Basic electronics and electrical first year engineering

FE Sem-I
Chapter- 5
Lecture 1

Electronics

p-n Junction Diode
• A p–n junction is formed at the boundary
between a p-type and n-type semiconductor
created in a single crystal of semiconductor by
doping
• Term diode means two electrode
• Arrow indicates direction of conventional current
through it

Formation of Depletion region
• After joining p-type and n-type material electrons near
the junction tend to diffuse into the p region.
• And leave positively charged ions (donors) in the n
region.
• Vice a versa, holes, leave fixed ions (acceptors) with
negative charge.
• The regions nearby the p–n junction gets
charged, forming the space charge region or depletion
layer
• Thus in p-n junction without an external applied
voltage, under thermal equilibrium ,a p. d. is formed
across the junction.
• This is known as barrier potential or junction potential

Biasing of diode
• With no external voltage applied to diode, the
depletion region available at junction
• Prevents the current to flow through it
• Thus required to be externally biased to make
current flow
• Two types
– Forward Biased
– Reversed Biased

Forward Biasing
• +ve terminal of battery is connected to the Ptype material and - ve terminal to the N-type
material.
• +ve potential repels holes toward the junction
where they neutralize some of the negative ions.
Vice a versa by –ve potential
• In case of f/w biased condition, conduction is by
MAJORITY current carriers

Reversed Biasing
• in case of reverse biasing, the –ve terminal connected
to the P-type material, and +ve to the N-type
• The -ve potential attracts the holes away from the edge
of the junction barrier on the P side, while the +ve
potential attracts the electrons away from the edge of
the barrier on the N side
• This action increases the barrier width
• This prevents current flow across the junction by
majority carriers.
• However, the current will not exact zero because of the
minority carriers crossing the junction

Reversed Biasing
• There are minority current carriers in both regions: holes
in the N material and electrons in the P material.
• With reverse bias, the electrons in the P-type material
are repelled toward the junction by the negative terminal
• As the electron moves across the junction, it will
neutralize a positive ion in the N-type material.
• vice a versa, the holes in the N-type material.
• This movement of minority carriers is called as reverse
saturation current
• It increases with the temperature
• It is nA for Si diode and A for Ge diode

V-I CHARACTERISTIC OF DIODE






IV characteristics for
forward bias
Point A corresponds to
zero-bias condition.
Point B corresponds to
where the forward voltage
is less than the barrier
potential of 0.7 V.
Point C corresponds to
where the forward voltage
approximately equals the
barrier potential and the
external bias voltage and
forward current have
continued to increase.

The diode DC or static resistance
RD


If forward biased :



If reverse biased:

RF

VF
IF

RR

VR
IR

VD
ID

AC or Dynamic Resistance


The dynamic.
resistance of a diode
is designated rd

rd

VF
IF

IV characteristics for reverse bias


The breakdown
voltage for a
typical silicon
pn junction can
vary, but a
minimum value
of 50 V is not
unusual

Diode Ratings
• Maximum average forward current
 This is the maximum amount of average current that
can be permitted to flow in the forward direction
without damaging .
 If this rating is exceeded, structure breakdown can
occur.

• Peak reverse voltage (PRV)
 It is one of the most important ratings and indicates
the maximum reverse-bias voltage that can applied to
a diode without causing junction breakdown
 Very important parameters when using diode as
rectifier
• Maximum power rating
 This is maximum power that can be dissipated at the
junction without damaging

ZENER DIODES
• The simplest of all voltage regulators is
the Zener diode voltage regulator.
• A Zener diode is a special diode that is
optimized for operation in the breakdown
region.

20

ZENER DIODE CHARACTERISTICS
• In the forward
region, the Zener
diode acts like a
regular silicon
diode, with a 0.7 volt
drop when it conducts.

21

• In the reverse bias
region, a reverse
leakage current flows
until the breakdown
voltage is reached.
• At this point, the
reverse
current, called Zener
current Iz, increases
sharply.
22

• Voltage after
breakdown is also
called Zener
voltage Vz.
• Vz remains nearly a
constant, even
though current Iz
varies
considerably.

