Multiple choice question for engineering
Set 1
1. Active devices like diodes and transistors become non-linear at high power levels due to:
a) Instability of transistor
b) Thermal noise
c) Gain compression
d) None of the mentioned
Answer
Answer: c [Reason:] All practical components become non-linear at high power level operations. Active devices like diode and transistor become non-linear at high power levels due to effects such as gain compression or the generation of spurious frequency components due to device non-linearities, but all devices ultimately fail at very high power levels.
2. The ________property of devices like diodes and transistors is responsible for their usage in amplifiers and frequency convertors.
a) Symmetry
b) Non linearity
c) Linearity
d) None of the mentioned
Answer
Answer: b [Reason:] Non linearity is of great utility for desirable functions such as amplification, detection and frequency and frequency conversion. But non-linearities in these devices lead to undesirable effects like gain compression and generation of spurious frequency components.
3. The output response of a non-linear circuit is given by the expression:
a) V0=a0+a1Vi1+a2Vi2+a3Vi3…
b) 0=a0+a1Vi1
c) V0=a2Vi2+a3Vi3
d) V0=a0
Answer
Answer: a [Reason:] The output of non-linear devices in terms of the applied output voltage is given as the Taylor series in terms of the input signal voltage. This expansion is given by V0=a0+a1Vi1+a2Vi2+a3Vi3….
And higher order terms. The constant term leads to rectification converting an AC input signal to DC.
4. As the degree of non-linearity of a transistor or a diode increases, the power gain of the device:
a) Increases
b) Decreases
c) Remains a constant
d) None of the mentioned
Answer
Answer: b [Reason:] Let V0cosωt be the input signal applies to a non linear device. The output of the device is given by V0=a0+a1Vi+a2Vi2+a3Vi3. Considering only up to power of 3 and expanding, we get terms, whose constants of third degree and higher order are negative. This results in fall in gain when the input signal levels are high.
5. The fall in gain of a device due to non-linearities is called:
a) Gain compression
b) Gain saturation
c) Impedance mismatch
d) None of the mentioned
Answer
Answer: a [Reason:] The fall in gain of a device due to non-linearities is called gain compression. Physically, this is usually due to fact that the instantaneous output voltage of an amplifier is limited by the power supply voltage used to bias the active device.
6. 1 dB compression point is defined as the power level for which the output power of the non-linear device is 1 dB
a) True
b) False
Answer
Answer: b [Reason:] 1 dB compression point is defined as the power level for which the output power is decreased by 1dB from the ideal linear characteristic. This power level is represented as P1dB and can be stated in terms of either input power or output power.
7. In a non-linear device, for a single input frequency the output will consist of:
a) Single frequency component
b) Harmonics of the input frequency
c) Constant gain
d) None of the mentioned
Answer
Answer: b [Reason:] Let V0cosωt be the input signal applies to a non linear device. The output of the device is given by V0=a0+a1Vi1+a2Vi2+a3Vi3. Substituting in this equation gives higher frequency components which is associated with higher order terms in the expansion. When the input consists of only one frequency component the higher frequencies are filtered out using band pass filter.
8. When the input frequency consists of more than one closely spaced frequency, it results in:
a) Intermodal distortion
b) Gain compression
c) Signal fading
d) Signal attenuation
Answer
Answer: a [Reason:] When the input frequency consists of more than one frequency, due to the non-linear effect of the device, higher frequency components are generated. When the two different frequencies are close enough to each other, due to the interference of multiple frequency components, intermodal distortion occurs.
9. Third-order intercept point is defined as the point at which the gain of a non linear device is 3 dB less than the maximum gain.
a) True
b) False
Answer
Answer: b [Reason:] Third-order intercept point is defined as hypothetical intersection point at which the first order and third order powers would be equal is called the third-order intercept point.
10. The relation between an intercept points referenced at the input versus the output is given by the relation:
a) OIP3=G (IIP3)
b) (IIP3) = G (OIP3)
c) OIP3=G (IIP3)2
d) None of the mentioned
Answer
Answer: a [Reason:] The relation between an intercept points referenced at the input versus the output is given by the relation OIP3=G (IIP3). Here OPI3 is the output third order intercept point; OIP3 is the input third order intercept point. G is the conversion loss in the device.
Set 2
1. The modes of wave propagation that a parallel plate waveguide can support are:
a) TEM, TE, TM modes
b) TM, TE modes
c) TEM, TM modes
d) TEM, TE modes
Answer
Answer: a [Reason:] Parallel plate waveguide is the simplest type of waveguide that can support TE and TM modes. It can also support a TEM mode since it is formed from two flat conducting plates.
