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# Multiple choice question for engineering

## Set 1

1. The _____________ of a MOSFET is affected by the voltage which is applied to the back contact.
a) Threshold Voltage
b) Output Voltage
c) Both threshold and output voltage
d) Neither of the threshold nor the output voltage

Answer: a [Reason:] The voltage difference between the source and the bulk, VBS changes the width of the depletion layer and therefore also the voltage across the oxide due to the change of the charge in the depletion region. This results in a difference in threshold voltage which equals the difference in charge in the depletion region divided by the oxide capacitance.

2. The variation of the threshold voltage with the applied bulk-to-source voltage is typically observed by plotting the _________________ as a function of the source-to-drain voltage.
a) drain current
b) square root of the drain current
c) square of the drain current
d) natural logarithm of the drain current

Answer: b [Reason:] The change in threshold current is directly proportional to the square root of the drain current. For further assistance check the mathematical expression for the same.

3. The SI units of the body effect parameter is
a) Volt
b) Volt X Volt
c) √Volt
d) It has no units

Answer: c [Reason:] Vt = Vt0 + k[√(Vsb + 2φf) – √2φf]. In this expression k is the body effect parameter hence its units can be determined.

4. An NMOS transistor has Vt0 = 0.8 V, 2 φf = 0.7 V, and γ = 0.4 V1/2. Find Vt when VSB = 3 V.
a) 0.12 V
b) 1.23 V
c) 2.34 V
d) 3.45 V

Answer: b [Reason:] Vt = Vt0 + k[√(Vsb + 2φf) – √2φf]. use this expression to obtain the desired result.

5. The threshold voltage is
a) Increases on increasing temperature
b) May increase or decrease on increasing temperature depending upon other factors
c) Temperature independent
d) Decreases on increasing temperature

Answer: d [Reason:] The threshold voltage depends only on the temperature and it decreases by roughly 2 mV for every degree Celsius increase in the temperature.

6. As the voltage on the drain is increased, a value is reached at which the pn junction between the drain region and substrate suffers avalanche breakdown known as
a) Weak avalanche
b) Strong avalanche
c) Weak storm
d) Punch-through

Answer: a [Reason:] As the voltage on the drain is increased, a value is reached at which the pn junction between the drain region and substrate suffers avalanche breakdown. This breakdown usually occurs at voltages of 20 V to 150 V and results in a somewhat rapid increase in current (known as a weak avalanche).

7. A breakdown effect that occurs in modern devices at low voltages (of around 20 V) is
a) Weak avalanche
b) Strong avalanche
c) Weak storm
d) Punch-through

Answer: d [Reason:] Punch-through occurs in devices with relatively short channels when the drain voltage is increased to the point that the depletion region surrounding the drain region extends through the channel to the source. The drain current then increases rapidly. Normally, punch-through does not result in permanent damage to the device.

8. At ______________ the drain current is no longer related to the Vgs by square law relationship.
a) When the temperature is high (around 700 Celsius)
b) When temperature is very low (around -50 Celsius)
c) Velocity saturation
d) None of the mentioned

Answer: c [Reason:] At velocity saturation the current depends linearly on the Vgs.

9. In MOSFETs a breakdown may occur at around 30 V. This is due to
a) Velocity saturation
b) Breakdown of the gate diode
c) Sudden decrease in the depletion region
d) Fall of the threshold voltage due to impurities

Answer: b [Reason:] The breakdown of the oxide at the gate may occur when the voltage is around 30 V. This may also permanently damage the device.

10. Which of the below issues may not be experienced when using MOSFETs?
a) Weak avalanche
b) Velocity saturation
c) Punch-through
d) All of the mentioned

Answer: d [Reason:] All of the mentioned are some of the common issues that one may face while dealing with MOSFETs.

## Set 2

1. A zener diode works on the principle of_________
a) tunneling of charge carriers across the junction
b) thermionic emission
c) diffusion of charge carriers across the junction
d) hopping of charge carriers across the junction

Answer: a [Reason:] Due to zener effect in reverse bias under high electric field strength, electron quantum tunneling occurs. It’s a mechanical effect in which a tunneling current occurs through a barrier. They usually cannot move through that barrier.

2. Which of the following are true about a zener diode?
1) it allows current flow in reverse direction also
2) it’s used as a shunt regulator
3) it operates in forward bias condition
a) 3 only
b) 1 and 2
c) 2 and 3
d) 2 only

Answer: b [Reason:] The operation of a zener diode is made in reverse bias when breakdown occurs. So, it allows currnt in reverse direction. The most important application of a zener diode is voltage or shunt regulator.

