1.1 Introduction to Semiconductors | unit 1 introduction to semiconductors

BEX

Module 01

Introduction to Semiconductors

1.1 Introduction to Semiconductors

N type Semiconductor:-

To increase the number of conduction band electrons inintrisic silicon , pentavalent imparity atoms are added. these are atoms with five valence electrons such as

i) arsenic (as)

ii) phosphors (p)

iii) Bismuth (Bi)

iii) Antimony ( sb)

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Pentavalent impurity atom in a silicon crystal. an antimony (sb) impurity atom shown above-

Each pentavalent atom forms covalent bonds with four adjustment silicon atoms, leaving one extra electron .

The pentavalent atom gives up on electron , it often called a donor atom.

Majority and minority carriers:- A type here means negative charge of an electron .electron are called the majority carriers in n-type material.

Hole in an n-type material are called minority carriers

P Type Semiconductor :-

To increase the number of holes in intrinsic silicon trivalent impurity atoms are added these are atoms with their valence electrons such as

i) Baron(B)

ii) Indium(IN)

iii)Gallium(GO)

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Trivalent impurity atom in a silicon crystal structure . A) boron (B) impurity atom is shown in the center.

The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon .

A Hole created by the doping process is not accompanied by a conduction (free) electron.

Trivalent Atom can take an electron it is often referred to as on accepted atom

1.2 Junction Diode: Principle of Diodes

Image result for a diode connected for forword bias diagram"

V Barrier

A Forward biased showing the flow of majority carriers and the voltage due to the barrier potential across the depletion region-

To bias a diode apply d,c vtg across it .

Forward bias is the condition that allows current through the PN junction.

Negative side of VBIAS is connected to the N -region of the diode and the positive side is connected to the P- region.

A selected requirement is that the bias voltage VBIAS must be greater than the barrier potential.

Because of like charges repel, the negative side of the bias voltage source pushes the free electrons , which are the majority carriers in the N-region towards the PN junction .the flow of free is called Electron Current.

The –Ve side of the source also provide a continues flow of electrons through the external connection (conductor).

into the N-region as show in fig-B

The bias voltage source imparts sufficient energy to the free electrons for them to overcome the barrier potential of the depletion region and move on through into the 'p' region once in the P-region. these conduction electrons have lost enough energy to immediately combine with holes in the valence band.

Now the e- are in the valance band in the P-region simply because they have lost too much energy overcoming the barrier potential to remain in the conduction band. since unlike charge attract, the positive side of the bias voltage source attracts the valence electrons toward the left end of the region.

The hole in the P-region provide the medium or "Pathway" for these valence electrons to move through the P-region.

The holes which are the majority carriers in the P-region, effectively (not actually) move to the right toward the junction as shown in fig-B.

The defective flow of holes is called the hole current.

As from Fig-B hole current as the flow of valence electrons through the P-region with the holes providing the only means for these electrons to flow.

As the electrons flow out of the P-region through the external connection and to the positive side of the bias in the P-region at the same time these electrons become conduction electrons in the mater conductor.

To these is a continues availability of holes effectively moving towards the PN junction stream of electrons as they come across the junction in to the P-region.

The effect of forward bias on the depletion region:-

dep reg f

Depletion Region

Forward bias narrows the depletion region & produce a vtg drop across pn junction equal to the barrier potential.

As more electrons flow into the depletion region , the number of positive ions is reduce . as more notes effectively flow into the depletion region on the other side of the pn junction ,the number of -ve ions is reduce this reduction in positive & -ve ions during forward bias causes the depletion region to narrow.

The Effect of the Barrier Potential during forward bias:-

The electronic field between the positive and negative ions in the depletion region on either side of the junction creates an energy bill, that prevent free R form diffusing across the junction at equilibrium this is known as the barrier potential

When forward bias is applied the free electrons are provided with enough energy from the bias voltage source to overcome the barrier potential and effectively climb the energy bill and cross the depletion region.

The energy that the electronics repair in order to pass through the depletion region is equal to the barrier potential.

Electron gives up an amount of energy equivalent to the barrier potential when they cross the depletion region.

