MAHALAKSHMI 2.pdf · SSathya Priyadharshini. Asst. Prof./EEE 2 6. Draw the input and output characteristics of a transistor in CE configuration and mark the cutoff, saturation and

SSathya Priyadharshini. Asst. Prof./EEE 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

QUESTION BANK

DEPARTMENT: EEE SEMESTER : III

SUBJECT CODE: EE2203 SUBJECT NAME: ELECTRONIC

DEVICES &CIRCUITS

UNIT 2

BJT AND ITS APPLICATIONS

PART A (2 Marks)

1. What are power transistors? List its applications. (AUC MAY 13)

Power transistors are constructed with n-layer, called drift region between p+ layer

and n+ layer. The voltage, current and power ratings are higher. Power diodes

operate at high speeds

2. Among CE,CB,CC configuration which one is more popular and why?

(AUC DEC’12)

CE configuration is more popular because of its medium input impedance and high

voltage gain

3. Calculate β when α is 0.98

β=α/(α-1) (AUC DEC’12)

β=0.98/(1-0.98)=48

4. Draw the h-parameter equivalent circuit of a CE BJT configuration. (AUC DEC’11)

5. What is the application of optocoupler? (AUC DEC’11)

The output stages in light activated SCR and light activated TRIAC used

optocouplers. Moreover when high load currents are to be switched, the optocoupler

output stage is used as a trigger circuit for high power devices.


6. Draw the input and output characteristics of a transistor in CE configuration

and mark the cutoff, saturation and active regions. (AUC MAY 09)

7. State the advantages of optocoupler. (Write any four). (AUC MAY’09)

8. Why is collector region wider than emitter region in BJT? (AUC DEC’09)

The collector has to collect the electrons from the emitter region so it is always wider

than the emitter region.

9. Name the operating modes of a transistor. (AUC NOV’10)

CE, CB, CC configuration

10. What are hybrid parameters? (AUC NOV’10)

Any linear circuit having input and output terminals can be analysed by four

parameters(one measured on ohm, one in mho and two dimensionless) called hybrid

or h-parameters..H parameters are used for ac analysis of a circuit.

11. Which of the BJT configuration is suitable for impedance matching applications?

Why? (AUC MAY’07)

CE and CB configuration.

12. What is meant by thermal run away? (AUC MAY’07)


As ICBO increases, it increases the collector current which raises the collector base

junction temperature. This effect is cummulative. This could produce a significant

shift in Q point or in the worst case Ic keeps on increasing until the collector base

junction overheats and burns out. This effect is called thermal runaway.

13. Why is it necessary to stabilize the operating point of transistor? (AUC NOV’05)

In order to make the transistor in the active region.

14. Derive the relationship between αdc and βdc. (AUC DEC’08)

β=α/(α-1) α=β/(1+β)

15. Draw and explain input and output characteristics of a transistor CB configuration

(AUC MAY’05)

16. List the comparison between CB, CE, CC amplifiers. (AUC DEC’07)


17. State the biasing conditions required for the three regions of operation of a BJT.

(AUC MAY’04)

Emitter base junction – forward biased & Collector base junction reverse biased.

18. For normal operation, how is emitter-base junction biased? Forward.

19 For normal operation, how is collector-base junction biased? Reverse

20. What is the relation between the currents of a transistor? IE =IB +IC

21. What are the types of circuit connections known as configurations, for

operating a transistor?

Common-Base (CB)

Common-Emitter (CE)

Common-Collector (CC)

22. What is the relation between α and β of a transistor ?

23. What are the regions used when BJT is used as a switch ? Saturation and cut-off

regions.

24. What is the thermal resistance of power BJT ?

Thermal resistance is the resistance to the flow of heat. Heat flows from the junction

to the surrounding air. Larger the transistor case, smaller the thermal resistance and

vice-versa. Thermal resistance is reduced by providing heat sink with the transistor.

25 Why must the base be narrow for the transistor (BJT) action ?

Beta (β) is the ratio of IC to IB .IB becomes less if the base width is narrow. Higher

value of β can be obtained with lower value of base current.


26.What is the value of cut-in voltage for a BJT ?