3. When the voltage across the zener diode increases_________
a) temperature remains constant and crystal ions vibrate with large amplitudes
b) temperature increases and crystal ions vibrate with large amplitudes
c) temperature remains constant and crystal ions vibrate with smaller amplitudes
d) temperature decreases and crystal ions vibrate with large amplitudes

Answer: b [Reason:] When voltage is increased, the tunnelling at reverse bias increases. The voltage rises temperature. The crystal ions with greater thermal energy tend to vibrate with larger amplitudes.

4. For the zener diode shown in the figure, the zener voltage at knee is 7V, the knee current is negligible and the zener dynamic resistance is 10Ω. If the input voltage (Vi) ranges from 10 to 16 volts, the output voltage (Vo) ranges from? a) 7 to 7.29V
b) 6 to 7V
c) 7.14 to 7.43V
d) 7.2 to 8V

Answer: c [Reason:] If i is the current flowing, then V0=10i+7 i=(VI-7)/210. By substituting, if VI=10V then i=1/70 and V0=(1/7)+7=7.14V if VI =16V then i=3/70 and V0=(3/7)+7=7.43V.

5. In the circuit below, the knee current of ideal zener diode is 10mA. To maintain 5V across the RL, the minimum value of RL is? a) 120
b) 125
c) 250
d) 100

Answer: b [Reason:] Here, IKNEE=10mA, VZ=5V. I=IL+IZ. I= (10-5)/100=50mA Now, 50=10+ILMAX . ILMAX=40mA. RLMIN=5/40mA=125 Ω.

6. The zener diode in the circuit has a zener voltage of 5.8V and knee current of 0.5mA. The maximum load current drawn with proper function over input voltage range between 20 and 30V is? a) 23.7mA
b) 20mA
c) 26mA
d) 48.3mA

Answer: a [Reason:] Here, I1MAX=IZMIN+ILMAX. IZMIN =0.5mA, I1MAX =(V1MAX-VZ )/RS . Putting the values we get , I1MAX =24.2mA. So, 24.2-0.5=23.7mA.

7. In the given limiter circuit, an input voltage Vi=10sin100πt is applied. Assume that the diode drop is 0.7V when it’s forward biased. The zener breakdown voltage is 6.8V.The maximum and minimum values of outputs voltage are _______ a) 6.1V,-0.7V
b) 0.7V,-7.5V
c) 7.5V,-0.7V
d) 7.5V,-7.5V

Answer: c [Reason:] With VI= 10V when maximum, D1 is forward biased, D2 is reverse biased. Zener is in breakdown region. VOMAX=sum of breakdown voltage and diode drop=6.8+0.7=7.5V. VOMIN=negative of voltage drop=-0.7V. There will be no breakdown voltage here.

8. The 6V Zener diode shown has zener resistance and a knee current of 5mA. The minimum value of R so that the voltage does not drop below 6V is? a) 1.2Ω
b) 80 Ω
c) 50 Ω
d) 70 Ω

Answer: b [Reason:] Here, Vz =6V, IZMIN=5mA.IS=IZMIN+ILMAX. 80=5+ILMAX . ILMAX=75Ma.RLMIN=VI/ILMAX=6/75mA =80 Ω.

9. Avalanche breakdown in zener diode is ______
a) electric current multiplication takes place
b) phenomenon of voltage multiplication takes place
c) electrons are decelerated for a period of time
d) sudden rise in voltage takes place.

Answer: a [Reason:] The carriers in transition region are accelerated by electric field to energies. That energies are sufficient to create electron current multiplication. A single carrier that is energized will collide with another by gaining energy. Thus an avalanche multiplication takes place.

10. The zener diode is heavily doped because______
a) to have low breakdown voltage
b) to have high breakdown voltage
c) to have high current variations
d) to maintain perfect quiescent point

Answer: a [Reason:] The value of reverse breakdown voltage at which zener breakdown occurs is controlled by amount of doping. If the amount of doping is high, the value of voltage at which breakdown occurs will decrease. Better doping gives a sooner breakdown voltage.

## Set 3

1. DC average current of a bridge full wave rectifier (where Im is the maximum peak current of input).
a) 2Im
b) Im
c) Im/2
d) 1.414Im

Answer: b [Reason:] Average DC current of half wave rectifier is Im. Since output of half wave rectifier contains only one half of the input. The average value is the half of the area of one half cycle of sine wave with peak Im. This is equal to Im.

2. DC power output of bridge full wave rectifier is equal to (Im is the peak current and RL is the load resistance).
a) 2 Im2RL
b) 4 Im2RL
c) Im2RL
d) Im2 RL/2

Answer: b [Reason:] DC output power is the power output of the rectifier. We know VDC for a bridge rectifier is 2Vm and IDC for a bridge rectifier is 2Im. We also know VDC=IDC/RL. Hence output power is 4Im2RL.