This energy loss results in a vtg drop across the pn junction equals to the barrier potential (0.7v)

An additional small vtg drop across the P & N regions due to the internal resistance of the material.

For doped Semiconductor material, this resistance called the dynamic resistance is very small and can usually be neglected.

REVERSE BIAS:-

Reverse bias is the condition the essentially prevents current through the diode.

A Diode connected for Reverse Biased-

Because unlike charges attract the positive side of the bia voltage source pulls the free election, which are the majority carriers in the N-region away from the PN junction.

AS the election flow towards the positive side of the voltage source additional positive ions are created

This results in a widening of the depletion region and a depletion of majority carriers.

The diode during the short transition time immediately after reverse bias vtg is applied-

In the P-region electrons from the negative side of the vtg source enter as valence electron and move from hole to hole toward the depletion region where the creators additional -ve ions.

This results in a widening of the depletion region and a depletion of majority carriers

As the depletion region widens the availability of majority carriers decreases

As more of the N & P regions become depleted of majority carriers, the electric field between the positive and -ve ions increase in strength the depletion region equals the bios vtg .this point the transition current essentially ceases except for a small reverse current that can usually be neglected.

Reverse Current

extremely small current that exist in reverse bias after the transition current dies act is caused by the minority carrier in the N& P region that are produced by the manly generated e hole pairs.

The conduction band in the P-region is at a higher energy level then the conduction band in the N-region. Therefore, the minority easily pass through the depletion region because they required no additional energy.

The Extremely small reverse current in a reverse biased diode is due to the minority carriers from thermally generated

e=hole pairs

The Diode - Before doping the p-type & N-type consisting silicon material atom acting as a neutral.

IF a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type, a junction forms at the bounded between the two regions and a diode is created.

Formation of the Depletion Region

pnj

At the instant of junction formation , free electrons in the N-region near the p-n junction being to diffuse across the junction and fall into holes near the junction in the P-region.

For every electron that defuse across the junction and combines with a hole , a positive charge is left in the region and a ve charge is created in the p-region firming a barriers potential. This action continues until the vtg of the barrier ripples further diffusion.

The Depletion region acts as a barriers to the farther movement of electrons across the junction

As positive ion & -ve ion across the junction produces a electric field across the junction -according to coulombs law.

The potential difference of the electric field across the depletion region is the amount of vtg required to move electronics through the electric field, this potential difference is called the barrier potential & is expressed in volt

The typical barrier potential is approximately 0.7 v for silicon & 0.3 v for germanium at 25c.

1.3 V-I characteristics of junction diode

A :- v=A Characteristic for forward bias:-

V-I Characteristics:-

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Graph shows how the dynamic resistance decrease as we move up the curve xd= DNF/DIF=

When the forward bias vtg is increased to a value where the vtg across the diode reaches approximately 0.7 v (barrier potential . the forward currant begins to increase rapidly

As we continue to increase the forward bias voltage the current continues to increase very rapidly ,but the voltage across the diode increase only gradually above 0.7 v.

This small increase in the diode vtg above the barrier potential is due to the voltage drop across the internal dynamic resistance of the semi conductive material.

Dynamic resistance:- A Resistance change as move along a V-I curve it is called dynamic or A. C resistance.

B) V - I Characteristics for Reverse biased :-

When the applied bias voltage is increased to a value where. the reverse vtg across the diode the reaches appropriately 0.7 v (barrier potential) the forward current begins to increase rapidly.

As we continue to increase the bias voltage the current continues to increase very rapidly bit the voltage across the diode increases very little above VBR.

IR increases little above VBR. resulting in overheating & possible damage.

1.4 AC and DC Resistance of Diode

Static or DC Resistance

It is the resistance offered by the diode to the flow of DC through it when we apply a DC voltage to it. Mathematically the static resistance is expressed as the ratio of DC voltage applied across the diode terminals to the DC flowing through it (shown by the black dotted line in Figure) i.e.

