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# Multiple choice question for engineering

## Set 1

1. When a load is matched to a transmission line, the condition that is satisfied when matched is:
a) ZL=Z0
b) ZL=2Z00
c) ZL=Zin
d) ZL=2Zin

Answer: A [Reason:] In order to deliver the maximum power from source to load, the transmission line has to be matched to the load. Hence for the transmission line to be matched to the load, the condition to be satisfied is ZL=Z0.

2. When a load ZL is matched to a line, the value of standing wave ratio is:
a) 1
b) 0
c) infinity
d) insufficient data to calculate SWR

Answer: A [Reason:] When the load is matched to the transmission line, they are said to be matched. Hence standing waves exist on the transmission line. Hence SWR is 1.

3. The value of reflection co efficient when a transmission line is matched to the load is:
a) 1
b) 0
c) 0.707
d) cannot be determined

Answer: B [Reason:] When the transmission line and the load are matched, no reflections occur in the transmission line and hence no voltage wave is reflected back. Hence, the reflection co-efficient for a matched line is 0.

4. The value of transmission co efficient when a transmission line is matched to a load is:
a) 1
b) 0
c) 0.5
d) 0.707

Answer: A [Reason:] Transmission co-efficient is defined as the ratio of the incident power to transmitted power at the load end. When the transmission line is matched, the incident power is completely transmitted. Hence, transmission co-efficient is 1.

5. The expression for power delivered to a load , when a line is matched and supplied with a source of Vg with generator impedance Rg +jXg is:
a) 0.5*Vg2/Rg
b) 0.5*Vg2Rg/4(Rg2+ Xg2)
c) Rg/4(Rg2+ Xg2)
d) generator impedance does not cause any losses

Answer: B [Reason:] Due to the generator impedance, there will be some power dissipated and hence the total source power is not transmitted. Hence that power dissipated due to generator impedance is also removed from the total power delivered.

6. If a transmission line is exited from a source of 4V at 1.2GHz frequency with a generator impedance of 4+j3 with a characteristic impedance of the transmission line 50Ω,then the power delivered to the load is:
a) 0.1 watt
b) 0.9 watt
c) 0.8 watt
d) 1watt

Answer: C [Reason:] The expression for total power delivered given the generator impedance is 0.5*Vg2Rg/4(Rg2+ Xg2). Substituting the given values in the above equation, the total power delivered is 0.8 watt.

7. If the generator impedance of a source connected to a transmission line is 50+j100Ω, then for conjugate matching to occur , the input impedance must be:
a) 50-j100 Ω
b) 50+100 Ω
c) 50 Ω
d) one of the mentioned

Answer: A [Reason:] The condition for conjugate matching is Zin=*Zg, where Zin is the input impedance of the transmission line and Zg is the generator impedance. For conjugate matching, taking the conjugate of the given impedance, the input impedance must be 50-100j Ω.

8. After conjugate impedance matching the input impedance used for matching after normalization was 1+j with the characteristic impedance of the transmission line being 100Ω, then the generator impedance must have been:
a) 100+100j
b) 1+j
c) 100-100j
d) 1-j

Answer: C [Reason:] After conjugate matching the input impedance of a transmission line after normalization is 1+j. hence the generator impedance will be the conjugate, that is 1-j. multiplying with the characteristic impedance, we get 100-100j.

9. For a matched transmission line with a generator impedance of 50Ω and the source being 4V,1GHZ,then the maximum power delivered to the line is:
a) 0.4 watt
b) 0.04 watt
c) 0.5 watt
d) no power is delivered

Answer: B [Reason:] The maximum power delivered to the load given the generator impedance is 0.5*Vg2Rg/4(Rg2+ Xg2). Substituting in the above equation the given values, power delivered is 0.04 watt.

10. If the power delivered to a load is 0.04w, then the normalized generator impedance if the source use is 4V at 2GHz and the generator impedance is real and characteristic impedance of the transmission line is 50Ω is:
a) 1 Ω
b) 1+j Ω
c) 1-j Ω
d) 50 Ω

Answer: A [Reason:] The maximum power delivered to the load given the generator impedance is 0.5*Vg2Rg/4(Rg2+ Xg2). Rearranging the equation and substituting the given value, Rg is 50Ω. To normalize, dividing the impedance by characteristic impedance, the impedance is 1 Ω.

