Generic selectors
Exact matches only
Search in title
Search in content
Search in posts
Search in pages
Filter by Categories
nmims post
Objective Type Set
Online MCQ Assignment
Question Solution
Solved Question
Uncategorized

Multiple choice question for engineering

Set 1

1. Thermal runaway is_________
a) an uncontrolled positive feedback
b) a controlled positive feedback
c) an uncontrolled negative feedback
d) a controlled negative feedback

View Answer

Answer: a [Reason:] Thermal runaway is a self destruction process in which an increase in temperature creates such a condition which in turn increases the temperature again. This uncontrolled rise in temperature causes the component to get damaged.

2. The thermal runway is avoided in a self bias because_________
a) of its independence on β
b) of the positive feedback produced by the emitter resistor
c) of the negative feedback produced by the emitter resistor
d) of its dependence on β

View Answer

Answer: c [Reason:] The self destruction of a transistor due to increase temperature is called thermal run away. It is avoided by the negative feedback produced by the emitter resistor in a self bias. The IC which is responsible for the damage is reduced by decreased output signal.

3. When the temperature is increased, what happens to the collector current after a feedback is given?
a) it remains same
b) it increases
c) it cannot be predicted
d) it decreases

View Answer

Answer: d [Reason:] Before the feedback is applied, when the temperature is increased, the reverse saturation increases. The collector current also increases. When the feedback is applied, the drop across the emitter resistor increases with decreasing collector current and the thermal runway too.

4. The thermal runway is avoided in a collector to base bias because_________
a) of its independence on β
b) of the positive feedback produced by the base resistor
c) of the negative feedback produced by the base resistor
d) of its dependence on β

View Answer

Answer: c [Reason:] The self destruction of a transistor due to increase temperature is called thermal run away. It is avoided by the negative feedback produced by the base resistor in a collector to base bias. The IC which is responsible for the damage is reduced by decreased output signal.

5. When the temperature is increased, what happens to the collector current after a feedback is given?
a) it remains same
b) it increases
c) it cannot be predicted
d) it decreases

View Answer

Answer: d [Reason:] Before the feedback is applied, when the temperature is increased, the reverse saturation increases. The collector current also increases. When the feedback is applied, the base current increases with decreasing collector current and the thermal runway too.

6. Discrete transistors T1 and T2 having maximum collector current rating of 0.75A are connected in parallel as shown in the figure. This combination is treated as a single transistor to carry a single current of 1A, when biased with a self bias circuit. When the circuit is switched ON, T1 had draws 0.55A and T2 draws 0.45A. If the supply is kept ON continuously, it is very likely that_________
electronic-devices-circuits-questions-answers-thermal-runaway-q6
a) both T1 and T2 get damaged
b) both T1 and T2 will be safe
c) only T1 gets damaged
d) only T2 gets damaged

View Answer

Answer: c [Reason:] The T1 transistor is having more power dissipation as it is drawing 0.55A. When power dissipation increases, the temperature increases and this leads to the ultimate further increase in the current drawn by T1. The current drawn by T2 will be reduced as the sum of currents drawn by T1 and T2 should be constant.

7. When the collector current is increased in a transistor_________
a) the reverse current is increased
b) the temperature is increased
c) collisions of electrons decrease
d) the emitter does not emit electrons

View Answer

Answer: b [Reason:] As the collector current is increased, the emitter releases more number of electrons. This causes more collisions of electrons at collector. This happens in a cycle and produces such a condition in which temperature is further more increased.

8. Which of the following are true?
a) TJ – TA = θPd
b) TJ – TA = θ/Pd
c) TJ – TA = θ+Pd
d) TJ – TA = θ-Pd

View Answer

Answer: a [Reason:] The TJ is called as junction temperature which varies and TA is called as the ambient temperature which is fixed. The difference between these temperatures is directly proportional to the power dissipation. Here, θ is called as thermal resistance which is proportionality constant.

9. When the power dissipation increases in a transistor, the thermal resistance_________
a) increases
b) cannot be predicted
c) decreases
d) remains same

View Answer

Answer: c [Reason:] The power dissipation is directly proportional to thermal resistance. We have, TJ – TA = θPd in which we can observe θ ∝ 1/Pd. So, a device with low power dissipation has high thermal resistance.