Dynamic or AC Resistance

It is the resistance offered by the diode to the flow of AC through it when we connect it in a circuit which has an AC voltage source as an active circuit element. Mathematically the dynamic resistance is given as the ratio of change in voltage applied across the diode to the resulting change in the current flowing through it. This is shown by the slope-indicating red solid lines in Figure and is expressed as

1.5 Diode Current Equation

ampers

Where ,

V= Applied voltage across the diode in volts

I=Current flow through the diode in amperes.

n= 2 for silicon P-N junction diode

=1 for germanium P-N junction diode.

IO = reverse saturation current flow through diode in amperes.

VT =Is the voltage equivalent of temperature in volts.

VT= K X T volt’s

K=Boltzmann's constant

K=8.62 * 10 -s ev/k

I= temperature in ok

The equation VI= K * T indicates that the current flow through the diode also depend upon the ambient temperature.

Room temperature =25 0c

T= 273+25=298K

VT=K * T

1.6 Equivalent circuit of Diode

1.7 Breakdown Mechanism

i) Avalanche breakdown:-As the magnitude of the reverse bias vtg is increased the kinetic energy of the minority carriers gets increased. while travelling the minority carriers collide with the stationery atoms which in turns results in breaking some of the covalent bond & generating free e- (carrier multiplication)

This process continues leading to a very swift multiplication giving rise to a large reverse current in just a few picoseconds. this effect is called as avalanche breakdown effect.

Topical Breakdown vtg is about 50v to 100v:-

i) Due to large power dissipation the junction temperature increase & may destroy the semiconductor device permanently.

ii) Zener Breakdown: - This type of breakdown occurs in heavily doped P-N junction in which the depletion region is very narrow.

All the applied reverse voltage appears across the depletion layer. the electric field is vtg per unit distance. it is very intense at the depletion region.

There for it can pull the electronic out of the valance bond by breaking the covalent bonds and producing the free electrons. This process is known as zener effect.

Due to this heavy current flow & diode may damage.

1.8 Zener Diode

Zener diode is a special type of p-n junction semiconductor diode in this diode the reverse breakdown voltage is adjusted precisely between 3v to 200v.

Its applications are based on this principle hence Zener diode is called as a breakdown diode.

The doping level of the imparity added to manufacture the zener diode is controlled in order to adjust the precise value of breakdown voltage.

PRINCIPLE OF OPERATION: - A zener diode can be forward biased or reverses biased. its operation in the forward biased mode is same as that of a p-n junction diode but its operation in the reverse biased mode is sustainably deferent.

Numerical:

A 5.0V stabilized power supply is required to be produced from a 12V DC power supply input source. The maximum power rating PZ of the zener diode is 2W. Using the zener regulator circuit above calculate:

a). The maximum current flowing through the zener diode.

Maximum current = Watts/ Voltage =2W/5V =400mA

b). The minimum value of the series resistor, RS

= 17.5 Ω

c). The load current IL if a load resistor of 1kΩ is connected across the zener diode.

d). The zener current IZ at full load.

Iz =Is -Il =440mA – 5mA = 395mA

1.9 Rectifier circuit

Half Wave rectifier

1) Due to the unidirectional current flow through the transformer there is a possibility of core saturation to avoid this transformer size must be increased.

2) Ripple factor is high.

3) Low rectification effecting.

4) Law TUF.

5) Law D.C O/P VTG & current.

6) Large filter component are required.

ADVANTAGES:-

1) Simple Construction.

2) Component required less.

3) Small size.

APPLICATION:-

Walkman, law cost power supply.

TRANSFORMER UTILISATION FACTOR (TUF):-It indicates how well the ilp transformer is being utilized

TUF= DC O/P Power / AC power rating of the transformer

Full Wave Rectifier

1) A centre tapped rectifier is a type of full wave rectifier that uses two diode connected to the secondary of a centre tapped transformer.

Center tapped full wave rectifier operation:-

I) During positive half cycle of i/p ac supply.

Diagram

D1.Is in forward biased & D2 is in reverse biased.

II)During -ve half cycle.

D1 Reverse biased

D2 Forward biased

ADVANTAGES

1) law ripple factor as Compared to HKR.

2)Better rectification efficiency.

3) Better TUF.

4)Higher value of average load vtg & avg load crt.

5)No possibility of transformer core saturation.