23

ZENER DIODE MODEL
• Zener model to be
applied

24

Basic Zener Regulator

V

VL

RLVi
R RL

25

• Unless until, applied voltage is greater than Vz
this will serve the purpose of voltage regulator
• Vi = IR + V0 = IR + Vz
• Suppose R is kept fixed and Vi increases result
into increase input current
• This rise in the current will increase the IR drop
• But voltage across zener (Vo remains constant)

26

• Now suppose IL changes with the Vi fixed
• Thus with increase IL will result into decrease
the diode current
• This will keep IR drop constant but if zener
operates above zener breakdown level then
voltage across zener will be constant
•

27

Bipolar Junction Transistors
(BJTs)
• The bipolar junction transistor is a semiconductor
device constructed with three doped regions.
• These regions essentially form two ‘back-to-back’
p-n junctions in the same block of semiconductor
material (silicon).
• The most common use of the BJT is in linear
amplifier circuits (linear means that the output is
proportional to input). It can also be used as a
switch (in, for example, logic circuits).

force – voltage/current
water flow – current
- amplification

Understanding of BJT

Basic models of BJT
npn transistor
Diode

Diode

pnp transistor
Diode

Diode

Qualitative basic operation of BJTs

Current Flow Convention

E

Emitter
(n-type)

Base
Collector
(p-type) (n-type)

IE

C

IC

IB

B

npn BJT Structure
• The emitter (E) and is heavily doped (ntype).
• The collector (C) is also doped n-type.
• The base (B) is lightly doped with opposite
type to the emitter and collector (i.e. p-type
in the npn transistor).
• The base is physically very thin for reasons
described below.

B-E and C-B Junctions
• The p-n junction joining the base and emitter
regions is called the base-emitter (B-E)
junction. (or emitter-base, it doesn’t really
matter)
• The p-n junction between the base and
collector regions is called the collector-base
(C-B) junction.(or base-collector)

BJT Operation
E
E (n)

B
(p)

C
C (n)

B

• The forward bias between the base and emitter
injects electrons from the emitter into the base and
holes from the base into the emitter.
• As the emitter is heavily doped and the base
lightly doped most of the current transport across
this junction is due to the electrons flowing from
emitter to base.

BJT Operation
• The base is lightly doped and physically very thin.
• Thus only a small percentage of electrons flowing
across the base-emitter (BE) junction combine
with the available holes in this region.

BJT Operation
• Most of the electrons (a fraction α which is close
to 1, e.g. 0.98) flowing from the emitter into the
base reach the collector-base (CB) junction.
• Once they reach this junction they are ‘pulled’
across the reverse biased CB junction into the
collector region i.e. they are collected.
• Those electrons that do recombine in the base give
rise to the small base current IB.

BJT Operation
• The electrons ‘collected’ by the collector at the CB junction essentially form the collector current in
the external circuit.
• There will also be a small contribution to collector
current, called ICO, from the reverse saturation
current across the CB junction.
• The base current supplies positive charge to
neutralise the (relatively few) electrons
recombining in the base. This prevents the build
up of charge which would hinder current flow.

Biased Transistor
• Biasing is the process of applying external voltage to
the transistor

Input characteristics of Transistor in CE
configuration

Output characteristics
• Cut –off region
Both the emitter-tobase and collector-tobase junction are reversed biased
IB = 0 and IC = ICEO
Thus region below IB is a cut off region

• Active Region
 The emitter-tobase junction is forward
biased and collector-to-base junction is
reversed biased
 IC increases slightly with increase in VCE and
largely depends upon IB
 Since IC = dc IB If IB increases then IC rises
substantially

• Saturation Region
 Both emitter-tobase junction and collectorto-base junction are forward biased
 IC increases rapidly with increase in VCE
• Output Resistance
• The dynamic output resistance(ro) can be
defined as the ratio of change in collectoremitter voltage ( VCE) to the change in collector
current ( IC) at constant IB

Rectifiers
• All electronic circuits required DC power supply
for their operation
• Where as standard supply available is 230V AC
• Thus need to rectified by using rectifier
• Types in your scope
• Half-Wave Rectifier
• Centre Tap Full -Wave Rectifier
• Bridge Rectifier

Half Wave Rectifier
•
•
•
•
•
•
•
•

During +ve half cycle, the diode is forward biased
This results into current through the diode
Assuming resistive load
Thus voltage across the load will therefore be the
same as the supply voltage ( Vs - Vf),
And it is sinusoidal for the first half cycle only so
Vout = Vs.
During -ve half cycle, the diode is reverse biased
Hence No current flows through the diode or
circuit
Result into Vout = 0.