10. Which of the following biasing techniques are prone to thermal runaway?
a) self bias
b) collector to base bias
c) fixed bias
d) the biasing technique is identified by temperature effect

View Answer

Answer: c [Reason:] The collector current of a fixed bias transistor is IC= β(VCC-VBE)/RB. When the temperature is increased, the reverse saturation increases. The collector current also increases. This in turn increases the current again which leads to damage of transistor.

Set 2

1. For a given transistor, the thermal resistance is 8°C/W and for the ambient temperature TA is 27°C. If the transistor dissipates 3W of power, calculate the junction temperature (TJ).
a) 51°C
b) 27°C
c) 67°C
d) 77°C

View Answer

Answer: a [Reason:] We know, TJ-TA=HPD TJ=TA+HPD=27+8*3=51°C.

2. Which of the following are true?
a) TJ-TA=θpd
b) TJ-TA=θ/pd
c) TJ-TA=θ+pd
d) TJ-TA=θ-pd

View Answer

Answer: a [Reason:] The TJ¬ is called as junction temperature which varies and TA is called as the ambient temperature which is fixed. The difference between these temperatures is directly proportional to the power dissipation. Here, θ is called as thermal resistance which is proportionality constant.

3. A silicon power transistor is operated with a heat sink HS-A=1.5°C/W. The transistor rated at 150W (25°C) has HJ-C=0.5°C/W and the mounting insulation has HC-S=0.6°C/W. What maximum power can be dissipated if the ambient temperature is 40°C and (TJ)MAX=200°C?
a) 70.6W
b) 61.5W
c) 37.8W
d) 56.9W

View Answer

Answer: b [Reason:] PD=(TJ-TA)/ HJ-C +HC-S +HS-A =200-40/0.5+0.6+1.5=61.5W.

4. The total thermal resistance of a power transistor and heat sink is 20°C/W. The ambient temperature is 25°C and (TJ)MAX=200°C. If VCE=4V, find the maximum collector current that the transistor can carry without destruction.
a) 3.67A
b) 7.56A
c) 2.19A
d) 4.16A

View Answer

Answer: c [Reason:] PD =(TJ-TA)/ H =200-25/20=8.75W. Now, VCEIC = 8.75/4=2.19A.

5. The total thermal resistance of a power transistor and heat sink is 20°C/W. The ambient temperature is 25°C and (TJ)MAX=200°C. If VCE=4V, find the maximum collector current that the transistor can carry without destruction. What will be the allowed value of collector current if ambient temperature rises to 75°C?
a) 3.67A
b) 7.56A
c) 2.19A
d) 1.56A

View Answer

Answer: d [Reason:] PD =(TJ-TA)/ H =200-75/20=6.25W. Now, IC = 6.25/4=1.56A.

6. Which of the following is true?
a) HC-A = HJ-C – HJ-A
b) HC-A = HJ-C + HJ-A
c) HJ-A = HJ-C – HC-A
d) HJ-A = HJ-C + HC-A

View Answer

Answer: d [Reason:] HJ-C is thermal resistance between junction and case and HC-A is thermal resistance between case and ambient. The circuit designer has no control over HJ-C. So, a proper approach to dissipate heat from case to ambient is through heat sink.

7. The condition to be satisfied to prevent thermal runaway?
a) ∂PC/∂TJ > 1/Q
b) ∂PC/∂TJ < 1/Q
c) ∂PC/∂TJ > 1/Q
d) ∂PC/∂TJ < 1/Q

View Answer

Answer: b [Reason:] PC is the power dissipated at the collector junction. TJ is junction temperature which varies. The difference between these temperatures is directly proportional to the power dissipation. Here, Q is called as thermal resistance which is proportionality constant.

8. Thermal stability can be obtained by_________
a) shifting operating point
b) increasing power supply
c) heat sink
d) decreasing current at collector

View Answer

Answer: c [Reason:] As power transistors handle large currents, they always heat up during operation. Generally, power transistors are mounted in large metal case to provide a large area from which the heat generated by the device radiates.

9. Thermal stability is dependent on thermal runaway which is_________
a) an uncontrolled positive feedback
b) a controlled positive feedback
c) an uncontrolled negative feedback
d) a controlled negative feedback

View Answer

Answer: a [Reason:] Thermal runaway is a self destruction process in which an increase in temperature creates such a condition which in turn increases the temperature again. This uncontrolled rise in temperature causes the component to get damaged.