2. The fringe effect can be neglected in a parallel plate waveguide is because of:
a) The dielectric material used
b) Width of the plates is greater than the distance between the plates
c) Material of the parallel plate waveguide used
d) None of the mentioned
Answer
Answer: b [Reason:] The strip width W of the parallel plate waveguide is assumed to be much greater than the separation d, hence the fringe effect or the fringing fields can be neglected.
3. The characteristic impedance of a parallel plate waveguide is given by:
a) η*D/W
b) η*W/D
c) D/ η*W
d) η*√(D/W)
Answer
Answer: a [Reason:] Characteristic impedance of a waveguide is the ratio of voltage and current. Defining voltage and current in the integral form of electric field and magnetic field respectively and solving the characteristic impedance is η*D/W. here η is the intrinsic impedance of the medium in the waveguide, D is the distance between the plates and W is the width of the rectangular plate.
4. If the width of a parallel plate waveguide is 30 mm and the distance between the parallel pates is 5 mm, with an intrinsic impedance of 377Ω, then the characteristic impedance of the wave is:
a) 50 Ω
b) 62.833 Ω
c) 100 Ω
d) None of the mentioned
Answer
Answer: b [Reason:] The expression for intrinsic impedance of a parallel plate waveguide is η*D/W. substituting the given values of intrinsic impedance and distance between plates and width of the plates, intrinsic impedance is 62.833Ω.
5. In TM mode, if the direction of wave propagation is in ‘z’ direction, then:
a) HZ=0
b) EZ=0
c) EY=0
d) HY=0
Answer
Answer: a [Reason:] In TM mode (transverse magnetic), when the wave propagation is along Z direction, magnetic field is absent in Z direction since the fields are transverse in nature. Hence HZ=0.
6. The wave impedance of a TM mode in a parallel plate waveguide is a:
a) Function of frequency
b) Independent of frequency
c) Proportional to square of frequency
d) Inversely proportional to square of frequency
Answer
Answer: a [Reason:] The wave impedance of a parallel plate waveguide in TM mode is β/k which is a function of frequency. The wave impedance is real for f>fC and purely imaginary for f<fC.
7. In a parallel plate waveguide, for a propagating mode, the value of β is:
a) Real
b) Complex
c) Imaginary
d) Cannot be defined
Answer
Answer: a [Reason:] The phase velocity and guide wavelength for a parallel plate waveguide are defined only for propagating modes. Propagating modes are those modes for which β are always positive. Hence β is always real for a parallel plate waveguide.
8. For TM2 mode, if the distance between two parallel plates of a waveguide are 40 mm, then the cut off wavelength for TM2 mode is:
a) 20 mm
b) 80 mm
c) 40 mm
d) 60 mm
Answer
Answer: c [Reason:] The cutoff wavelength of a TMn mode in a parallel plate waveguide is 2d/n, where d is the distance between the plates and n signifies the mode of operation. For the given condition, substituting the given values, cut off wavelength is 40 mm.
9. For a parallel waveguide, the dominant mode for TM propagation is:
a) TM0 mode
b) TM1 mode
c) TM2 mode
d) Dominant mode does not exist
Answer
Answer: b [Reason:] The mode of propagation for which the cutoff wavelength for wave propagation is maximum is called dominant mode. In TM mode of propagation, TM0 mode is similar to TEM mode of propagation. Hence, TM1 mode is the dominant mode.
10. Phase velocity of the plane waves in the two direction in a parallel plate waveguide is:
a) 1/√(μ∈)secant θ
b) 1/√(μ∈)cosecant θ
c) 1/√(μ∈)tangent θ
d) 1/√(μ∈)cosine θ
Answer
Answer: a [Reason:] The phase velocity of each plane wave along its direction of propagation (θ direction) is 1/√(μ∈), Which is the speed of light in the material filling the guide. But, the phase velocity of the plane waves in the z direction is 1/√(μ∈)secant θ.
11. For a parallel plate waveguide, which of the following is true?
a) No real power flow occurs in the ‘z’ direction
b) Power flow occurs in ‘z’ direction
c) No power flow occurs in any direction
d) Wave propagation in z direction is not possible in any mode
Answer
Answer: a [Reason:] The superposition of the two plane waves in Z direction is such that complete cancellation occurs at y=0 and y=d, to satisfy the boundary conditions that EZ=0 at these planes. As f decrease to fc, β approaches 0 so that θ approaches 90⁰. The two plane waves are then bouncing up and down, with no motion in +z direction, and no power flow occurs in the z direction.