Disadvantages of HWR
• Low output because one half cycle only delivers
output
• A.C. component more in the output
• Requires heavy filter circuits to smooth out the
output

Peak Inverse Voltage
• In HWR, during the negative half cycle of the
secondary voltage, the diode is reverse biased.
• No voltage across the load RL during this half cycle
• Thus whole secondary voltage will come across the
diode.
• When the secondary voltage reaches its maximum
Vm, in the negative half cycle the voltage across the
diode is also maximum.
• This maximum voltage is known as peak inverse
voltage (PIV).
• It is the maximum voltage the diode must withstand
during the reverse bias half cycle of the input
• In the case of HWR, PIV =Vm

R.M.S. Value
• The R.M.S. value is the effective value of the
current flowing through the load and is given by

R.M.S. Value
• This is the rms value of the total load current
which include d.c. value and a.c. components
• In the out put of rectifier, the instantaneous value
of a.c fluctuation is the difference of the
instantaneous total value and the d.c. value
• Thus instantaneous a.c. value is given as
• ′= −

Ripple factor( )
• The purpose of the rectifier is to convert a.c.
voltage to d.c., but no type of rectifier convert
a.c. to perfect d.c.
• It produces pulsating d.c.
• This residual pulsation is called ripple.
• The ripple factor indicates the effectiveness of a
rectifier in converting a.c. to perfect d.c
• It is the ratio of the ripple voltage to the d.c.
voltage.

Ripple factor( )

• In case of HWR the a.c. component exceeds
the d.c. component.
• Thus the HWR is a poor rectifier

DC or average value of load current (Idc)

Transformer Utilization Factor
• The d.c. power to be delivered to the load in a
rectifier circuit decides the rating of the
transformer used in the circuit. So, transformer
utilization factor is defined as
• The factor which indicates how much is the
utilization of the transformer in the circuit is called
Transformer Utilization Factor (TUF).

• The a.c. power rating of transformer = Vrms Irms

The dc power delivered to the load,

Full Wave Rectifer
• In FWR, current flows through the load during both half
cycles of the input a.c. supply.
• Like the HWR circuit, a FWR circuit produces an output
voltage or current which is purely DC or has some
specified DC component.
• FWR have some fundamental advantages over their
HWR counterparts.
• The average (DC) output voltage is higher than for HWR
• The output of the FWR has much less ripple than that of
the HWR producing a smoother output waveform.
• There are two types of FWR
• Centre Tap rectifier
• Bridge Rectifier

Centre Tap Full Wave (CTFW) Rectifier
• In this circuit two diodes are used
• One for each half of the cycle
• A transformer is used whose secondary winding is
split equally into two halves with a common center
tapped connection, (C)
• This configuration results in each diode conducting
in turn when its anode terminal is positive w.r. to the
transformer center point C producing an output
during both half-cycles, twice that for the half wave
rectifier so it is 100% efficient

• This FWR consists of two diodes connected to a
single load resistance (RL) with each diode taking it
in turn to supply current to the load
• When point A of the transformer is +ve w. r. to point
B, diode D1 conducts in the forward direction
• When point B is +ve (in the -ve half cycle) with
respect to point A, diode D2 conducts in the forward
direction and the current flowing through resistor R
is in the same direction for both circuits.
• As the output voltage across the resistor R is the
phasor sum of the two waveforms combined, this
type of FWR circuit is also known as a "bi-phase"
circuit.