3. Ripple factor of bridge full wave rectifier is?
a) 1.414
b) 1.212
c) 0.482
d) 1.321

Answer: c [Reason:] Ripple factor of a rectifier measures the ripples or AC content in the output. It’s obtained by dividing AC rms output with DC output. For full wave bridge rectifier it is 0.482.

4. If input frequency is 50Hz then ripple frequency of bridge full wave rectifier will be equal to_________
a) 200Hz
b) 50Hz
c) 45Hz
d) 100Hz

Answer: d [Reason:] Since in the output of bridge rectifier one half cycle is repeated, the frequency will be twice as that of input frequency. So, f=100Hz.

5. Transformer utilization factor of bridge full wave rectifier _________
a) 0.623
b) 0.812
c) 0.693
d) 0.825

Answer: b [Reason:] Transformer utilization factor is the ratio of AC power delivered to load to the DC power rating. This factor indicates effectiveness of transformer usage by rectifier. For bridge full wave rectifier it’s equal to 0.693.

6. If peak voltage on a bridge full wave rectifier circuit is 5V and diode cut in voltage os 0.7, then the peak inverse voltage on diode will be_________
a) 4.3V
b) 9.3V
c) 8.6V
d) 3.6V

Answer: d [Reason:] PIV is the maximum reverse bias voltage that can be appeared across a diode in the circuit. If PIV rating of diode is less than this value breakdown of diode may occur.. Therefore, PIV rating of diode should be greater than PIV in the circuit, For bridge rectifier PIV is Vm-VD = 5-1.4=3.6.

7. Efficiency of bridge full wave rectifier is_________
a) 81.2%
b) 50%
c) 40.6%
d) 45.33%

Answer: a [Reason:] It’s obtained by taking ratio of DC power output to maximum AC power delivered to load. Efficiency of a rectifier is the effectiveness of rectifier to convert AC to DC. It’s usually expressed inn percentage. For bridge full wave rectifier, it’s 81.2%.

8. In a bridge full wave rectifier, the input sine wave is 40sin100t. The average output voltage is_________
a) 22.73V
b) 16.93V
c) 25.47V
d) 33.23V

Answer: c [Reason:] The equation of sine wave is in the form Emsinωt. Therefore, Em=40. Hence output voltage is 2Em=80V.

9. Number of diodes used in a full wave bridge rectifier is_________
a) 1
b) 2
c) 3
d) 4

Answer: d [Reason:] The model of a bridge rectifier is same as Wein Bridge. It needs 4 resistors. Bridge rectifier needs 4 diodes while centre tap configuration requires only one.

10. In a bridge full wave rectifier, the input sine wave is 250sin100t. The output ripple frequency will be_________
a) 50Hz
b) 200Hz
c) 100Hz
d) 25Hz

Answer: c [Reason:] The equation of sine wave is in the form of Emsinωt. So, ω=100 and frequency (f)=ω/2=50Hz. Since output of bridge rectifier have double the frequency of input, f=100Hz.

## Set 4

1. For NMOS transistor which of the following is not true?
a) The substrate is of p-type semiconductor
b) Inversion layer or induced channel is of n type
c) Threshold voltage is negative
d) None of the mentioned

Answer: c [Reason:] The threshold voltage is positive for NMOS.

2. Process transconductance parameter is directly proportional to
a) Electron mobility only
b) (Electron mobility)-1 only
c) Oxide capacitance only
d) Product of oxide capacitance and electron mobility

Answer: d [Reason:] It is the product of the electronic mobility with the oxide capacitance (F/m2).

3. The SI Units of the Process transconductance Parameter (k’) is
a) V2/A
b) A/V2
c) V/A
d) A/V

Answer: b [Reason:] k’ = μn Cox where μn is electronic mobility (m2/Vs) and Cox is oxide capacitance is (F/m2).

4. Aspect ratio of the MOSFET has the units of
a) No units
b) m
c) m2
d) m-1

Answer: a [Reason:] It is the ratio of the induced channel width (w) to the induced channel length (l).

5. The MOSFET transconductance parameter is the product of
a) Process transconductance and inverse of aspect ratio
b) Inverse of Process transconductance and aspect ratio
c) Inverse of Process transconductance and inverse of aspect ratio
d) Process transconductance and aspect ratio

Answer: d [Reason:] This statement only satisfies the mathematical expression.

6. With the potential difference between the source and the drain kept small (VDS is small), the MOSFET behaves as a resistance whose value varies __________ with the overdrive voltage
a) Linearly
b) Inversely
c) Exponentially
d) Logarithmically

Answer: b [Reason:] For small VDS, resistance r is given by R = 1 / ((μn Cox)(w/l)(VOV)).

7. For a p channel MOSFET which of the following is not true?
a) The source and drain are a p type semiconductor
b) The induced channel is p type region which is induced by applying a positive potential to the gate
c) The substrate is a n type semiconductor
d) None of the mentioned

Answer: b [Reason:] The induced channel is p type region which is induced by applying a negative potential to the gate.