## Set 2

1. Silicon and germanium are called ___________ semiconductors.
a) direct gap
b) indirect gap
c) band gap
d) indirect band gap

Answer: b [Reason:] The forbidden energy gap for silicon and germanium are respectively 1.21 eV in Si and 0.79 eV in germanium. Silicon and germanium are called indirect gap semiconductors because the bottom of the conduction band does not lie directly above the top of the valence band.

2. GaAs is used in the fabrication of GUNN diodes because:
a) GaAs is cost effective
b) It less temperature sensitive
c) it has low conduction band electrons
d) less forbidden energy gap

Answer: d [Reason:] In GaAs, the conduction band lies directly above the top of the valence band. The lowest energy conduction band in GaAs is called as primary valley. GaAs consists of six secondary valleys. The bottom of one of the secondary valley is at an energy difference of 0.35 eV with the bottom of the primary valley in conduction band.

3. In a GaAs n-type specimen, the current generated is constant irrespective of the electric filed applied to the specimen.
a) true
b) false

Answer: b [Reason:] In a GaAs n-type specimen, when the electric field applied reaches a threshold value of Eth, the current in the specimen becomes suddenly oscillatory and with respect to time and these oscillations are in the microwave frequency range. This effect is called Gunn Effect.

4. When the electric field applied to GaAs specimen is less than the threshold electric field, the current in the material:
a) increases linearly
b) decreases linearly
c) increases exponentially
d) decreases exponentially

Answer: a [Reason:] When the electric field applied is less than the threshold value of electric field, the electrons jump from the valence band to the primary valley of the conduction band and current increases linearly with electric field.

5. When the applied electric field exceeds the threshold value, electrons absorb more energy from the field and become:
a) hot electrons
b) cold electrons
c) emission electrons
d) none of the mentioned

Answer: a [Reason:] When the applied electric field exceeds the threshold value, electrons absorb more energy from the field and become hot electrons. These electrons jump into the lowest secondary valley in the conduction band. When the electrons become hot, their mobility reduces.

6. GaAs is used in fabricating Gunn diode. Gunn diode is:
a) bulk device
b) sliced device
c) made of different type of semiconductor layers
d) none of the mentioned

Answer: a [Reason:] A GUNN diode is a bulk device, that is, it does not contain any junction but it is a slice of n-type GaAs. P-type GaAs does not exhibit Gunn Effect. Hence it is a reversible and can be operated in both directions.

7. The electrodes of a Gunn diode are made of:
a) molybdenum
b) GaAs
c) gold
d) copper

Answer: a [Reason:] Gunn diode is grown epitaxially onto a gold or copper plated molybdenum electrode, out of gallium arsenide doped with silicon, tellurium or selenium to make it n-type.

8. When either a voltage or current is applied to the terminals of bulk solid state compound GaAs, a differential ______ is developed in that bulk device.
a) negative resistance
b) positive resistance
c) negative voltage
d) none of the mentioned

Answer: a [Reason:] When either a voltage or current is applied to the terminals of a sample of bulk solid state compound formed by group 5 and 3 elements of periodic table, a differential resistance is developed in the bulk device. This fundamental concept is called RWH theory.

9. The number of modes of operation for n type GaAs is:
a) two
b) three
c) four
d) five

Answer: c [Reason:] n-type GaAs used for fabricating Gunn diode has four modes of operation. They are Gunn oscillation mode, limited space charge accumulation mode, and stable amplification mode bias circuit oscillation mode.

10. The free electron concentration in N-type GaAs is controlled by:
a) effective doping
b) bias voltage
c) drive current
d) none of the mentioned

Answer: a [Reason:] The free electron concentration in n-type GaAs is controlled through effective doping so that they range from 1014 to 1017 per cc at room temperature. The typical specimen of n-type GaAs has the dimensions 150 µm by 150 µm.

11. The modes of operation of a Gunn diode are illustrated in a plot of voltage applied to the Gunn diode v/s frequency of operation of Gunn diode.
a) true
b) false

Answer: b [Reason:] A graph of plot of product of frequency and the length of the device plotted along y-axis versus the product of doping concentration and length along X- axis. These are the parameters on which the four modes of operation of Gunn diode are explained.