(Full Wave) Bridge rectifier
• This circuit uses four individual rectifying diodes
connected in a closed loop "bridge"
configuration
• The main advantage of this bridge circuit is that
it does not require a special centre tapped
transformer, thereby reducing its size and cost
• The single secondary winding is connected to
one side of the diode bridge network and the
load to the other side

• The four diodes labeled D1 to D4
are arranged in "series pairs"
with only two diodes conducting
current during each half cycle.
• During the +ve half cycle of the
supply, D1 and D2 conduct
• While D3 and D4 are reverse
biased and the current flows
through the load as shown
• During the -ve half cycle of the
supply, D3 and D4 conduct
• But D1 and D2 switch off as they
are now reverse biased
• The current flowing through the
load is the same direction as
before.

• During the +ve half cycle of the supply, D1 and
D2 conduct
• While D3 and D4 are reverse biased and the
current flows through the load as shown

Peak inverse Voltage
• In the case of centre tapped FWR, PIV =Vm +
Vm=2Vm
• Where, the first Vm is the maximum voltage
across the load when one diode conducts which
must appear at the cathode of the other diode,
• the second Vm is the maximum reverse voltage
appear at the anode of second (OFF) diode
• Hence the peak inverse voltage across the
second (OFF) diode in the positive half
cycle=2Vm.
• In the case of Bridge FWR, PIV =Vm.

• During the -ve half cycle of the supply, D3 and D4 conduct
• But D1 and D2 switch off as they are now reverse biased
• The current flowing through the load is the same direction as
before.
• As the current flowing through the load is unidirectional, so
the voltage developed across the load is also unidirectional
the same as for the previous two diode full-wave rectifier

RMS Value of load current
• the instantaneous a.c. value is given as
• ′= −
• Here the rms value of the a.c component is
given by,

Ripple factor( )

• It shows that in case of a FWR output d.c. components
exceeds the a.c component
• The FWR is good in rectification process.

• TUF is defined as the ratio of d.c. output power
to a a.c. power supplied to it by the secondary
winding.
•
=
/
(
)
• In case of a Bridge FWR, the rated voltage of
the secondary winding=
/ 2
• and rms value of current flowing through the
secondary winding= Im/ 2

• The average TUF in full wave rectifying circuit is
determined by considering primary and secondary
winding separately. There are two secondaries. Each
secondary has a TUF of 0.287.

Advantages of Bridge FWR
• The peak inverse voltage (PIV) across each
diode is Vm and not 2Vm as in the case of FWR.
Hence the voltage rating of the diodes can be
less.
• Centre tapped transformer is not required.
• There is no D.C. current flowing through the
transformer since there is no centre tapping and
the return path is to the ground.
• So the transformer utilization factor is high.

Dis-advantages of Bridge FWR
• Four diodes are to be used.
• There is some voltage drop across each diode
and so output voltage will be slightly less
compared to CT FWR.
• But these factors are minor compared to the
advantages.

Rectifier With Filter
• The output of the FWR contains both ac and dc
components.
• A majority of the applications, which cannot
tolerate a high value ripple,
• Thus requires further processing of the rectified
output.
• The undesirable ac components i.e. the
ripple, can be minimized using filters.

FWR with C Filter
•

A capacitor filter connected directly across the load is
shown
• The property of a capacitor is that it allows ac
component and blocks dc component
• The operation of the capacitor filter is to short the ripple
to ground but leave the dc to appear at output when it is
connected across the pulsating dc voltage.
• During the positive half cycle, the capacitor charges upto
the peak vale of the transformer secondary voltage, Vm
and will try to maintain this value as the full wave input
drops to zero.

FWR with C Filter
• Capacitor will discharge through RL slowly until the
transformer secondary voltage again increase to a value
greater than the capacitor voltage.
• The diode conducts for a period, which depends on the
capacitor voltage. The diode will conduct when the
transformer secondary voltage becomes more than the
diode voltage. This is called the cut in voltage.
• The diode stops conducting when the transformer
voltage becomes less than the diode voltage. This is
called cut out voltage.

Have a nice day!
Best of luck for Exams
and for bright future.

Basic electronics and electrical first year engineering

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