For Silicon BJT - 0.7V For Germanium - 0.3V

27. Why an ordinary transistor is called bipolar ?

Because the transistor operation is carried out by two types charge

carriers— majority and minority carriers.

28. Why transistor (BJT) is called current controlled device ?

The output voltage, current or power is controlled by the input current in

a transistor. So , it is called the current controlled device.

29. What are “emitter injection efficiency” and “base transport factor” of a

transistor?

The ratio of current of injected carriers at emitter junction to the total emitter

current is called the emitter injection efficiency.

Transport factor, β = Ic ⁄IB

30. Why silicon type transistors are more often used than Germanium type ?

Because silicon has smaller cut-off current ICBO , small variations in ICBO due

to variations in temperature and high operating temperature as compared to those

in case of Germanium.

31.Why collector is made larger than emitter and base?

Collector is made physically larger than emitter and base because collector is

to dissipate much power.

32. Why the width of the base region of a transistor is kept very small as

compared to other regions?

Base region of a transistor is kept very small and lightly doped so as to pass most of

the injected charge carriers to the collector.

33. Why emitter is always forward biased with respect to base? To supply majority

charge carrier to the base.

34. Why collector is always reverse biased with respect to base? To remove the

charge carriers away from the collector-base junction.

35. Why CE configuration is most popular in amplifier circuits?

Because its current, voltage and power gains are quite high and the ratio of output

impedance and input impedance are quite moderate.


36. Why is CC configuration seldom used? Because its voltage gain is always less

than unity.

37. How many h-parameters are there for a transistor? Four – hi, ho, hr hf or h11,

h12, h21, h22.

38. What are the units for h11 and h22? h11 – ohm; h22 – mho (or) siemen.

39. What are the parameters hr and h0 called? hr – reverse transfer voltage.

ho – output admittance.

40. Why h-parameters are called hybrid parameters? Because they have different

units are mixed with other parameters.

41. Which is the smallest of the four h-parameters of a transistor? h0 or h12

42. What is the typical value of hie? 1 kΩ

43. A transistor connected in common base configuration has a -________ input

resistance and a ________ output resistance.

Low input resistance.

Very high output resistance.

4 4 . Which of the BJT configuration is suitable for impedance matching

application and why?

CC configuration is suitable for impedance matching applications because of

very high input impedance and low output impedance.

45. Give the current gain expression for a common emitter transistor

configuration?

γ = ΔIE/ΔIB

46. What are the tools used for small signal analysis of BJT? i. h – Parameter circuit

model.ii. z – Parameter circuit model. iii. y – Parameter circuit model.

iv. Transconductance parameter circuit model. v. Physical model.

vi. T-model.

47. What is the significance of ICBO and ICO?

ICBO is the leakage current from the collector to base with emitter open. ICO is the

leakage current from collector to emitter with base open (ICO = ICEO).


48. For a non-transistor IE = 12mA and β = 140. Determine the value of IB and Ic.

IE = 12mA, β = 140

IB = IE / (1+ β)= 0.085mA

IE = IB + IC

= 12 X 10-3 / (1+140)

IC = IE - IB = 12 - 0.085 X 10-3 = 11.915mA.

49. Differentiate FET and BJT (any two).

FET

FET

BJT

BJT

1. Unipolar device (that is current conduction by

only one type of either electron or hole).

1. Bipolar device (current conduction by both

electron and hole).

2. High input impedance due to reverse bias.

2. Low input impedance due to forward bias.

3. Gain is characterized by trans conductance

3. Gain is characterized by voltage gain.

50. What are the biasing conditions to operate transistor in active region? Emitter-

base junction has to be forward biased and collector-base junction to be reverse

biased.

51. What is thermal runaway?

The power loss in transistor is primarily at the collector junction because the voltage

there is high compared to the low voltage at the forward biased emitter junction. If the

collector current increases, the power developed tends to raise the junction

temperature. This causes an increase in β and α further increase in collector current

in temperature may occur resulting in “thermal run away.”

52. In a transistor operating in the active region although the collector junction is

reverse biased, the collector current is quite large. Explain.