12. The mode of operation in which the Gunn diode is not stable is:
a) Gunn oscillation mode
b) limited space charge accumulation mode
c) stable amplification mode
d) bias circuit oscillation mode

Answer: a [Reason:] In Gunn oscillation mode, the device is unstable due to the formation of accumulation layer and field domain. This high field domain moves from cathode to anode.

13. The frequency of oscillation in Gunn diode is given by:
a) vdom/ Leff
b) Leff/ Vdom
c) Leff/ WVdom
d) none of the mentioned

Answer: a [Reason:] In Gunn oscillation mode, the frequency of oscillation is given by vdom/ Leff, where vdom is the domain velocity, Leff is effective length that the domain moves from the time it is formed until the time a new domain is formed.

14. In Gunn diode oscillator, the Gunn diode is inserted into a waveguide cavity formed by a short circuit termination at one end
a) true
b) false

Answer: a [Reason:] The Gunn diode is mounted at the centre of the broad wall of a shorted waveguide since for the dominant TE10 mode; the electric field is maximum at the centre.

15. In a Gunn diode oscillator, the electron drift velocity was found to be 107 cm/second and the effective length is 20 microns, then the intrinsic frequency is:
a) 5 GHz
b) 6 GHz
c) 4 GHz
d) 2 GHz

Answer: a [Reason:] The intrinsic frequency for a Gunn oscillator is given by Vd/L. Here VD is the drift velocity and L is the effective length. Substituting the given values in the above equation, intrinsic frequency is 5 GHz.

## Set 3

1. BJTs are bipolar junction transistors. The name bipolar is given because:
a) they are made of n type and p type semiconductor
b) they have holes as charge carriers
c) they have electrons as charge carriers
d) none of the mentioned

Answer: d [Reason:] In bipolar junction transistors, both electrons and holes are charge carriers and both of them together constitute current flow in transistors. Since both carriers result in current, they are called bipolar devices.

2. BJTs are suitable for RF applications because:
a) good performance in terms of frequency
b) power capacity
c) noise characteristics
d) all of the mentioned

Answer: d [Reason:] BJTs designed to operate at certain frequency can be operated over a wide range of frequencies hence offering higher bandwidth. Also they have high power handling capacity and very good noise characteristics.

3. Bipolar junction transistors have _______ 1/f characteristics hence making them suitable for oscillators.
a) high
b) low
c) constant
d) decreasing exponential

Answer: b [Reason:] Bipolar junction transistors have very low 1/f noise. 1/f noise is nothing but thermal noise. Hence BJTs are not very temperature and can be used at high temperature applications as well.

4. Silicon junction transistors are used as amplifiers at frequency range of about:
a) 5-10 MHz
b) 2-10 GHz
c) 40-50 MHz
d) 12-45 GHz

Answer: b [Reason:] Silicon junction transistors have unconditional stability as a two port device at a wide range of frequencies. They are more suitable as amplifiers in the frequency range of about 2-10 GHz. Junction transistors when used as oscillators are used in the frequency range of about 20 GHz.

5. At frequency range of about 2-4 GHz, BJTs are preferred over FETs.
a) true
b) false

Answer: a [Reason:] At about 2-4 GHz frequency range, BJTs have higher gain as compared to FETs, power capacity is high and biasing can be done using a single power supply. Because of these advantages, BJTs are preferred over FETs.

6. One major disadvantage of BJTs over FETs is that:
a) they have low gain
b) they do not have a good noise figure
c) low bandwidth
d) none of the mentioned

Answer: b [Reason:] Bipolar junction transistors are subject to shot noise as well as thermal noise effects, so their noise figure is not as good as that of FET. Noise figure can pose serious problems at high operating frequencies.

7. Bipolar junction transistor is a ________ driven device.
a) current
b) voltage
c) power
d) none of the mentioned

Answer: a [Reason:] Bipolar junction transistor is a current driven device where the collector output current directly depends on the input base current. Base current modulates the collector current of the device.

8. The upper frequency limit of BJT depends on the:
a) collector length in the transistor
b) base length
c) emitter length
d) driving voltage

Answer: b [Reason:] The upper operating frequency limit of a BJT depends on the base length of the transistor. Typical base length of a transistor is in the range of a 0.1 µm. the operating frequency is a few GHz for this base length.