10. Which of the following biasing techniques are affected by thermal runaway?
a) self bias
b) collector to base bias
c) fixed bias
d) the biasing technique is identified by temperature effect

View Answer

Answer: c [Reason:] The collector current of a fixed bias transistor is IC= β(VCC-VBE)/RB. When the temperature is increased, the reverse saturation increases. The collector current also increases. This in turn increases the current again which leads to damage of transistor.

Set 3

1. Which gas is used to fill the chamber in the grown junction type transistor construction?
a) helium
b) boron
c) nitrogen
d) oxygen

View Answer

Answer: c [Reason:] In the process of transistor construction, a crucible is placed in the chamber. This chamber consists of hydrogen or nitrogen. These gases help in the prevention of oxidation. It also contains purified Ge or Si at a temperature few degrees above its melting point.

2. In a grown junction type construction, the method used form a junction transistor is_________
a) alloy type diffusion
b) mesa type
c) speed variation method
d) fused junction type

View Answer

Answer: c [Reason:] The grown junction may be formed by suddenly varying the rate of pulling the seed crystal from the melt. This method is based on the fact that proportion in which N and P type impurities crystallise i.e.., enter the grown crystal depends on the rate of pulling.

3. Which of the following methods take impurity variation method for transistor construction?
a) alloy type diffusion
b) grown junction type
c) epitaxial type
d) mesa type

View Answer

Answer: b [Reason:] In impurity variation method, the impurity content of the semiconductor is altered in its type as well as the quantity. For example, in making NPN germanium grown junction transistor, a small type of N type impurity is added to molten germanium and the crystal growth is started.

4. Which of the following is true about grown junction type construction?
a) N type impurity is added to P type impurity
b) Boron helps in the prevention of oxidation
c) The seed is pulled to a large distance for a correct growth
d) Slow pulling leads to the formation of P type crystal

View Answer

Answer: d [Reason:] This method is based on the fact that proportion in which N and P type impurities crystallise i.e.., enter the grown crystal depends on the rate of pulling. If the pulling rate is small, a P type crystal is grown. If the pulling rate is fast, an N type crystal is grown.

5. What is the melting point of indium in alloy type transistors?
a) 300°C
b) 200°C
c) 155°C
d) 100°C

View Answer

Answer: c [Reason:] This is similar to soldering and PNP transistor is generally is made by this process. In this method, first of all N type germanium is obtained. The N type wafer and indium dots are placed in a furnace and heated to about 500°C.

6. The non rectifying base contact is made from_________
a) welding a strip
b) germanium
c) indium
d) graphite

View Answer

Answer: a [Reason:] Leads for emitter and collector are soldered to the dots making non rectifying contacts. Further, non rectifying base contact is usually made from a welding a strip or loop of gold plated wire to the base plate.

7. What is the thickness of wafer in the alloy type transistors?
a) 1-2m inch
b) 3-5m inch
c) 5-6m inch
d) 4-7m inch

View Answer

Answer: b [Reason:] The wafer of crystal has a 3-5m inch thickness and 80m inch square. This is placed in a graphite jig with a dot of prepared indium. One dot of an indium is 3 times larger than the other.

8. The larger dot of the indium is used as_________
a) base
b) emitter
c) control pin
d) collector

View Answer

Answer: d [Reason:] The wafer is placed in a graphite jig with a dot of prepared indium. One dot of an indium is 3 times larger than the other. Finally the larger dot is used as collector. The smaller dot is used as emitter.

9. The electrical properties of a transistor in alloy type construction is determined by_________
a) space between the junctions in the wafer
b) proportions of N and P type impurities
c) the pulling rate of crystal
d) uniformity of the crystal lattice

View Answer

Answer: a [Reason:] Large area collector junction helps in collecting most of the holes emitted from the emitter ensuring that the collector current almost equals the emitter current. The spacing between two junctions inside germanium wafer is very small and determines the electrical properties.

10. The grown junction type transistors is generally used for_________
a) PNP transistors
b) NPN transistors
c) Both transistors
d) Depends on the material used

View Answer

Answer: b [Reason:] Grown junction type transistors are manufactured through growing single large crystal which is slowly pulled from the melt in crystal growing furnace. This is generally used for NPN transistors.