Forward biasing the input side and reverse biasing the output side are the

requirements of a transistor in the active region. The collector current is

experimentally equal to the emitter current. Therefore the collector current will be

large as emitter current is large on the other hand, in CE operation IB is multiplied by

β, hence we get large collector current.


53. Why CE configuration is considered to be the most versatile one? The common

emitter configuration provides very good voltage gain about 500CE

configuration finds excellent usage in audio frequency applications, hence used in

receivers and transmitter.

54. Define bipolar junction transistors.

These devices operate with both holes and electrons and hence are called bipolar

junction.

55. Write the junction transistor operation may be drawn from the analysis. 1. The

major charge carriers in the PNP junction transistor are holes.

2. The major charge carriers in the NPN junction transistor are electrons

PART B(16 Marks)

1. Explain the construction, principle of operating, characteristics and applications of

Power transistors and optocouplers. (AUC MAY 09,NOV 09,NOV 12)

OPTOCOUPLERS:

Operation & construction:

Optocoupler is a phototransistor and LED combined in one pack. when the current

flows in the LED, the emitted light is directed to the phototransistor, producing current

flow in the transistor. The coupler may be operated as a switch, in which both LED

and phototransistor are normally Off. A pulse of LED causes the transistor to be

switched On for the duration of the pulse. The coupling is optical, so there is a high

degree of electrical isolation between the input and output terminals and so the term

optoisolator is used.

1 6

5

2 4

3


Applications:

The circuit of an optoisolator in dc or pulse type coupling is shown. The diode

current is switched on & off by the action of transistor Q1 operating from a 24V

supply. Transistor Q2 is turned on into saturation when D1 is energized. The collector

current of Q2 provides the load (sinking) current and the current through resistor R2.

Pull up resistors is necessary to ensure that the load terminal is held at the 5V supply

level when Q2 is off.


A linear application of an optocopler is shown. The 5V supply provides a Dc bias

current to D1 via R2 and the AC signal coupled via C1 and R1 increases and

decreases the diode current. Transistor Q1 is biased into an On state by direct

current through D1, and its emitter current is increased and decreased by the

variation in light level produced by the al;ternating current in D1. An output voltage is

developed across R3.


2. Draw the circuit for determining the transistor common base characteristics and

explain how the characteristics are measured and draw the graphs.

(AUC NOV 10 DEC 11)

Characteristics curve of CB transistor

The curves representing the relation between different DC currents and voltages of a

transistor helps in studying the operation of a transistor, when they are connected in

a circuit. The three important characteristics of a transistor are,

Input characteristics


Output characteristics

Constant current characteristics

Input Characteristics:

IE varies with VBE when VCB is held constant. VBE is varied and the corresponding

variation in IE is noted. This characteristic curve is used to find the input resistance

of a transistor.Rin=(∆VBE/∆IE) when VCB constant.

For a pnp transistor the largest current components are due to holes. Holes flow

from emitter to collector and few holes flow down towards ground out of the base

terminal.

(IE = IC + IB ).

For a forward biased junction, VEB is positive and for a reverse biased junction VCB is

negative. The complete transistor can be described by the following two relations,

which give the input voltage VEB and output current IC in terms of the output voltage

(VCB) and input current IE.

VEB = f1(VCB, IE)

IC= f2(VCB, IE)

Output Characteristics:

Here IC varies with VCB when IE is held constant. VCB is varied when IE is

constant in steps and the corresponding value of IC is noted. Next VCB is reduced

back to zero and IE is increased to a little higher value than before and the whole

procedure is repeated to get the family of curves as shown.

This characteristic is used to find output resistance of a transistor.

Rout=(∆VCB/∆IC) when IE is constant.

It can be seen that even when IE is zero IC flows in very small amount and this is

called leakage current flowing due to the minority charge carriers in reverse biased

junction.. IC depends on VCB. If VCB is increased beyond certain value, IC

increases rapidly due to avalanche breakdown.

(1). Active region:

In this region the collector diode is reverse biased and the emitter diode is forward

biased. Consider first that the emitter current is zero. Then the collector current is

small and equals the reverse saturation current ICO of the collector junction

considered as a diode.