9. In the hybrid –π model of a BJT, the capacitance Cc between the base and collector in the hybrid –π model is ignored.
a) true
b) false

Answer: a [Reason:] The capacitance Cc in the hybrid –π model is small and can be neglected. This has the effect of making the S12 parameter of the BJT equal to zero, implying that the power flows only in one direction through the device.

10. with the increase in the operating frequency of a BJT, the S22 parameter of the transistor:
a) increases
b) decreases
c) remains constant
d) none of the mentioned

Answer: b [Reason:] With increase in the operating frequency of the transistor, S22 parameter of the transistor decreases. S22 parameter signifies the voltage reflected back to port 2. S22 parameter has a value of about 0.93 at 0.1 GHz frequency and 0.33 at 4 GHz frequency.

## Set 4

1. The material used to fabricate IMPATT diodes is GaAs since they have the highest efficiency in all aspects.
a) true
b) false

Answer: b [Reason:] IMPATT diodes can be fabricated using silicon, germanium, GaAs or indium phosphide. Out of these materials, GaAs have highest efficiency, low noise and high operating frequencies. But GaAs has a major disadvantage of complex fabrication process and higher cost. So, GaAs are not preferred over silicon and germanium.

2. When a reverse bias voltage exceeding the breakdown voltage is applied to an IMPATT diode, it results in:
a) avalanche multiplication
b) break down of depletion region
c) high reverse saturation current
d) none of the mentioned

Answer: a [Reason:] A reverse bias voltage exceeding the breakdown voltage is applied to an IMPATT diode, a high electric field appears across the n+ p junction. This high field imparts sufficient energy to the holes and also to valence electrons to raise themselves to the conduction band. This results in avalanche multiplication of electron hole pair.

3. To prevent an IMPATT diode from burning, a constant bias source is used to maintain _______ at safe limit.
a) average current
b) average voltage
c) average bias voltage
d) average resistance

Answer: a [Reason:] Avalanche multiplication is a cumulative process resulting in rapid increase of carrier density. To prevent the diode from burning due to this increased carrier density, a constant bias source is used to maintain average current at safe limit.

4. The number of semiconductor layers in IMPATT diode is:
a) two
b) three
c) four
d) none of the mentioned

Answer: c [Reason:] IMPATT diode consists of 4 layers according to the construction. It consists of a p+ region and n+ layers at the two ends. In between these layers, a p type layer and an intrinsic region is sandwiched.

5. The resonant frequency of an IMPATT diode is given by:
a) Vd/2l
b) Vd/l
c) Vd/2πl
d) Vdd/4πl

Answer: a [Reason:] The resonant frequency of an IMPATT diode is given by the expression Vd/2l. Here VD is the carrier drift velocity; L is the length of the intrinsic region in the IMPATT diode.

6. If the length of the intrinsic region in IMPATT diode is 2 µm and the carrier drift velocity are 107 cm/s, then the drift time of the carrier is:
a) 10-11 seconds
b) 2×10-11 seconds
c) 2.5×10-11 seconds
d) none of the mentioned

Answer: b [Reason:] The drift time of the carrier is defined as the ratio of length of the intrinsic region to the carrier drift velocity. Substituting the given values in this relation, the drift time of the carrier is 2×10-11 seconds.

7. If the length of the intrinsic region in IMPATT diode is 2 µm and the carrier drift velocity are 107 cm/s, then the nominal frequency of the diode is:
a) 12 GHz
b) 25 GHz
c) 30 GHz
d) 24 GHz

Answer: b [Reason:] Nominal frequency is defined as the ratio of the carrier drift velocity to twice the length of the intrinsic region. Substituting the given values in the above equation, the nominal frequency is 25 GHz.

8. IMPATT diodes employ impact ionization technique which is a noisy mechanism of generating charge carriers.
a) true
b) false

Answer: a [Reason:] IMPATT devices employ impact ionization techniques which is too noisy. Hence in order to achieve low noise figure, impact ionization is avoided in BARITT diodes. The minority injection is provided by punch through of the intermediate region.

9. An essential requirement for the BARITT diode is that the intermediate drift region be completely filled to cause the punch through to occur.
a) true
b) false

Answer: b [Reason:] An essential requirement for the BARITT diode is that the intermediate drift region be completely filled to cause the punch through to the emitter-base junction without causing avalanche breakdown of the base collector junction.