Set 4

1. In which region a transistor acts as an open switch?
a) cut off region
b) inverted region
c) active region
d) saturated region

View Answer

Answer: a [Reason:] In this mode, both the junctions are reverse biased. The transistor has practically zero current because the emitter does not emit charge carriers to the base. There is negligibility current due to minority carriers. In this mode the transistor acts as an open switch.

2. In which region a transistor acts as a closed switch?
a) cut off region
b) inverted region
c) active region
d) saturated region

View Answer

Answer: d [Reason:] In this mode, both the junctions are forward biased. The negative terminal of the battery is connected to the emitter. The collector current becomes independent of base current. In this mode the transistor acts as a closed switch.

3. Which of the following circuits act as a switch?
a)electronic-devices-circuits-questions-answers-transistor-switch-q3
b)electronic-devices-circuits-questions-answers-transistor-switch-q3a
c)electronic-devices-circuits-questions-answers-transistor-switch-q3b
d)electronic-devices-circuits-questions-answers-transistor-switch-q3c

View Answer

Answer: b [Reason:] This is an inverter, in which the transistor in the circuit is switched between cut off and saturation. The load, for example, can be a motor or a light emitting diode or any other electrical device.

4. The current which is helpful for LED to turn on is_________
a) emitter current
b) base current
c) collector current
d) depends on bias

View Answer

Answer: c [Reason:] Depending on the type of load, a collector current is induced that would turn on the motor or LED. The transistor in the circuit is switched between cut off and saturation. The load, for example, can be a motor or a light emitting diode or any other electrical device.

5. Which of the following statements is true?
a) Solid state switches are applications for an AC output
b) LED’s can be driven by transistor logics
c) Only NPN transistor can be used as a switch
d) Transistor operates as a switch only in active region

View Answer

Answer: b [Reason:] Output devices like LED’s only require a few milliamps at logic level DC voltages and can therefore be driven directly by the output of a logic gate. However, high power devices such as motors or lamps require more power than that supplied by an ordinary logic gate so transistor switches are used.

6. The base emitter voltage in a cut off region is_________
a) greater than 0.7V
b) equal to 0.7V
c) less than 0.7V
d) cannot be predicted

View Answer

Answer: c [Reason:] From the cut off characteristics, the base emitter voltage (VBE) in a cut off region is less than 0.7V. The cut off region can be considered as ‘off mode’. Here, VBE > 0.7 and IC=0. For a PNP transistor, the emitter potential must be negative with respect to the base.

7. In saturation region, the depletion layer_________
a) increases linearly with carrier concentration
b) decreases linearly with carrier concentration
c) increases by increasing the emitter current
d) decreases by decreasing the emitter voltage drop

View Answer

Answer: d [Reason:] Here, the transistor will be biased so that maximum amount of base current is applied, resulting in maximum collector current resulting in minimum emitter voltage drop which results in depletion layer as small as possible and maximum current flows through the transistor.

8. The base emitter voltage in a saturation region is_________
a) greater than 0.7V
b) equal to 0.7V
c) less than 0.7V
d) cannot be predicted

View Answer

Answer: d [Reason:] From the saturation mode characteristics, the transistor acts as a single pole single throw solid state switch. A zero collector current flows. With a positive signal applied to the base of transistor it turns on like a closed switch.

9. The switching of power with a PNP transistor is called_________
a) sourcing current
b) sinking current
c) forward sourcing
d) reverse sinking

View Answer

Answer: a [Reason:] Sometimes DC current gain of a bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors is used. The load is connected to ground and the transistor switches the power to it.

10. The switching of power with a NPN transistor is called_________
a) sourcing current
b) sinking current
c) forward sourcing
d) reverse sinking

View Answer

Answer: b [Reason:] Sometimes DC current gain of a bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors is used. The load is connected to supply and the transistor switches the power to it.

Set 5

1. The collector current will not reach the steady state value instantaneously because of_________
a) stray capacitances
b) resistances
c) input blocking capacitances
d) coupling capacitance

View Answer

Answer: a [Reason:] When a pulse is given, the collector current will not reach the steady state value instantaneously because of stray capacitances. The charging and discharging of capacitance makes the current to reach a steady state value after a given time constant.

