Transistors a Chitra

8/2/2019 Transistors a Chitra

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Session 2

Session Name: Operating Princ iple and

Cha rac teristics of Bipolar Junc tion Transistors

Author Name: A. Chitra

Department: ECE

Subjec t/ Course: Elec tron Devices


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Page 2 Ver 1:. 01 Operat ing Pri nciple and Character ist ics of Bipolar Junction TransistorsA. Chit ra IFET, Vil lupuram

Session Objec tives

At the e nd of this session, the lea rner w ill be ab le to:

Expla in w orking princ ip le of transisto rs.

Clea rly unde rstand how the transistor ac ts as a switc h a nd amp lifier.

Expla in why it is current c ontrolled devic e.List the cha rac teristics of transisto r c irc uit.

Tea c hing Learning Ma terial

BoardPresentation


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Operat ing Pri nciple and Character isti cs of Bipolar Junction Transistors Page 3A. Chit ra IFET, Villupuram

Session Plan

Time(in min)

ContentLearning Aid

andMethodology

FacultyApproach

Typical StudentActivity

Skill andCompetencyDeveloped

05Recap

PictureVisualization

Conducts,NarratesShows

Part icipates,ListensIdentifies

IntrapersonalLinguisticVisualKnowledgeComprehension

10Basic operat ingprinciple of transistor

Analogydemonstration,PPT, Interaction

Explains byQuestioning andcorrelates

Listens, answers

IntrapersonalInterpersonalLinguisticspatial

KnowledgeComprehension

05Operationsperformed bytransistor

Brainstorming Facilit ates Part icipates

IntrapersonalInterpersonalKnowledge

20Configurati ons ofTransistor

PPT, Board andchalk

Explains Listens

IntrapersonalLinguisticLogicalKnowledgeComprehension

10Observat ions fromthe input and outputcharacteri st ics

I challenge youFacilitates,Monitors

Part icipates,presents

InterpersonalIntrapersonalverbalKnowledgeComprehensionApplicat ion

05Conclusion

Board and chalk,ppt

Explains Listens

IntrapersonalLinguisticKnowledgeComprehension


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Session Inputs

Recap

We can start the session with an activity to recap the previous

session by conducting innovative quiz.

Sugg ested Ac tivity: Pic ture Visualization

Refe r to the p ic tures given b elow and ide ntify the keyword used during the

session 1.

1.

2.


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3.

N+ denotes ---------------------------

4.

5.

E N+ P N

B

C


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Basic Operating Princ iple o f Transistor

Having recapped the previous session we can now give an

ana logy to explain the b asic op erating p rinc ip le of transistor.

Swamiji was prea c hing one day that, it is not tha t I am g oing to

prea c h you wha t I lea rnt, but it is tha t w hat you w ant. Tank is filled

with full of water. If a person is coming with a small glass, he will

get only a glass of water. If a person is coming with a big

conta iner, he will be g etting tha t full. Similarly the t ransfer of

know led ge also ha pp ens.

Now let us see a wa ter system:

Here the wa ter system is taken a s ana logy and it is dem onstra ted . A

water filter with a tap can be taken to class and the working can

be de monstrated .

Suggested Ac tivity: Analog y dem onstration

To d emonstrate the above g iven analogy, a wate r filter with a tap can be

taken to the c lass. We can invo lve 3 lea rners to g ive demonstra tion.

Learners1 is asked to come and open the tap fully and then learner1 willsay;

Learner1: Wate r is flow ing .

Next, Learner2 will be asked to come and close the tap and then he willsay;

Learner2: No water is flowing. Finally, learner3 will be asked to come and

tank

Container

tapValve


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op en the tap p artia lly and then lea rner3 will say;

Learner3: Water is flow ing slow ly.

Now let us see how this ana logy is rela ted to our transistor.

Relating the analogy and transistor:

Wate r syste m Transisto rB p ipe line Base

C p ipe line Collec tor

E p ipe line Emitte r

Wate r reservoir a t C Tap Supply voltag e a t Co llec tor end base

Sma ll wa ter po ured into B

Black plunger moveme nt

Supply voltage a t b ase end Base

current

Wate r flow from C to E Flow of collec tor Current and emitte r

current

What we have learnt from this analogy?