12. TE mode is characterized by:
a) EZ=0
b) HZ=0
c) Ex=0
d) Ey=0
Answer
Answer: a [Reason:] In TE mode of wave propagation, the electric field is in transverse direction and hence electric field component in the direction of wave propagation is 0. Hence, EZ=0.
13. If in a parallel plate waveguide, PL=4 mW/m and Pₒ=10 mW/m, then what is the conduction loss?
a) 0.5
b) 0.4
c) 0.1
d) 0.2
Answer
Answer: d [Reason:] Conductor loss of a parallel plate waveguide is given by PL/2Pₒ. Substituting the given values in the above equation, conductor loss is 0.2.
14. If the distance between the two plates of a parallel plate waveguide is 20 mm and is operating in TE₂ mode, then the cut off frequency of TE₂ mode is:
a) 20 MHz
b) 15 GHz
c) 5 GHz
d) None of the mentioned
Answer
Answer: b [Reason:] The cutoff frequency for TEn mode is n/2d√(∈μ) for a parallel plate waveguide. Substituting the given values, the cutoff frequency is 15 GHz.
15. The wave impedance for a non-propagating mode in TE mode is:
a) K/β
b) Imaginary
c) Zero
d) Non-existing
Answer
Answer: b [Reason:] Wave impedance of a parallel plate waveguide for TEN modes is k/β. This expression is valid and real only for propagating modes. For non propagating modes, impedance becomes imaginary.
Set 3
1. A PIN diode consists of ______number of semiconductor layers.
a) Three
b) Two
c) Four
d) One
Answer
Answer: a [Reason:] PIN diode is a p-type, intrinsic, n-type diode consisting of narrow layer of p-type semiconductor and a narrow layer of n-type semiconductor material, with a thicker region of intrinsic or very lightly n doped semiconductor sandwiched between them.
2. The material out of which PIN diode is made is:
a) Silicon
b) Germanium
c) GaAs
d) None of the mentioned
Answer
Answer: a [Reason:] Silicon is the semiconductor normally used because of its power handling capability and it offers high resistivity for the intrinsic region. But depending on the application, these days GaAs is also used in fabricating PIN diodes.
3. The behavior of a PIN diode is entirely different from normal diodes at all frequency of operation.
a) True
b) False
Answer
Answer: b [Reason:] PIN diode acts as a ordinary diode at frequencies up to about 100MHz. at high frequencies it stops to rectify and then acts as a variable resistance.
4. The junction resistance and capacitance of the intrinsic region in a PIN diode are connected______ in the equivalent circuit of PIN diode.
a) Series
b) Parallel
c) Connected across package capacitance
d) None of the mentioned
Answer
Answer: b [Reason:] The junction capacitance Cj and junction resistance Rj of a PIN diode are connected in parallel in the equivalent circuit of a PIN diode. The package resistance and package capacitance are connected in series to these junction parameters.
5. The resistance of the PIN diode with positive bias voltage:
a) Increases
b) Decreases
c) Remains constant
d) Insufficient data
Answer
Answer: b [Reason:] When the bias is varied on the PIN diode, its microwave resistance RJ changes from a typical value of 6 KΩ under negative bias to perhaps 5 Ω under forward bias. Thus if the diode is mounted on a 50Ω coaxial line, it will not significantly load this line.
6. A PIN diode can be used in either a series or a shunt configuration to form a __________
a) Single pole single throw switch
b) Single pole double throw switch
c) Amplifier
d) Oscillator
Answer
Answer: a [Reason:] A PIN diode can be used in either a series or a shunt configuration to form a single pole single throw switch. In the series configuration, the switch is on when the diode is forward biased and off when the diode is reverse biased.
7. The working principle of series and shunt configuration single pole single throw switch is the same.
a) True
b) False
Answer
Answer: b [Reason:] In the series configuration, the switch is on when the diode is forward biased and off when the diode is reverse biased. In the shunt configuration, forward biasing the diode cuts-off the transmission while reverse biasing the diode ensures transmission from input to output.
8. Under ideal conditions, when a PIN diode is used as a switch, the switch must have _______ insertion loss in the ON state.
a) Maximum
b) Zero
c) Average
d) Insertion loss cannot be defined for a switch
Answer
Answer: b [Reason:] Ideally, when PIN diode is used as switch, the switch should have zero insertion loss in the ON state and infinite attenuation in the OFF state. These are ideal conditions. But practically a good operating switch must have low insertion loss.