If the forward current IB is increased, then a fraction of IE ie. adcIE will reach the

collector. In the active region, the collector current is essentially independent of


collector voltage and depends only upon the emitter current. Because adc is, less

than one but almost equal to unity, the magnitude of the collector current is slightly

less that of emitter current. The collector current is almost constant and work as a

current source.

The collector current slightly increases with voltage. This is due to early effect. At

higher voltage collector gathers in a few more electrons. This reduces the base

current. The difference is so small, that it is usually neglected. If the collector voltage

is increased, then space charge width increases; this decreased the effective base

width. Then there is less chance for recombination within the base region.

(2). Saturation region:

The region to the left of the ordinate VCB = 0, and above the IE = 0, characteristic in

which both emitter and collector junction are forward biased, is called saturation

region.

When collector diode is forward biased, there is large change in collector current

with small changes in collector voltage. A forward bias means, that p is made

positive with respect to n, there is a flow of holes from p to n. This changes the

collector current direction. If diode is sufficiently forward biased the current changes

rapidly. It does not depend upon emitter current.

(3). Cut off region:

The region below IE = 0 and to the right of VCB for which emitter and collector

junctions are both reversed biased is referred to cutoff region. The characteristics IE

= 0, is similar to other characteristics but not coincident with horizontal axis. The

collector current is same as ICO. ICBO is frequently used for ICO. It means collector to

base current with emitter open. This is also temperature dependent.

Current Transfer characteristics.

Here IC varies with IE and VCE is held constant. First VCB is set to a convenient

value & IE is increased in steps & corresponding values of IC is noted.

αdc=(∆IC/∆IB)

usually αdc is found from the output characteristics rather than from transfer

characteristics.

CB connection is rarely employed for AF circuits because,

Its current gaion is less than unity.

Its input and output resistance are so different

3. For a common emitter circuit draw the h-parameter equivalent circuit and write the

expressions for input impedance, output impedance and voltage gain.

(AUC APR 09, NOV 10,DEC11)


Common Emitter circuit:

Consider the transistor amplifier circuit shown in the figure. The capacitors act as AC

short circuit. Circuit circuit input terminals are base and emitter and output terminals

are collector and emitter. The current and voltage waveforms show that there is 180

degree phase shift between input and output waveforms. This is due to the fact that

as Vi increases in positive direction, it increases VBE. The increase in VBE increases

IC, thereby increasing the voltage drop across Rc and thus reducing the level of

collector voltage VC.Similarly when VS changes in negative direction, the resultant

decrease in VBE reduces IC level, thereby reducing VRC and producing a positive

output.

H parameter Equivalent circuit:

The first step in AC analysis of a transistor circuit is to draw the AC equivalent circuit

by substituting short circuit in the place of power supplies and capacitors. When this

is done the AC equivalent circuit is,


The h parameter circuit is now drawn by replacing the transistor in the equivalent

circuit with its h parameter model. The effect of hreVc is unimportant for practical

purpose in CE ccircuit.

The current and voltage polarities in the above figure are those that occur when the

instantaneous level of input voltage moving in positive direction.

Input Impedance

The input impedance of transistor terminal is Zib ≈ hie

Typical value of hie for a low current transistor is 1.5 KΩ.

Hie=(1+hfe)hib

Zb is the input impedance of the device base terminal. at the circuit input terminal,

resistors R12 & R2 is seen parallel to Zb.

The input impedance is,

Zi=R1R2Zb.

Output Impedance:

The ouput impedance at the transistor collector is Zc≈1/hoe

At the circuit output terminal Rc is parallel with Zc. So the circuit output impedance is

Zo=RcZc = Rc(1/hoe)

Because 1/hoe is typically 50KΩ. Rc is usually much less than 50KΩ, the circuit

output impedance is approximately equal to Rc.


Voltage Gain:

The circuit voltage gain is Av=(Vo/Vi)

Vi=Ib*hie

Vo=-Ic(Rc Rl)

So, Av=[-Ic(Rc Rl)]/[ Ib*hie]

Ic/Ib can be replaced with hfe . the voltage gain equation now becomes,

Av=[-hfe(Rc Rl)]/[ hie]

‘-‘ sign indicates that Vo is 180 degree out of phase with Vi.