DISADVANTAGES

PIV of diode is 2vm , more size costly.

APPLICATION

I) Battery charges.

2)power supply at laboratory, high current, electronic ckt.

1.10 Clipper and Clamper

Diode Clipper or Limiting Circuits :-

Diode C kts called limiters or clippers are sometimes used to clip off portions of signal voltages above or below certain levels.

Another type of diode ckt called a clamper is used to add or restore a d.c level to an electrical signal.

Below fig a) that limits or clips the positive part of the input voltage As the ilp vtg goes positive the diode become forward biased Because the cathode is at ground potential (ov)the anode cannot exceed 0.7v (for si)

So point A is limited to 0.7v the diode is reverse biased and appears as an open. The O/P VTG looks like the with a magnitude determined by the voltage divider formed by R1& the load resistor RL.

Vout=(RL/R1+RL) vin

If R1 small compared to RL then vout = vin

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Clipper of the positive alternation. The diode is F,B during the +ve alternate RB during –ve alternation:-

Negative Clipper:-

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Limiting of the negative alternation the diode is PB during the –ve alternation R.B during the +ve alternation.

During Reverse bias the +ve part of the ilp vtg is clipped off when the diode is forward biased during the –ve part of the ilp vtg is clipped off. when the diode is F.B during the –ve part of the ilp vtg point A is held at 0.7v by the diode is no longer forward biased and a voltage appears across RL proportional to the ilp vtg.

Problem:- What would you expect to see displayed on oscilloscope connected across RL in the Limiter.

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The diode is forward biased and conducts when then the i/p vtg goes below -0.7v so for the –ve limiter , the peak o/p vtg across RL can be determined by the following

Equation -

Volt =(RL/R+RL) Vin

=(1k/100+1k)10v

= 1K/1.1K*10

=1*103/1.1*103*10

= 1*104/11*102*10

= 1*104*10-2*10

=10/11 10*10*10/11

Volt = 10000/11 = 9.09V

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Biased parallel /Clipper :-1) The level to which an act voltage is limited can be adjusted by adding a bias vtg in series with the diode shown in fig.

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The voltage at point A mast equal VBINS+0.7v before the diode will become forward biased & conduct

Once the diode begins to conduct the vtg at point A is limited to VBINS +0.7 so that all ilp above this level is clipped off

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To limit a vtg to a specified –ve level the diode & bios vtg must be connected. In this case the vtg at point ‘A’ must go below –VBIN-0.7v to forward biased the diode & initiate limiting action

Prob:- fig shows a ckt combing a positive clipper with a-ve clipper .Determine the o/p vtg c/f

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When the voltage at point A riches +5.7V diode A conducted and limits the waveform to +5.7v diode D2 does not conduct until the voltage reaches -5.7V

Therefore positive voltages above +5.7V & -ve voltages below -5.7V are clipped off

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Diode Clampers:-

1) Positive Clamper:-

Diagram:-

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Assumptions :-

1) The V/p is a perfect sine waveform

2)The value of R& C are chosen such that the Line constant T=RC is large equal

3)The diode is an ideal one

4)The Rc time constant is much longer as compared to one cattle period ‘T’ of the input.

RC >100T

Operation :-I n the first negative half cycle after turning on the Ckt the diode acts as a closed S/W & charges the capacitor to peak ilp vtg VM with the polarity shown below

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The diode reverse biased in both half cycle so it remains off.

2)Negative Clamper:-

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In the first positive half cycle the capacitor will charge through the forward biased to peak vtg vm

The charging takes place very quickly as the diode resistances negligibly small

Once the capacitor charges to vm the diode is reverse biased and stops conducting.

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Diagram non ideal diode

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Biased Clamper:-

diagram:-

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-Clamper with additional dc source:-

Diagram-

$C:\Users\i7\Desktop\Unit 1\65.jpg$

The load vtg W/F shows d.c level shift is positive but less than +vm

Level shift is given by

DG shift = vm - v

Operation:-

In the –ve half cycle ilp the diode will be forward biased and capacitor gets charged to a vtg (vm-v) volts with the polarities shown.