9. When PIN diode is used as a switch, the expression for insertion loss of the switch is given by:
a) 10 log (Po/PL)
b) 10 log (PL/P0)
c) 10 log (PL. Pₒ)
d) None of the mentioned
Answer
Answer: a [Reason:] Insertion loss of a switch is defined as the ratio of incident power applied to the load when switch is absent to the actual power delivered to the load.
10. For a shunt configuration switch, the diode impedance is 40 Ω and the terminated line characteristic impedance is 50 Ω. Then the insertion loss of the switch is:
a) 2.2 dB
b) 4.2 dB
c) 8.4 dB
d) 3.6 dB
Answer
Answer: b [Reason:] Insertion loss of a shunt configuration switch is given by 20 log (2ZD+Z0/2ZD). Substituting the given values in the above expression, the insertion loss of the shunt configuration switch is 4.2 dB.
11. In the series configuration of a PIN diode switch, the terminated load impedance was found to be 50 Ω and the diode impedance was 60 Ω. Then the insertion loss of the switch is:
a) 4 dB
b) 2 dB
c) 3.6 dB
d) 4.8 dB
Answer
Answer: a [Reason:] Insertion loss of a shunt configuration switch is given by 20 log (2ZD+Z0/2ZD). Substituting the given values in the above equation, the insertion loss is 4 dB.
12. The number of PIN diodes used in SPST switch and SPDT switch are the same.
a) True
b) False
Answer
Answer: b [Reason:] The number of PIN diodes in SPST switch is one, while the number of PIN diodes used in single pole double throw switch is two.
Set 4
1. _______________ are used in the final stages of radar and radio transmitters to increase the radiated power level.
a) Power amplifiers
b) Oscillators
c) Transistors
d) Attenuators
Answer
Answer: a [Reason:] Power amplifiers are used in the final stages of radar and radio transmitters to increase the radiated power level. Output of power amplifiers are in the range of 100-500 mW.
2. Important factors to be considered for power amplifier design are:
a) Efficiency
b) Gain
c) Thermal effect
d) All of the mentioned
Answer
Answer: d [Reason:] As per the application requirement and considering various aspects of an amplifier like efficiency, gain, thermal efficiency and inter modulation distortion, amplifiers need to be designed.
3. Amplifier efficiency is the ratio of RF output power to DC input power. This parameter determines the performance of an amplifier.
a) True
b) False
Answer
Answer: a [Reason:] Power amplifier is the primary consumer of DC power in most hand-held wireless devices, so amplifier efficiency is an important consideration. Amplifier efficiency is the ratio of RF output power to DC input power.
4. Gain of power amplifiers __________ with increase in operating frequency.
a) Increases
b) Decreases
c) Increases exponentially
d) Decreases exponentially
Answer
Answer: b [Reason:] Silicon bipolar junction transistor amplifiers in the cellular telephone band of 800-900 MHz band have power added efficiencies of about 80%. But this efficiency drops quickly with increase in the operating frequency.
5. ___________ amplifiers are linear circuits, where the transistor is biased to conduct over the entire range of the input signal cycle.
a) Class A amplifiers
b) Class B amplifiers
c) Class C amplifiers
d) None of the mentioned
Answer
Answer: a [Reason:] Class A amplifiers are linear circuits, where the transistor is biased to conduct over the entire range of the input signal cycle. Because of this, class A amplifiers theoretically have a maximum efficiency of 50%.
6. A class B amplifier consists of _______ transistors in order to conduct the input signal over the entire cycle.
a) 1
b) 2
c) 4
d) 6
Answer
Answer: b [Reason:] Class B amplifier is biased to conduct only during one-half of the input signal cycle. 2 complementary transistors are operated in a class B push pull amplifier to provide amplification over the entire cycle.
7. Power amplifiers in the increasing order of efficiency is:
a) Class A, B, C
b) Class C, A, B
c) Class B, A, C
d) Efficiency of all the 3 amplifiers is the same
Answer
Answer: a [Reason:] Class A amplifiers have an efficiency of about 50%. Class B amplifiers have an efficiency of about 78%, class C amplifiers can achieve efficiencies up to 100%. In the increasing order of efficiency, C > B> a)
8. Behavior of a transistor in power amplifiers is unpredictable at all input signal levels.
a) True
b) False
Answer
Answer: b [Reason:] A transistor behaves linearly for signal powers below 1dB compression point and so, the small –signal scattering parameters should not depend either on the input power level or the output termination impedance.
