current will flow from source to ground through the load.

74. Transistor as an Oscillator: (PNP – Tuned Collector)
I
I0
t
0
Saturation current
Saturation current
Output RF Signal
●
●
Ece
● ●
L’
Ebe
IC
E
B
C
●
●
K
C
L
L’’
IB
IE
●●
●
N
N
P
Feedback
network
Amplifier
Input
Output

75. ●
●
CL
Ece
● ●
L’
Transistor as an Oscillator:
(PNP– Tuned Base)
Ebe
Ic
E
B
C
P
P
N
●●
●
●
●
K
L’’
Ib
Ie
I
I0
t
0
Saturation current
Saturation current
Output RF Signal
An oscillator is a device which can produce undamped electromagnetic
oscillations of desired frequency and amplitude.
It is a device which delivers a.c. output waveform of desired frequency from
d.c. power even without input signal excitation.

76. NPN Transistor as Common Emitter Amplifier:
Ie
Ece
Vce RL
E
C
N
N
P
●●
●Ebe
●
●
Input Signal
IcRL
+Vce
-V Output
Amplified Signal
ce
Ib B
Ic
The process of increasing the amplitude of input signal without its wave shape
and without changing its frequency is known as amplification.
A device which increases the amplitude of the input signal is called amplifier.
A transistor used as a amplifier in active mode.

79. Logic Gates:
The digital circuit that can be analysed with
the help of Boolean Algebra is called logic
gate or logic circuit.
A logic gate can have two or more inputs
but only one output.
There are 3 fundamental logic gates namely
OR gate, AND gate and NOT gate.
Truth Table:
The operation of a logic gate or circuit can
be represented in a table which contains all
possible inputs and their corresponding
outputs is called a truth table.
If there are n inputs in any logic gate, then
there will be n2 possible input
combinations.
0 and 1 inputs are taken in the order of
ascending binary numbers for easy
understanding and analysis.
A
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
B
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
C
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
D
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Eg. for 4 input gate

80. D1
D2 RL
A ●
●●
Y
●
5 V
+
E
●B
●
5 V
+
E
E
●
E
A B Y = A + B
0 0 0
0 1 1
1 0 1
1 1 1
Truth Table
●
A ●
B ●
Y
Digital OR Gate:
The positive voltage (+5 V)
corresponds to high input
i.e. 1 (state).
The negative terminal of
the battery is grounded and
corresponds to low input
i.e. 0 (state).
Case 1: Both A and B are
given 0 input and the diodes do
not conduct current. Hence no
output is across RL. i.e. Y = 0
Case 2: A is given 0 and B is given 1. Diode D1 does
not conduct current (cut-off) but D2 conducts. Hence
output (5 V) is available across RL. i.e. Y = 1
Case 3: A is given 1 and B is given 0. Diode D1
conducts current but D2 does not conduct. Hence
output (5 V) is available across RL. i.e. Y = 1
Case 4: A and B are given 1. Both the diodes
conduct current. However output (only 5 V) is
available across RL. i.e. Y = 1

81. Digital AND Gate:
RL
D1
A ●
D2
●●
Y
●
+
E 5 V
+
E
●
E
5 V
●B ●
+
5 V
E
A B Y = A . B
0 0 0
0 1 0
1 0 0
1 1 1
Truth Table
●
A●
B●
Y
Case 1: Both A and B are given 0
input and the diodes conduct
current (Forward biased). Since
the current is drained to the earth,
i.e.hence, no output across RL.
Y = 0
Case 2: A is given 0 and B is
given 1. Diode D1 being forward
biased conducts current but D2
does not conduct. However, the
current from the output battery is
drained through D1. So, Y = 0
Case 3: A is given 1 and B is given 0. Diode D1 does
not conduct current but D2 being forward biased
conducts . However, the current from the output
battery is drained through D2. Hence, no output is
available across RL. i.e. Y = 0
Case 4: A and B are given 1. Both the diodes do not
conduct current. The current from the output battery
is available across RL and output circuit. Hence,
there is voltage drop (5 V) across RL. i.e. Y = 1

82. ●
Rb
E
Digital NOT Gate:
●
5 V
+
E
●
Y
E
RL
●●
●
●E
B
C
N
N
P
A
●
5 V
+
E
Truth Table
A Y=A′
0
1
1
0
●
Y
NPN transistor is connected to biasing
batteries through Base resistor (Rb)
and Collector resistor (RL). Emitter is
directly earthed. Input is given
through the base and the output is
tapped across the collector.
Case 1: A is given 0 input. In the
absence of forward bias to the P-type
base and N-type emitter, the transistor
is in cut-off mode (does not conduct
current). Hence, the current from the
collector battery is available across the
output unit. Therefore, voltage drop of
5 V is available across Y. i.e. Y= 1 A ●
Case 2: A is given 1 input by connecting the +ve terminal of the
input battery. P-type base being forward biased makes the
transistor in conduction mode. The current supplied by the
collector battery is drained through the transistor to the earth.
Therefore, no output is available across Y. i.e. Y = 0

83. NOR Gate:
E
RL
●
●
Y
5 V
+
E
E
●●
●
●E
B
C
N
N
P
RbD1
RLD2
A ●
●●
E
●
5 V
●B ●
+
+
E
E
5 V
Truth Table
A B A + B Y = (A + B)′
0 0 0 1
0 1 1 0
1 0 1 0
1 1 1 0
● ●
A ●
B ●
A + B Y = (A + B)′
●
A ●
B ●
Y = (A + B)′
Symbol:
Circuit:

84. E
●
●
Y
5 V
+
RL
E
E
●●
●
●E
B
C
N
N
P
RbD1
D2 RL
A ●
●●
●
5 V
+
E
5 V
+
E
●B ●
5 V
+
E
NAND Gate:
Truth Table
A B A . B Y = (A . B)′
0 0 0 1
0 1 0 1
1 0 0 1
1 1 1 0
●
A ●
B ●
● ●
A . B Y = (A . B)′
●
A●
B●
Y = (A . B)′
Symbol:
Circuit:

85. NOR Gate as a Building Block:
OR Gate:
AND Gate:
NOT Gate:
A ●
B ●
(A + B)′
●
Y = A + B
●
●
A
′A●
●
B
′
B ●
Y = A . B
●
●
●
A′
B′
●
Y = A′
A ●
A B (A + B)′ A + B
0 0 1 0
0 1 0 1
1 0 0 1
1 1 0 1
A B A′ B′ A′+B′ (A′+B′)′
0 0 1 1 1 0
0 1 1 0 1 0
1 0 0 1 1 0
1 1 0 0 0 1
A A′
0
1
1
0

86. NAND Gate as a Building Block:
A ● ●
A
′
B ● ●
B′
Y = A + B
●
●
●
A′
B′
OR Gate:
AND Gate:
●
Y = A . B
●
A●
B●
(A . B)′
NOT Gate:
●
Y = A′
A●
A B A′ B′ A′.B′ (A′ . B′)′
0 0 1 1 1 0
0 1 1 0 0 1
1 0 0 1 0 1
1 1 0 0 0 1
A B (A . B)′ A . B
0 0 1 0
0 1 1 0
1 0 1 0
1 1 0 1
A A′
0
1
1
0

87. XOR Gate:
●
●
●
●
●
A ●
A′
B ●
B′
A
B
A′
B
AB′
A B A′ B′ A′B AB′
0 0 1 1 0 0
0 1 1 0 1 0
1 0 0 1 0 1
1 1 0 0 0 0
Y = A′B + AB′
= A B
0
1
1
0
A ●
B ●
● Y = A B
Y = A′B + AB′
= A B