$C:\Users\i7\Desktop\Unit 1\67.jpg$

$C:\Users\i7\Desktop\Unit 1\69.jpg$

In above diagram during positive half cycle diode will remain permanently off therefore .the job of the diode is only to charge the capacitor.

Vo=vc +vi

Vo=(vm-v)+vi

Series clipper CKT:-

1)Series Negative clipper :- Ideal diode

$C:\Users\i7\Desktop\Unit 1\68.jpg$

$C:\Users\i7\Desktop\Unit 1\71.jpg$

Operation:-

In the positive half cycle of the sinusoidal ilp the diode is forward biased. Being an ideal diode , it acts as a closed switch & connects the load across the ilp the load vtg there fore equal to the ilp vtg in the positive half cycle.

In the –ve half cycle of the diode is reverse biased acts as an open ckts/w the load vtg is therefore zero during the –ve half cycle.

2) Series Positive clipper:-

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Series clipper with a D.C supply biased [clippers] :-

1) Biased Series – ve Clipper:-

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$C:\Users\i7\Desktop\Unit 1\74.jpg$

Operation:-

The operation of this ckt can be divided in to there intervals

1) Operation when in is +ve but less than v:-in the positive half cycle of the ilp as long as vin v the diode is not forward biased, therefore from to and then from t2 to t/2 shown in fig below the diode will remain in off state and the o/p voltage will be zero.

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Fig -a) Equivalent ckt for +vin>v -

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Fig b) Equivalent ckt for vin -ve

1.11 Avalanche Diode

An avalanche diode is a type of semiconductor diode which is designed to experience avalanche breakdown at a specified reverse bias voltage. The pn junction of an avalanche diode is designed to prevent current concentration and resulting hot spots so that the diode is undamaged by the avalanche breakdown.

The avalanche breakdown that occurs is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current when compared to a non-avalanche diode.

The construction of the avalanche diode is similar to the Zener diode, and indeed both Zener breakdown and Avalanche breakdown are present in these diodes. Avalanche diodes are optimized for avalanche breakdown conditions, so they exhibit small but significant voltage drop under breakdown conditions, unlike Zener diodes that always maintain a voltage higher than breakdown.

This feature provides better surge protection than a simple Zener diode and acts more like a gas discharge tube replacement. Avalanche diodes have a small positive temperature coefficient of voltage, where diodes relying on the Zener effect have a negative temperature coefficient.

The normal diode allows an electric current in one direction i.e. forward direction. Whereas, avalanche diode allows the current in both direction i.e. forward and reverse direction but it is specially designed to work in reverse bias condition.

1.12 Bipolar Junction Transistor: Transistor Operation

BJT:- The BJT is constructed with the three draped semiconductors region’s separate by two PN junctions as shown below

-Basic epitaxial planner structures.

-Three terminal with region’s are called emitter, base and collector.

-The physical representation of the two types of BJT’s,

One type consists between two regions separated by a P region (npn) and other type consists of two p regions separated by an n region (pnp).

-The Pn junction joining the base region and the emitter region is called the base emitter junction.

-The Pn junction joining the base region and the collector region is called the base collector junction.

-The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions.

*Base Transistor Operation:-

In order for the transistor to operate properly as an amplifier the two pn junction must be correctly biased with the external D.C vtg.

-The next figure shows the proper bias arrangement for both the npn and pnp transistors for active operation as an amplifier.

-In both the cases the base emitter

(BE) junction is forward biased & the base collector junction (BC) junction is reverse biased

As from above figure consider n-p-n transistor. The forward bias from base to emitter narrow’s the BE depletion region and the reverse bias from base to collector widens the BC depletion region shown in figure.

The heavily doped N-TYPE emitter region is full with conduction band(frep) electron’s that easily diffuse through the forward biased BE junction into the p-type base region where they become minority carrier’s same as forward biased diode region

The base region is lightly doped & very thin so that it has a very limited number of holes.

Those only a small percentage of all the e-flowing the BE junction can combine with the available holes in the base.

The relatively few recombined flow out of the base lead as valance electrons, forming as small base current.

Most of the e flowing from the emitter into the thin lightly dooped base region do not recombine but diffuse into the BC depletion region.