A tiny amount of c urrent flow ing into "B" a llow s a large amount

to flow from "C" to "E" so we have an "amp lific a tion effec t". We

c an c ontrol a BIG flow of c urrent w ith a SMALL flow of c urrent.

If we c ontinually change the sma ll am ount of w a ter flow ing

into "B" the n we c ause c orrespond ing changes in the LARGE

amo unt o f water flow ing from "C" to "E".

For example, if we measure the c urrent flow in ga llons/ minute:

Suppose 1 gallon/ minute flowing into "B" a llow s 100

ga llons/ minute to flow from "C" to "E" then w e c an say that thetransisto r ha s a "ga in" or "amp lifica tion" fac tor of 100 time s. In a

rea l transistor we measure c urrent in thousand ths of anAmpere or "milliamps". So 1mA flowing into "B" would allow

100mA to flow from "C" to "E".

The amo unt o f curren t tha t c an flow from "C" to "E" is lim ited by

the "p ipe d iame ter". So , no m atte r how muc h current we p ush


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into "B", there will be a po int beyond whic h we c an't ge t any

more current flow from "C" to "E". The only wa y to solve this

p rob lem is to use a larger transisto r. A "po wer transisto r".

The transistor can b e used to switch the current flow on and

off. If we put suffic ient c urren t into "B" the transisto r w ill a llowthe maximum amount o f current to flow from "C" to "E". The

transisto r is switched fully "on".

If the c urrent into "B" is red uced to the point whe re it can no

longer lift the b lac k plunge r thing, the transistor w ill be "off".

Only the sma ll "lea kage" curren t from "B" will be flow ing . To turn

it fully off, we must stop all current flowing into "B".

You can control the current flowing from the emitter to the

collec to r, with a sma ll change in the base c urrent. Thus it is ca lled

as a current controlled device. Now let us visualize the transistor

am plific at ion function.

Operations Performed by Transistor

Now the session can be ma de m ore interac ting . The lea rners ma y

be a b le to define the func tions of BJT like amp lifica tion and

switc hing. This can be d one b y questioning the lea rners.

FB diode


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Suggested Activity: Brainstorming

Possible questions can be:

A. What is the func tion of a switc h?B. What is meant by m od ulation?

Possib le a nswers may be:

A.1. Switc h ha s two sta tes on a nd off2. it ha s tw o p ositionsB.1. Variation

2. Inc rea sing or dec reasing

3.Change etc

Yes. You are right.

Switc h is capab le of p roviding two sta tes on a nd off.

Modulation or amplification is state between off and on where

the signal strength/amplitude is modulated/varied or amplified or

inc rea sed . Depend ing on the b iasing as we saw in the last session .

If both the juns are FB it is fully ON. If both the juns are RB it is fully

OFF. If EB is FB and CB is RB it p rovides Mo dula tion.

Various Configurations of Transistor

We can now e xpla in the va rious configurat ions of t ransistor.

Various configurat ions of transistor:

1. Comm on Base mod e2. Com mon emitter mod e3. Comm on c ollec tor mode


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Page 10 Ver 1:. 01 Operat ing Principle and Character ist ics of Bipolar Juncti on TransistorsA. Chit ra IFET, Vil lupuram

Comm on-base configuration (CB)

Fig below show s the c ommo n-base c onfigura tion for pnp and npn transistor.

CB is derived from the fa c t tha t the:

1. Base is com mon to both i/ p a nd o / p o f the configuration.2. Base is usua lly the termina l c losest to or a t ground potential

Ana lysis of Com mo n-ba se c onfigura tion fo r pnp.

