- UGC-NET Electronic Science Question Paper With Answer Key Download Pdf [Feb 2023]
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UGC-NET Electronic Science Feb 2023 Answers with Explanations
1. (B): A photodiode operates by absorbing photons and converting them to a current across the device. When it is reverse-biased and kept in the dark, it doesn't receive any photons. Hence, there is no photo-generated current. However, due to thermal energy, a tiny amount of current called the reverse saturation current (or dark current) flows. This is the minimum current flowing through the device when no photons are absorbed. It's essential to keep in mind that the reverse saturation current is a very small value, and it is the baseline current for a reverse-biased photodiode in the dark.
2. (C) 3. (C)
4. (D): Ripple rejection refers to the ability of a circuit, often a power supply or amplifier, to suppress or reduce the amount of ripple voltage from its input to its output. A higher ripple rejection ratio indicates a better suppression of input ripple. In mathematical terms, ripple rejection is given as:
Ripple Rejection (dB) = 20 log ( Vripple (input)Vripple (output) )
This essentially measures the effectiveness of a circuit in filtering out unwanted fluctuations (ripple) from a desired signal.
5. (C): The oxide capacitance for a MOS transistor is dependent on its mode of operation. In the saturation mode, the drain-to-gate overlap results in an effective capacitance called Cgd. This capacitance arises due to the overlap of the gate over the drain and can be represented as:
Cgd = COX × W × LDWhere:
- COX is the oxide capacitance per unit area.
- W is the width of the MOS transistor.
- LD is the overlap length.
This capacitance can influence the dynamic behavior of the MOSFET, especially at high frequencies, and is an essential parameter in RF circuit design and analysis.
6. (B): When an n+ region is diffused into a p-type substrate in a MOS transistor, the junction created between the channel and the drain is between the n+ region and the p substrate. Therefore, the type of junction formed is n+/p.
7. (B): For an n-channel MOS transistor, the value of the K parameter, also known as the transconductance parameter, is given by:
K = μn × Cox × W2L
Given,
Cox = 7 × 10-8 F/cm2
W = 40 pm = 4 × 10-5 cm
L = 4 pm = 4 × 10-6 cm
Substitute these values in the formula:
K = 600 × 7 × 10-8 × 4 × 10-52 × 4 × 10-6
K = 42 × 10-5 A/V2.
8. (D): In a resistive load inverter, the critical voltage VOH (Output High voltage) is the minimum voltage level at the output that is still considered HIGH. For a resistive load inverter, this voltage is usually close to VDD because the output only drops slightly due to the resistance of the load. Given that VDD = 5V, the most likely value for VOH is also 5V.
9. (B) 10. (B) 11. (C)
12. (A)
13. (B): The quiescent state of a transistor refers to its steady-state operation when it's biased appropriately but no signal (neither AC nor varying DC) is applied to it. In this state, the
transistor's operating point (also called the Q-point) is set. This Q-point determines the DC collector current and collector-to-emitter voltage when no signal is present. The right biasing ensures the transistor operates in its active region, ready to amplify any applied signal.
14. (D)
15. (D): Given the op-amp setup, the differential input voltage is:
In differential mode, the output voltage is:
The common mode input voltage is:
Given CMRR in dB = 100 dB,
Common mode output is:
Since the op-amp amplifies the difference between the two inputs, the output voltage will be
So, the closest option to this value is 2.57 V, which corresponds to option D.
16. (A): Operational amplifiers (op-amps) can be employed in many circuit configurations. When an op-amp is utilized in rectification, it's generally for precision rectification which eliminates diode voltage drops. [ ... ]
precision half-wave rectification. A half-wave rectifier allows only one half of an AC wave to pass through to the load. If the op-amp was used in a full-wave configuration, it would precisely rectify both halves of the waveform.
17. (A): To convert the decimal fraction (0.125(10)) to octal:
Start by multiplying the fraction by 8:
The integer part is 1.
So, 0.12510 = 0.18.
This means the fraction 0.125 in decimal is equivalent to 0.1 in octal, making option A the correct choice.
18. (B) 19. (B)
20. (B): The D flip-flop, also known as a data or delay flip-flop, has an output Qn that takes the state of the input D at the moment of a positive edge at the clock input. So, the characteristic equation of a D flip-flop is simply Qn+1 = Dn. From the options given, the one that most closely matches this expression is B. The other options don't capture the fundamental behaviour of the D flip-flop. This flip-flop is primarily used for storage or data transfer applications in digital circuits.
21. (B)
22. (B): For the 8086 microprocessor:
Initially, AL = 19 BCD and BL = 36 BCD
Adding these: 19 + 36 = 55
However, in BCD, a single digit can't exceed 9. So, after the ADD operation, the result is 55, which isn't a valid BCD. The DAA (Decimal Adjust After Addition) instruction then adjusts the result to obtain a valid BCD. After DAA, AL will have a value of 55 (which is now a valid BCD).
