basic electronics and electrical first year engineering

Basic Electrical and Electronics Engineering

FE Sem-IChapter- 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

Formation of Depletion region

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 P-type 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

Forward Biasing

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

Reversed Biasing

V-I characteristics of 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

If forward biased :

If reverse biased:

F

FF I

VR

D

DD I

VR

R

RR I

VR

AC or Dynamic Resistance

The dynamic. resistance of a diode is designated rd

F

Fd I

Vr

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

Silicon versus Germanium

Transition capacitance

• In reverse biased, majority carrier move away from junction

• This movement results change in charge (dq) with the change in voltage (dv)

• This increase in charge caused by increase in voltage is defined as transition capacitance

• It given as

Diffusion capacitance

• In forward biased, barrier potential is reduces and majority charge carrier crosses the junction

• This rate of change of injected charge with applied voltage is called as diffusion capacitance

• It given as

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

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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.

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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.

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• 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.

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• Voltage after breakdown is also called Zener voltage Vz.

• Vz remains nearly a constant, even though current Iz varies considerably.

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ZENER DIODE MODEL• Zener model to be

applied

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Basic Zener Regulator

L

iLL RR

VRVV

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Basic Zener Regulator• 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)

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Basic Zener Regulator• 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

•

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).

Understanding of BJT

force – voltage/currentwater flow – current - amplification

Basic models of BJT

Diode

Diode

Diode

Diode

npn transistor

pnp transistor

Qualitative basic operation of BJTs

Basic models of BJT

Current Flow Convention

EEmitter (n-type)

Base (p-type)

Collector (n-type)

B

C

IE

IB

IC

npn BJT Structure

• The emitter (E) and is heavily doped (n-type).

• 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

• 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.

E

E (n) B

(p) C (n)

B

C

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 C-B 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

Transistor Configuration

Circuit for CE configuration

Input characteristics of Transistor in CE configuration

• Input Resistance• It is the ratio of change in the

voltage to the change in input current

Output characteristics


• Cut –off regionBoth the emitter-to- base and collector-to-

base junction are reversed biasedIB = 0 and IC = ICEO

Thus region below IB is a cut off region


• Active Region The emitter-to- base 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-to- base junction and collector-

to-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 collector- emitter 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

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

• ′ 𝑖 = 𝑖𝐿 − 𝐼𝑑𝑐

R.M.S. Value

Ripple factor( )𝜸• 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( )𝜸

• 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).

Transformer Utilization Factor• 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

(Full Wave) Bridge rectifier• 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.

(Full Wave) Bridge rectifier• 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.

(Full Wave) Bridge rectifier• 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

DC Output voltage

• Vdc = Idc RL

• Since Rd<<<RL

RMS Value of load current

RMS Value of load current

• the instantaneous a.c. value is given as • 𝑖′ = 𝑖𝐿 − 𝐼𝑑𝑐 • Here the rms value of the a.c component is

given by,

Ripple factor( )𝜸

• It shows that in case of a FWR output d.c. components exceeds the a.c component

• The FWR is good in rectification process.

Rectifier Efficiency

Transformer Utilization Factor• TUF is defined as the ratio of d.c. output power

to a a.c. power supplied to it by the secondary winding.

• 𝑇𝑈𝐹 = / ( ) 𝑃𝑑𝑐 𝑃𝑎𝑐 𝑟𝑎𝑡𝑒𝑑• In case of a Bridge FWR, the rated voltage of

the secondary winding= / 𝑉𝑚 2 • and rms value of current flowing through the

secondary winding= Im/ 2

Transformer Utilization Factor• 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.

Comparison of Rectifiers

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

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.

Ripple Factor

• The ripple factor of a FWR with C-Filter given as

• The ripple factor of a HWR with C-Filter given as

• For good filtering C and RL should be high

Have a nice day!Best of luck for Exams and

for bright future.

basic electronics and electrical first year engineering

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