10. If the RMS peak current in an IMPATT diode is 700 mA and if DC input power is 6 watt, with the load resistance being equal to 2.5 Ω, the efficiency of the diode is:
a) 10.1 %
b) 10.21 %
c) 12 %
d) 15.2 %

Answer: b [Reason:] Efficiency of IMPATT diode is defined as the ratio of output RMS power to the input DC power. Calculating the RMS output power from the given RMS current and substituting in the equation of efficiency, the efficiency is 10.21%.

11. If the critical field in a Gunn diode oscillator is 3.2 KV/cm and effective length is 20 microns, then the critical voltage is:
a) 3.2 V
b) 6.4 V
c) 2.4 V
d) 6.5 V

Answer: b [Reason:] Critical voltage of a Gunn diode oscillator is given by the expression lEc where l is the effective length and Ec is the critical field. Substituting the given values in the above equation, critical voltage is 6.4 volts.

## Set 5

1. The major disadvantage of coupled line coupler is:
a) complex construction
b) low power gain
c) higher loss
d) none of the mentioned

Answer: d [Reason:] Coupling in a coupled line coupler is too loose to achieve coupling factors of 3 or 6 dB. Since higher efficiency is not achievable, this a major disadvantage. One method of improving coupling is designing in such a way that the fringing field contributes to coupling.

2. Lange couplers consist of four parallel lines that are ______ coupled.
a) tightly
b) loosely
c) partially
d) none of the mentioned

Answer: a [Reason:] Lange couplers consist of four parallel lines. These parallel lines are tightly coupled to achieve 3 dB coupling ratio. Achieving 3dB coupling implies that 70% of the applied power is coupled.

3. Lange coupler is a type of Quadrature hybrid.
a) true
b) false

Answer: a [Reason:] Lange coupler ha two output ports. The output voltage measured at these ports is 900 out of phase with one another. Hence, they are called a type of Quadrature hybrid.

4. A major disadvantage of Lange coupler is:
a) high power loss
b) complex construction
c) low gain
d) none of the mentioned

Answer: b [Reason:] The main disadvantage of Lange coupler is probably practical, as the lines are very narrow and close together, and the required bonding wires across the lines increases complexity.

5. For even mode excitation of a Lange coupler, all the four lines of the Lange coupler are at:
a) equal potential
b) zero potential
c) unequal potential
d) negative potential

Answer: a [Reason:] during the analysis of Lange coupler, when they are excited in the even mode, all the four conductors of the coupler are at equal potential. Hence the capacitance Cm has no effect and need not be considered for further analysis.

6. Even mode characteristic impedance of a Lange coupler:
a) (Vp.Cep)-1
b) Vp.Cep
c) (Vp.Lep)-1
d) Vp.Lep

Answer: a [Reason:] The even mode characteristic impedance of a Lange coupler is given by (Vp.Cep)-1. Here Vpp is the phase velocity and Cep is the capacitance of any of the four conductors of the Lange coupler measured during even mode of analysis.

7. In a Lange coupler having 4 lines, the adjacent lines are connected so as to act as two conductors in parallel.
a) true
b) false

Answer: b [Reason:] In a Lange coupler consisting of 4 lines, the alternating lines are connected together. This simplifies the computation effort for finding equivalent capacitance and the construction is simpler. Also ports are easily distinguished and helps in flow of power in required direction.

8. For a 4 wire Lange coupler, if the odd mode and even mode capacitances are 12pF and 9pF respectively, then the equivalent capacitance in terms of two wired coupled line in even mode is:
a) 15.1 pF
b) 16.7 pF
c) 12 pF
d) 9 pF

Answer: b [Reason:] Equivalent capacitance in terms of two wired coupled model is Ce (3Ce+C0)/(Ce+C0), Where Ce and C0 are the equivalent capacitances in even mode and odd mode for 4 wire model. Substituting the given values in the above expression, the equivalent capacitance is 16.7 pF.

9. For a 4 wire Lange coupler, if the odd mode and even mode capacitances are 12pF and 9pF respectively, then the equivalent capacitance in terms of two wired coupled line in odd mode is:
a) 18.56 pF
b) 25.71 pF
c) 42 pF
d) 31.6 pF