23. (C): The 8086 microprocessor has a segmented memory architecture, which means it accesses memory using a segment and an offset. Both segment and offset are 16 bits wide. However, the actual physical address is calculated by shifting the segment address 4 bits to the left and adding the offset. This results in a 20-bit physical address. Thus, the Bus Interface Unit (BIU) of the 8086 microprocessor sends out a 20-bit address.
24. (D): The 8086 uses the Stack Segment (SS) and Stack Pointer (SP) registers to handle stack operations. When you execute the PUSH BX instruction, the data in BX (which consists of BH and BL) is stored on the stack. Before this happens, SP is decremented by 2 (since it's a 16-bit register).
So, SP = 2000H - 2 = 1FFEH.
Given SS = 3000H, the physical address is calculated as:
Physical Address = (SS * 10H) + SP
= (3000H * 10H) + 1FFEH
= 30000H + 1FFEH = 31FFEH
So, BH is stored at 31FFEH and BL is stored at 31FFFH.
25. (B)
26. (B): The modification made by Maxwell to the existing electromagnetic laws was to Ampere's Law. He introduced a term known as the "displacement current" to account for the changing electric field in a capacitor. This modification was crucial to make Ampere's Law consistent with the conservation of charge in situations where the electric field is changing, such as in a charging or discharging capacitor. With this modification, Maxwell's equations became a complete set that could describe all electromagnetic phenomena, leading to the prediction and subsequent discovery of electromagnetic waves.
27. (C): The gradient (or grad) of a scalar function V is a vector field that points in the direction
of the greatest rate of increase of V, and its magnitude is the maximum rate of change. For V = 2xyz2,
Differentiating with respect to x, y and z, we get:
Thus, the gradient of V is represented by the above expression.
28. (D): The dBm is a unit of power in decibels (dB) relative to 1 mW. So, when 0 dBm power is transmitted, it means the actual power transmitted is equivalent to the reference power, which is 1 mW. It's essential to note that dBm provides a relative measure and is useful in communication systems where power levels can vary significantly and a logarithmic scale provides a more intuitive sense of the power levels.
29. (D): The numerical aperture (NA) of an optical fiber is defined by the square root of the difference of the squares of the refractive indices of the core (η1) and the cladding (η2). That is,
Given, NA = 0.3 and η2 = 1.6, we can rearrange the formula:
Substituting the given values:
So, the refractive index of the core material is 1.63.
30. (B): In coherent binary FSK (Frequency Shift Keying), orthogonal sinusoidal signals are used. The bit interval (Tb) is the inverse of the difference between the two frequencies. Given frequencies of 20 kHz and 50 kHz,
The maximum possible bit interval is the inverse of this frequency difference:
= 0.0000333 sec
or 0.0333 ms or 0.0166 m sec.
Thus, the maximum possible bit interval for the coherent binary FSK system given the frequencies is 0.0166 m sec.
31. (D): The most efficient way to demodulate SSB (Single Side Band) signals is by using a Coherent Detector. SSB signals do not have a carrier, so an envelope detector would not work as it does for AM signals. A coherent detector, on the other hand, re-inserts the carrier signal, allowing the original message signal to be recovered through multiplication and filtering. This method ensures accurate phase and frequency matching with the original signal.
32. (D): In Wideband FM (WBFM), the modulation index (m) typically exceeds unity. Modulation index is defined as the ratio of the frequency deviation to the modulating frequency. For WBFM, this deviation is large, and as a result, the modulation index is high. This leads to multiple sidebands, which give WBFM its wide bandwidth. On the contrary, for narrowband FM (NBFM), the modulation index is typically less than unity, resulting in fewer sidebands and a more confined frequency spectrum.
33. (D): For most practical control systems, the damping factor (ζ) should lie in the range 0.28 < ζ < 0.7. The damping factor determines how oscillations in a system decay in response to a disturbance or setpoint change. A system with a damping factor of 1 (or higher) is overdamped and responds slowly. A system with a damping factor of 0 is undamped and will oscillate indefinitely. The range 0.28 to 0.7 ensures that the system responds quickly without excessive oscillation, making it suitable for many practical applications.
36. (A): For a series resonant circuit, the resonant frequency (fr) is given by:
Where L is the inductance and C is the capacitance. If the inverter operates under resonant conditions, then:
Given, fr = 5 kHz and C = 1 μF
This gives L = 40 mH.
37. (B): When converting a galvanometer into an ammeter, we use a shunt resistor to bypass the majority of the current, allowing only a small fraction to flow through the galvanometer. The shunt resistance (Rs) is given by:
Where, Rg is the galvanometer resistance, Ig is the full-scale deflection current of the galvanometer, and I is the full-scale deflection current of the ammeter.