The BC depletion region diffuse e is being pulled across the reverse biased BC junction by the attraction of the collector supply vtg.

The electrons now move through the collector region, out through the collector lead into the +ve terminal of the collector vtg source. This forms the collector electrons current.

The collector current is much larger than the base current.

This is the reason transistor exhibit current gain.

Current Equation in n-p- n, p-n-p transistors & amplifier

Transistor Current:-

Transistor’s Characteristic’s and parameters: -

-The transistors is connected to d.c bias vtg for both the npn&pnp types VBB forward biases the base emitter junction &Vcc reverse biases the base collector junction.

*Biasing conditions for different regions of operation:-

Sr. no	Region of operation	BE junction	CB junction	work
1	Cutoff region	R.B	R.B	S/w
2	Active region	F.B	R.B	Ampr
3	Saturation region	F.B	F.B

Transistor’s Configuration:-

1) Common Base configuration(C.B)

2) Common emitter Configuration(C.E)

3) Common Collector Configuration(C.C)

Common Base configuration(C.B):-

-The I/p is applied between the emitter and the base. The base acts as a common terminal between the I/p and o/p.

-The input vtg is therefore VEB and the input current is IE.

-The output is taken between the collector and the base therefore the output vtg is VCB and the output current is IC.

* Current relation’s in CB configuration:-

-The collector current is IC of the common base configuration is given by

Ic=Ic(INI)+ICBO

-Where the Ic(INI) called the injected collectors current and it is due to the number of electrons crossing the collectors base junction.

-ICBO :- This is the reverse saturation current flowing due to the minority carrier’s between the collector and base when the emitter is open

-ICBO flow’s due to the reverse biased collector’s base junction. As ICBO negligible as compared to Ic(INI) we can neglect it in practice.

.’. Ic=Ic(INI)………………………practically

Ic=ICBO………………with emitter open

Emitter is open

ICBO

Collector is to base control

-Since the ICBO flow’s due to terminally generated minority carrier’s it increases with increase the temperature.

-It doubles it’s value for every 100c rise in temperature.

-Current amplification factor or current gain (ddc):-

Current amplification factor or current gain is the ratio of collector current due to the injection to the total emitter current

αd.c = Ic(INI)

-The value of the ddc for CB configuration will always be less than 1.This is because

Ic(INI)<IE.

-Typically the value of d.d.c ranges between 0.95 to 0.995 depending upon the thickness of the base region.

-Larger the thickness of the base region smaller the value of the d.d.c

Ic(INI)=d.d.c.IE

Hence the expression for IC is given by

IC=αd.c IE + ICBo--------------------I

But the ICBo is negligibly small

ICd.d.c IE

* Expression for IB:-

IE=IB+IC

IE=αd.cIE+ICBo+IB…………………from I

IB=(1-αd.c)IE-ICBo

Neglecting ICBo

IB=(1-αd.c)IE

Characteristics of a transistors in a common base configuration:-

1. Input Characteristic:-

A. Input Characteristic: is always a graph of input current verses input vtg. For common base (CB) configuration input current is the emitter current (IE) & I/p vtg. is the emitter to the base vtg (VEB)

The I/p Characteristic is plotted at a constant O/p vtg. VCB

B. Output Characteristic:

Output Characteristic is always a graph of O/p current versus O/p Vtg.

For the CB configuration the O/p current is collector current (IC) of the output voltage is collector to base vtg. (VCB)

Output Characteristic is plotted for a constant value of I/p current (IE)

O/p Characteristic of a n-p-n transistor in CB Configuration

Dynamic O/p resistance (ro)

ro = / I constant

in the active region Ic does not depend on VCB. It depends only on the I/p current IE. That is why the transistor is called as a current controlled or current operated device.

Feature of CB configuration :

Common terminal : base

Input current : IE

O/p current IC

I/P Vtg. : VEB

O/P Vtg. VCB

Current gain :

( less than 1)

Vtg. Gain : medium

Input resistance : very low (20-)

O/P resistance : very high (1 m-2)

Application : as preamplifier

1.13 Load line Analysis

The load line solution was found by superimposing the actual characteristics on a plot of network equation involving the same network variables.