8. When the voltage across the drain and the source (VDS) is increased from a small amount (assuming that the gate voltage, VG with respect to the source is higher than the threshold voltage, Vt), then the width of the induced channel in NMOS (assume that VDS is always small when compared to the Vov)
a) Will remain as was before
b) Will become non uniform and will take a tapered shape with deepest width at the drain
c) Will become non uniform and will take a tapered shape with deepest width at the source
d) Will remain uniform but the width of the channel will increase

Answer: c [Reason:] The voltage across the source will be VOV and the voltage will decrease linearly to VOV – VDS as we reach the drain end. The width of the induced channel is proportional to the voltage.

9. The saturation current of the MOSFET is the value of the current when
a) The voltage between the drain and drain becomes equal to the overdrive voltage
b) The voltage between the drain and drain becomes equal to the threshold voltage
c) The voltage between the drain and drain becomes equal to the voltage applied to the gate
d) The voltage between the drain and drain becomes equal to difference the overdrive voltage and the threshold voltage

Answer: a [Reason:] By definition of the MOSFET saturation current.

10. At channel pinch off
a) The width of the induced channel becomes non linear
b) The width of the induced channel becomes very large (resulting in very large resistance and very low, practically zero, current)
c) width becomes 1/e times the maximum possible width
d) The width of the induced channel becomes zero and the current saturates

Answer: d [Reason:] It is a characteristics of a channel pinch off.

## Set 5

1. In a shunt capacitor filter, the mechanism that helps the removal of ripples is_________
a) The current passing through the capacitor
b) The property of capacitor to store electrical energy
c) The voltage variations produced by shunting the capacitor
d) Uniform charge flow through the rectifier

Answer: b [Reason:] Filtering is frequently done by shunting the load with capacitor. It depends on the fact that a capacitor stores energy when conducting and delivers energy during non conduction. Throughout this process, the ripples are eliminated.

2. The cut-in point of a capacitor filter is_________
a) The instant at which the conduction starts
b) The instant at which the conduction stops
c) The time after which the output is not filtered
d) The time during which the output is perfectly filtered

Answer: a [Reason:] The capacitor charges when the diode is in ON state and discharges during the OFF state of the diode. The instant at which the conduction starts is called cut-in point. The instant at which the conduction stops is called cut-out point.

3. The rectifier current is a short duration pulses which cause the diode to act as a_________
a) Voltage regulator
b) Mixer
c) Switch
d) Oscillator

Answer: c [Reason:] The diode permits charge to flow in capacitor when the transformer voltage exceeds the capacitor voltage. It disconnects the power source when the transformer voltage falls below that of a capacitor.

4. A half wave rectifier, operated from a 50Hz supply uses a 1000µF capacitance connected in parallel to the load of rectifier. What will be the minimum value of load resistance that can be connected across the capacitor if the ripple% not exceeds 5?
a) 114.87Ω
b) 167.98Ω
c) 115.47Ω
d) 451.35Ω

Answer: c [Reason:] For a half wave filter, ϒ=1/2√3 fCRL=1/2√3*50*10-3*RL RL=103/5√3=115.47Ω.

5. A 100µF capacitor when used as a filter has 15V ac across it with a load resistor of 2.5KΩ. If the filter is the full wave and supply frequency is 50Hz, what is the percentage of ripple frequency in the output?
a) 2.456%
b) 1.154%
c) 3.785%
d) 3.675%

Answer: b [Reason:] For a full wave rectifier, ϒ=1/4√3 fCRL =1/4√3*50*10-3*2.5 =0.01154. So, ripple is 1.154%.

6. A full wave rectifier uses a capacitor filter with 500µF capacitor and provides a load current of 200mA at 8% ripple. Calculate the dc voltage.
a) 15.56V
b) 20.43V
c) 11.98V
d) 14.43V

Answer: d [Reason:] The ripple factor ϒ=IL/ 4√3 fCVDC VDC=200*10-3/ 4√3 *50*500*8 =14.43.

7. The charge (q) lost by the capacitor during the discharge time for shunt capacitor filter.
a) IDC*T
b) IDC/T
c) IDC*2T
d) IDC/2T

Answer: a [Reason:] The ‘T’ is the total non conducting time of capacitor. The charge per unit time will give the current flow.

8. Which of the following are true about capacitor filter?
a) It is also called as capacitor output filter
b) It is electrolytic
c) It is connected in parallel to load
d) It helps in storing the magnetic energy

Answer: b [Reason:] The rectifier may be full wave or half wave. The capacitors are usually electrolytic even though they are large in size.

9. The rms ripple voltage (Vrms) of a shunt filter is_________
a) IDC/2√3
b) IDC2√3
c) IDC/√3
d) IDC√3