Current Gain:

The transistor current gain is ,

Hfe =Ic/Ib

This is device current gain and not the transistor current gain. Is is divided between

hie and the bias resistors RB= R1R2. The output current from collector Ic is divided

between Rc & Rl. The overall circuit current gain is,

Ai=[hie*Rc*Rb)/[(Rc+Rl)*(Rc+hie)]

In practical applications the current gain of a transistor amplifier is not an

importantquantity.

Power Gain:

Ap=Av*Ai.

Like the current gain, power gain is also unimportant.

4. Explain the switching characteristics of transistor with neat sketch. (AUC NOV’10)

Diode Connected BJT:

Consider the circuit which shows a BJT with its collector and base terminals

connected together. This is referred to as diode connected. The total current into

the base collector terminal is (Ic+Ib), which equals IE flowing out of Q1. The total

voltage drop across the BJT is the normal base emitter voltage, VBE, which is

typically 0.7 V for a silicon device. So the device behaves exactly like a diode.

In the block representation of a transistor, the charge carriers flow across the

forward biased base emitter junction. But since the collector junction has a zero

voltage external bias, the charge carriers cross the junction to constitute collector

current.the barrier voltage of an unbiased pn junction is + on n side and – on p

side. The barrier voltage is the result of charge carriers crossing the junction to


create the depletion region, and it exists even when the collector & base terminals

are not connected together. This barrier voltage polarity + on n side and – on p side

pulls the minority charge carriers from the base into the collector. So, a collector

current will flow when the collector and base terminals are connected together,

when the collector base voltage is zero.

BJT Saturation:

A BJT may be used as a switch as well as for amplification. The switching circuit is similar to

a amplifier circuit, except that a pulse waveform is given as a input instead of bias voltage

and ac signal source. When the input voltage Vi is at zero level the base current IB is also

zero and consequently IC is zero and VCE equals VCC.

VCE=VCC-(IC*R2), with IC=0, VCE=VCC

When Vi is at a positive level, a base current flows. In the switching circuit Ib is made large

enough to produce an Ic level that will cause the voltage drop across R1 to approximately

equal the supply voltage, VCC. With IC*R2≈VCC

VCE ≈ VCC – Ic*R2 ≈ 0


If VCE went down to exactly zero volts, the collector base junction would become forward

biased by 0.7V. in this case, the collector base barrier voltage would overcome and charge

carriers from the emitter would be repelled from the collector base junction. Consequently

there would be zero collector current. But if IC becomes zero, there would be no voltage

drop across R2 and the collector emitter junction would not be forward biased. So IC does

not become large enough to make VCE equal zero.

In the diode connected BJT IC flows when VCB equals zero. Itr is found that even when the

collector base junction becomes parytially forward biased, the barrier voltage is not

completely overcome, and IC continues to flow. In this situation the collector emitter voltage

is termed the saturation voltage VCEsat and it is the minimum level of collector emitter


voltage that exists when the voltage drop across the collector resistor R2 tends to drive VCE

towards zero.

In a switching circuit, a small base current is used to control a much larger collector current

to switch the trasnsistor between on & off.

5. Draw the input & output characteristics of a BJT in CE configuration.

(AUC MAY 09)


The curves representing the relation between different DC currents and voltages of a

transistor helps in studying the operation of a transistor, when they are connected in

a circuit. The three important characteristics of a transistor are,

Input characteristics

Output characteristics

Constant current characteristics

Input Characteristics:

IB varies with VBE when VCE is held constant. VBE is varied and the corresponding

variation in IB is noted. This characteristic curve is used to find the input resistance

of a transistor.Rin=(∆VBE/∆IB) when VCE constant.

Output Characteristics:

Here IC varies with VCE when IB is held constant. VCE is varied when IB is

constant in steps and the corresponding value of IB is noted. Next VCE is reduced

back to zero and IB is increased to a little higher value than before and the whole

procedure is repeated to get the family of curves as shown.

This characteristic is used to find output resistance of a transistor.

Rout=(∆VCE/∆IC) when IB is constant.