It is known as load line analysis as load of network defined the slope of the straight line connecting the points defined by network parameters.

The smaller the load resistance, the steeper the slope of the network load line.

Fig.: DC Load Line (Ref. 2)

The output equation is given by,

The characteristic curve is drawn as

Fig.: DC Load Line characteristic curve (Ref. 2)

When Ic = 0mA then

If we choose VCE = 0V then,

Numerical

Determine the values of VCC RC and RB of a fixed bias configuration for a given load line and defined Q point.

Solution:

1.14 DC Biasing (Fixed bias and Voltage Divider)

Base bias is the simplest way of biasing a BJT transistor. It ensures that the voltage applied at the base, VBB, is correct, which then supplies this current so that the BJT has enough base current to switch on the transistor.

VBB is used to provide sufficient current to turn on the transistor. RB is used to provide the desired value of base current IB. VCC is the collector supply voltage required for a transistor to have sufficient power to operate. This voltage, reverse-biases the transistor, so that the transistor has sufficient power to have an amplified output collector current. The collector resistor, RC provides the desired voltage in the collector circuit

The base current is found by dividing the voltage across resistor RB shown below:

IB= (VBB - VBE)/RB

Since the voltage drop across a silicon junction is 0.7V, the value of VBE=0.7V. Therefore, IB equals:

IB= (VBB - VBE)/RB= (5v - 0.7v)/56kΩ= 76.78µA

The collector current IC can be calculated next:

IC=βdc x IB= 100 x 76.78µA≈ 7.68mA

With IC then known, the collector-emitter voltage, VCE can be calculated.

VCE= VCC - IC x RC= 15v - (7.68mA x 1KΩ)= 7.32v

Base biasing can also be done with a single supply voltage, VCC, omitting VBB. So instead of using VBB in calculations, you use VCC. The result of the calculations remains the same.

Disadvantages:

βdc of a transistor is one of the most unstable and unpredictable parameters of a transistor.

βdc is also susceptible to changes due to temperature.

Voltage divider bias

In the previous configurations, the bias current and the collector emitter voltage were a function of current gain β of a transistor.

Since β is temperature sensitive, actual value could not be defined .

Hence it is very much desired to develop a bias circuit which is independent of β.

This gives a Voltage divider configuration.

Fig.: Voltage Divider Bias (Ref. 2)

When analysed on exact basis, the sensitivity to changes in β is very small and if circuit parameters are nicely chosen then it can even become independent of β.

IB changes with the change in β but the operating point on the characteristics defined by Ic and Vce remain fixed if proper parameters are employed.

Applying exact analysis, the above circuit is redrawn as,

Fig.: Analysis of Voltage Divider circuit (Ref. 2)

The voltage is replaced by a short circuit equivalent (Ref. 2)

Now calculating open circuit thevenin’s voltage and applying voltage divider rule we have (Ref. 2)

Applying kirchoff’s voltage law , (Ref. 2)

Put IE = ( β + 1 ) IB

Once IB is determined then rest of the quantities can be found out (Ref. 2)

The remaining equations for collector, emitter and base voltages are same as obtained for emitter- bias configuration.

Numerical

Determine dc bias voltage VCE and current IC for voltage divider configuration. (Ref. 2)

Solution:

1.15 Introduction to Amplifiers.

It is commonly known as Op-amp.

It requires an external power source hence is an active device.

It is a versatile device that can be used to amplify ac as well as dc input signals.

It can perform mathematical operations like addition, subtraction, multiplication, integration etc.

Hence was named as Operational Amplifier due to its capability of performing mathematical operations.

When external feedback is provided then the op-amp can be used as ac-dc signal amplifier, oscillator, regulator, active filter etc.

References:

1. Electronic Devices Circuit Theory - by Rober L. Boylestad 11th Edition, Pearson Publication, 2014

2. Microelectronic Circuits by A. S. Sedra and Kenneth C. Smith 7th Edition, Oxford University Press. 2017

3. Digital Design by M. Morris Mano, 5th Edition, Pearson Publication, 2016.

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