Step 1: B-E junc tion m ust b e forward b ias

Step 2:

1. B-C junc tion must b e reve rse b ias

C

B

E

pnp

C

E

npn

B

Fig. 4.3: Symbols used with the CB configuration

IC

IEIB

IC

IEIB

p pn

VEB

E

B

C B

E

pnp

IIB

VEB


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2. ICBO=ICO=0 A is a reve rse saturat ion current and norma lly know n as

leakage

urrent

Now with EB junction FB and CB junction RB, we can relate the

currents in the c irc uit b y app lying simp le KCL. Now two c ts are are

coming out o f the tr and one c t is ente ring into the tr.

p pn

VCB

E

B

C

C

B

pnp

ICB

IB


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Current relat ions in CB mode

Current base, IB (A) is small compare to current emitter, IE (mA) and current

collector,IC (mA).

The relat ionship amo ng these current can be ana lyse w ith KCL :

IE = IB + IC.

Now with EB junction FB and CB junction RB, we know that the

outp ut c urrent is grea ter than the input current . The fa c tor by

which the input c urrent is ma gnified is ca lled ga in fac tor.

Current ga in:

The ratio o f outp ut c urrent to input current is ca lled current ga in and is

rep resented by .

=

Ideally = 1, but in rea lity it is between 0.9 and 0.998

Expression for output current:

Ic = Ic (ma jority) + Ic (m inority)

Ic = IE + IcBo

Ic = IE (ignore IcBo d ue to small va lue)

E

C

I

I


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These c urves are the input a nd the outp ut c harac teristic s for

co mmon ba se mode.

Observations from the Input and Output Charac teristics

Having und erstood the Various Co nfigurat ions of Transistor, we

can now conduct a group activity namely I challenge you by

dividing the learners into 4 groups. We can facilitate and guide

the entire proc ess.

Suggested Ac tivity: I challenge you

1. All the groups are given 05 minutes for observing and listing the

ob servations on the input and outp ut charac teristics.

2. Then in the next 10 minutes they c an d iscuss their observations with atea m lead er from ea ch group or all in the g roup .

The hints g iven to Group A & B:

You c an relate the input c harac teristics to a devic e w hic h we lea rnt in theprevious sessions and you list out the observations from these input curves.

A


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The hints g iven to Group C & D:

Can you list out status of person with money and without money? You

should relate the output c harac teristic s to tha t sta tus.

Now let us consolida te the o bservations ma de b y a ll the group s.

Observations from input curves

Varia tion of VBE and IE:

1. For VBE < VA (VA is the vo ltage a t the p oint A= 0.5V), IE ~ 02. For VBE > VA, IE inc rea ses with inc rea se in VBE. But IE a t a

faster ra te com pared to VBE.

Effec t of output voltag e Vc B on input curve:1. As VcB is inc reased the curve shifts upwards.2. This is bec ause of high RB inc reases thr DLW and henc e the

base w id th dec rea ses and this is ca lled BWM or EE

Henc e w e c an ob serve tha t the input c harac teristics a re similar toFB d iod e charac teristics.

Ob serva tions from output c urves. There a re three regions in outp ut

curve:

1. Saturation region (Both CB and EB juns are FB) ON2. Cut Off reg ion (Both CB and EB juns are RB) OFF

3. Ac tive reg ion (EB jun FB and CB jun RB) Amplifier


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Conclusion

We can conc lude the session by sta ting the fo llowing line .

We can ob serve tha t Transistor can be emp loyed as a switc h or

as an amplifier de pend ing on the b iasing . In the next c lass we w ill

see the FETs.


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Summary

In this session, we learnt to:

Clearly understand the functioning of transistor

Explain the various configurations of transistor

Sketc h a nd ana lyze the input a nd outp ut c harac teristics of transistor.


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Assignment

1. Ge nerate any examp le for transistor op erating p rinc ip le.2. Ca rry out the ana lysis for CE c onfigura tion c harac teristics.3. Carry out the analysis for CC configuration characteristics.4. Tabula te the c om parison betw een the three c onfigura tions.


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Page 18 Ver 1:. 01 Operat ing Principle and Character ist ics of Bipolar Juncti on TransistorsA Chit ra IFET Vil lupuram

References

Elec tronic Devic es and Circ uits G.K. Mitha l

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Transistors a Chitra