2. For the BJT, β=∞, VBEon=0.7V VCEsat=0.7V. The switch is initially closed. At t=0, it is opened. At which time the BJT leaves the active region?
electronic-devices-circuits-questions-answers-transistor-switching-times-q2
a) 20ms
b) 50ms
c) 60ms
d) 70ms

View Answer

Answer: b [Reason:] At t < 0, the BJT is OFF in cut off region. IB=0 as β=∞, so IC=IE. When t > 0, switch opens and BJT is ON. The voltage across capacitor increases. From the input loop, -5-VBE-I(4.3K)+10=0 and gives I=1mA. IC1=1-0.5=0.5mA. VC1=0.7+4.3+10=-5V. IC1=C1dVC1/dt. From this equation, we get t=50ms.

3. The technique used to quickly switch off a transistor is by_________
a) reverse biasing its emitter to collector junction
b) reverse biasing its base to collector junction
c) reverse biasing its base to emitter junction
d) reverse biasing any junction

View Answer

Answer: c [Reason:] The technique used to quickly switch off a transistor is by reverse biasing its base to collector junction. It is demonstrated in a high voltage switching circuit. The advantage of this circuit is that it is not necessary to have high voltage control signal.

4. The disadvantage of using the method of reverse biasing base emitter junction is_________
a) high voltage control signal
b) low voltage control signal
c) output swing
d) incomplete switching of output

View Answer

Answer: d [Reason:] This method is used to quickly switch off a transistor is by reverse biasing its base to collector junction. It is demonstrated in a high voltage switching circuit. The disadvantage of using the method of reverse biasing base emitter junction is that the output does not switch completely to GND due to forward voltage drop of the diode.

5. Which of the following circuits helps in the applications of switching times?
a)electronic-devices-circuits-questions-answers-transistor-switching-times-q5a
b)electronic-devices-circuits-questions-answers-transistor-switching-times-q5b
c)electronic-devices-circuits-questions-answers-transistor-switching-times-q5c
d)electronic-devices-circuits-questions-answers-transistor-switching-times-q5d

View Answer

Answer: b [Reason:] This is an inverter, in which the transistor in the circuit is switched between cut off and saturation. The load, for example, can be a motor or a light emitting diode or any other electrical device.

6. Which of the following helps in reducing the switching time of a transistor?
a) a resistor connected from base to ground
b) a resistor connected from emitter to ground
c) a capacitor connected from base to ground
d) a capacitor connected from emitter to ground

View Answer

Answer: a [Reason:] Connecting a resistor connected from base of a transistor to ground/negative voltage helps in reducing the switching the switching time of the transistor. When transistor saturate, there is stored charge in the base that must be removed before it turns off.

7. The time taken for a transistor to turn from saturation to cut off is _________
a) inversely proportional to charge carriers
b) directly proportional to charge carriers
c) charging time of the capacitor
d) discharging time of the capacitor

View Answer

Answer: b [Reason:] When sufficient charge carriers exist, the transistor goes into saturation. When the switch is turned off, in order to go into cut off, the charge carriers in the base region need to leave. The longer it takes to leave, the longer it takes for a transistor to turn from saturation to cut off.

8. The switching of power with a PNP transistor is called _________
a) sourcing current
b) sinking current
c) forward sourcing
d) reverse sinking

View Answer

Answer: a [Reason:] Sometimes DC current gain of a bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors is used. The load is connected to ground and the transistor switches the power to it.

9. The base emitter voltage in a cut off region is _________
a) greater than 0.7V
b) equal to 0.7V
c) less than 0.7V
d) cannot be predicted

View Answer

Answer: c [Reason:] From the cut off characteristics, the base emitter voltage (VBE) in a cut off region is less than 0.7V. The cut off region can be considered as ‘off mode’. Here, VBE < 0.7 and IC=0. For a PNP transistor, the emitter potential must be negative with respect to the base.

10. Switching speed of P+ junction depends on _________
a) Mobility of minority carriers in P junction
b) Life time of minority carriers in P junction
c) Mobility of majority carriers in N junction
d) Life time of minority carriers in N junction

View Answer

Answer: d [Reason:] Switching leads to move holes in P region to N region as minority carriers. Removal of this accumulation determines switching speed. P+ regards to a diode in which the p type is doped excessively.

.woocommerce-message { background-color: #98C391 !important; }