It can be seen that even when IB is zero IC flows in very small amount and this is

called leakage current flowing due to the minority charge carriers in reverse biased

junction.. IC depends on VCE. If VCE is increased beyond certain value, IC

increases rapidly due to avalanche breakdown.


(1) Active Region:

In this region collector junction is reverse biased and emitter junction is forward

biased. It is the area to the right of VCE = 0.5 V and above IB= 0. In this region

transistor current responds most sensitively to IB. If transistor is to be used as an

amplifier, it must operate in this region.

If adc is truly constant then IC would be independent of VCE. But because of early

effect, adc increases by 0.1% (0.001) e.g. from 0.995 to 0.996 as VCE increases from

a few volts to 10V. Then bdc increases from 0.995 / (1-0.995) = 200 to 0.996 / (1-

0.996) = 250 or about 25%. This shows that small change in a reflects large change

in b. Therefore the curves are subjected to large variations for the same type of

transistors.

(2) Cut Off:


Cut off in a transistor is given by IB = 0, IC= ICO. A transistor is not at cut off if the

base current is simply reduced to zero (open circuited) under this condition,

IC = IE= ICO / ( 1-αdc) = ICEO

The actual collector current with base open is designated as ICEO. Since even in the

neighborhood of cut off, a dc may be as large as 0.9 for Ge, then IC=10

ICO(approximately), at zero base current. Accordingly in order to cut off transistor it is

not enough to reduce IB to zero, but it is necessary to reverse bias the emitter

junction slightly. It is found that reverse voltage of 0.1 V is sufficient for cut off a

transistor. In Si, the a dc is very nearly equal to zero, therefore, IC = ICO. Hence even

with IB= 0, IC= IE= ICO so that transistor is very close to cut off.

In summary, cut off means IE = 0, IC = ICO, IB = -IC = -ICO , and VBE is a reverse

voltage whose magnitude is of the order of 0.1 V for Ge and 0 V for Si.

Reverse Collector Saturation Current ICBO:

When in a physical transistor emitter current is reduced to zero, then the collector

current is known as ICBO (approximately equal to ICO). Reverse collector saturation

current ICBO also varies with temperature, avalanche multiplication and variability

from sample to sample. Consider the circuit shown in below figure. VBB is the

reverse voltage applied to reduce the emitter current to zero.

IE = 0, IB = -ICBO

If we require, VBE = - 0.1 V

Then - VBB + ICBO RB < - 0.1 V


(3).Saturation Region:

In this region both the diodes are forward biased by at least cut in voltage. Since the

voltage VBE and VBC across a forward is approximately 0.7 V therefore, VCE = VCB +

VBE = - VBC + VBE is also few tenths of volts. Hence saturation region is very close to

zero voltage axis, where all the current rapidly reduces to zero. In this region the

transistor collector current is approximately given by VCC / R C and independent of

base current. Normal transistor action is last and it acts like a small ohmic

resistance.

Current Transfer characteristics.

Here IC varies with IB and VCE is held constant. First VCE is set to a convenient

value & IB is increased in steps & corresponding values of IC is noted.

Large Signal Current Gain βdc :-

The ratio Ic / IB is defined as transfer ratio or large signal current gain bdc

Where IC is the collector current and IB is the base current. The bdc is an indication if

how well the transistor works. The typical value of bdc varies from 50 to 300.

In terms of h parameters, b dc is known as dc current gain and in designated hfE ( b dc

= hfE). Knowing the maximum collector current and bdc the minimum base current

can be found which will be needed to saturate the transistor.

This expression of bdc is defined neglecting reverse leakage current (ICO).

Taking reverse leakage current (ICO) into account, the expression for the bdc can be

obtained as follows:

bdc in terms of adc is given by


Since, ICO = ICBO

Cut off of a transistor means IE = 0, then IC= ICBO and IB = - ICBO. Therefore, the

above expression bdc gives the collector current increment to the base current

change form cut off to IB and hence it represents the large signal current gain of all

common emitter transistor.

usually βdc is found from the output characteristics rather than from transfer

characteristics.

Documents

MAHALAKSHMI 2.pdf · SSathya Priyadharshini. Asst. Prof./EEE 2 6. Draw the input and output characteristics of a transistor in CE configuration and mark the cutoff, saturation and