9. If the output power of an amplifier is 10 V, and the input power supplied to the amplifier is 0.229 V given that the DC voltage used is 38.5 V, efficiency of the power amplifier is:
a) 25%
b) 50%
c) 75%
d) 35%
Answer
Answer: a [Reason:] Efficiency of a power amplifier is (Pout- Pin)/ PDc Substituting the given values in the above expression, efficiency of the power amplifier is 25%.
10. If a power amplifier has an output power of 10 W, and an amplifier gain of 16.4 dB, then the input drive power is:
a) 400 mW
b) 225 mW
c) 229 mW
d) 240 mW
Answer
Answer: c [Reason:] Input drive power required to get an output of 10 W is Pout (dBm)- G (dB). G is the gain of the amplifier. Substituting the given values in the above equation, 229 mW.
Set 5
1. Example of a non reciprocal device:
a) Branch line coupler
b) Wilkinson coupler
c) Magic-T hybrid
d) Circulator
Answer
Answer: d [Reason:] Non reciprocal device is the one in which the response between any two ports I and j of a component depends on the direction of signal flow. Circulator is a device that allows power flow either in clockwise direction or counter clockwise direction.
2. A microwave network can be called non reciprocal only if it contains anisotropic materials like ferrite materials.
a) True
b) False
Answer
Answer: b [Reason:] A microwave network consisting of active non linear devices like transistor amplifiers, ferrite phase shifters and more. Presence of active devices or anisotropic materials can make a microwave network non reciprocal.
3. This is not an example of anisotropic material:
a) Yttrium aluminum garnet
b) Aluminum
c) Cobalt
d) Silicon
Answer
Answer: d [Reason:] Yttrium aluminum garnet is a ferromagnetic compound. Aluminum and cobalt are iron oxides that are anisotropic. Silicon is a non metal that is isotropic in nature.
4. The magnetic properties of a material are due to the existence of ___________
a) Electrons in atoms
b) Electric dipole moment
c) Magnetic dipole moment
d) None of the mentioned
Answer
Answer: c [Reason:] The magnetic properties of ferromagnetic materials are due to the existence of magnetic dipole moments, which arise primarily from electron spin. The magnetic dipole moment of an electron is 9.27×10-24 A-m2.
5. ___________ is a measure of the relative contributions of the orbital moment and the spin moment to the total magnetic moment.
a) Lande’s factor
b) Gibbs factor
c) Newton’s ratio
d) None of the mentioned
Answer
Answer: a [Reason:] An electron in orbit around a nucleus gives rise to an effective current loop and thus an additional magnetic moment, but this effect is negligible compared to the magnetic moment due to spin. Lande’s factor is a relative measure of these orbital moments.
6. Lande’s factor for all ferromagnetic materials is in the range of 0 to 1.
a) True
b) False
Answer
Answer: b [Reason:] Lande’s factor (g) is one when the moment is only due to orbital motion and 2 when the moment is only due to spin. For most microwave ferrite materials, g lies between 1.98 and 2.01.
7. The variation of magnetic moment of a ferromagnetic material with applied bias field is linear.
a) True
b) False
Answer
Answer: b [Reason:] With the increase in the applied bias field to a ferromagnetic material, the magnetic moment increases exponentially initially, after a certain applied bias field magnetic moment remains a constant.
8. A permanent magnet is made by placing the magnetic material in a strong magnetic field.
a) True
b) False
Answer
Answer: a [Reason:] A permanent magnet is made by placing the magnetic material in a strong magnetic field and then removing the field to leave the material magnetized in a remanent state.
9. The operating point of a permanent magnet is in the:
a) First quadrant
b) Second quadrant
c) Third quadrant
d) Fourth quadrant
Answer
Answer: b [Reason:] Unless the magnet shape forms a closed path, the demagnetization factors at the magnet ends will cause a slightly negative H field to be induced in the magnet. Thus the “operating point “of a permanent magnet will be in the second quadrant. This portion of the curve is called demagnetization curve.
10. After demagnetization of a magnetic material, the residual magnetization retained in the magnetic material is called:
a) Remanence
b) Residue
c) Retardation
d) None of the mentioned
Answer
Answer: a [Reason:] The residual magnetization called remanence characterizes the strength of the magnet, so magnetic material with large remanence is chosen.