Given Rg = 100 Ω, Ig = 1 mA, and I = 1 A:
38. (C): The Schering Bridge is particularly designed for measuring the capacitance and its associated power factor or dissipation factor. It is especially suitable for measuring the capacitance of a cable. The bridge provides a null indication when the ratio of the products of the impedances in the arms is equal. Dielectric loss, capacitance, and loss angle are all properties that can be measured using a Schering Bridge.
42. (A)
- (a) In a transistor, the emitter is the most heavily doped, followed by the collector, and then the base. This ensures efficient operation and minimizes the base width modulation.
- (b) Common Base (CB) configuration isn't usually used as an amplifier. It's the Common Emitter (CE) configuration that's a good current amplifier.
- (c) In CB configuration, the output and input signals are in phase.
- (d) Common Collector (CC) configuration, often known as an emitter follower, has high input impedance and low output impedance.
43. (C)
- (a) In NMOS, the Fermi level is closer to the conduction band, making the Fermi potential negative.
- (b) This contradicts statement (a).
- (c) For NMOS, substrate bias voltage VSB is positive to induce an n-type inversion layer. For PMOS, VSB is negative to induce a p-type inversion layer.
- (d) This contradicts statement (c).
44. (A):
- (a) The behavior of the target chip is described algorithmically to begin the design process. This provides a high-level representation of what the chip should accomplish.
- (b) Floor planning in VLSI involves mapping the architecture of the processor onto the chip surface, determining where modules will be placed, and planning interconnect routes.
- (c) The behavioral domain defines the functional aspects of the chip, including Finite State Machines (FSMs) which dictate how the chip will respond to various inputs.
- (d) Individual modules are indeed implemented using leaf cells, which are the building blocks in a standard cell library.
45. (C):
- (a) For an RLC circuit, Kirchhoff's Voltage Law (KVL) can be applied to determine the relationship between voltages in the circuit. Using this, one can derive an equation for current i(t).
- (b) Kirchhoff's Current Law (KCL) relates the currents entering and leaving a node. While it's valuable in many contexts, for an RLC circuit in series, KVL is more directly applicable.
- (c) The Fourier Transform allows conversion from the time domain to the frequency domain, and it can be used to solve circuits with sinusoidal sources.
- (d) The Laplace Transform is a powerful tool for solving linear differential equations, like those derived from RLC circuits.
- (e) The Fourier series is used to represent periodic functions as a sum of sines and cosines, and while it's useful in analyzing periodic signals, it might not be the primary method for solving the current in an RLC circuit.
46. (A)
47. (B):
- (a) The IC 741 is a widely recognized operational amplifier, but it comes in an 8-pin package, not 10 pins.
- (b) The gain for a non-inverting operational amplifier configuration is 1 + RfRi, where Rf is the feedback resistor and Ri is the resistor connected to the inverting input.
- (c) CMRR (Common-Mode Rejection Ratio) measures an op-amp's ability to reject common-mode signals. For an ideal amplifier, the CMRR should be infinite, not 0.
- (d) The slew rate defines the maximum rate of change of the output voltage and is indeed a parameter that limits the bandwidth for large signals.
48. (C):
- (a) The RMS value of current for a half-wave rectifier is lower than that of a full-wave rectifier. The exact value would depend on the input waveform.
- (b) In a full-wave bridge rectifier, the Peak Inverse Voltage (PIV) of a diode is equal to the peak voltage, Vm.
- (c) TUF (Transformer Utilization Factor) and rectification efficiency are two different parameters. While both are measures of performance, they evaluate different aspects.
- (d) Rectifiers convert AC to rippled DC, and further filtering is required to get a smooth DC output.
49. (A)
50. (B):
- (a) RTL (Resistor-Transistor Logic) is an older technology and has a relatively low fan-out.
- (b) ECL (Emitter-Coupled Logic) is known for its high speed and has a greater fan-out compared to TTL (Transistor-Transistor Logic).
- (c) DTL (Diode-Transistor Logic) was a predecessor to TTL and typically has a fan-out comparable to or less than TTL.
- (d) CMOS (Complementary Metal-Oxide-Semiconductor) logic circuits are characterized by low power consumption and have a fan-out greater than TTL. This is because CMOS devices consume power mainly during the switching and not when static.
51. (C): A decoder is a combinational circuit that converts binary information from n input lines to a maximum of 2n unique output lines.
- (a) Decoders are similar to demultiplexers. In fact, they can be viewed as a special type of demultiplexer where there is no data input and only address lines.
- (b) This is correct. Decoders don't have a "data input" per se. Instead, they take an n-bit input and direct it to one of the 2n outputs.
- (d) This is also correct. An n-bit decoder will have 2n output lines. For example, a 2-bit decoder will have 4 output lines.
- (e) This is incorrect. Decoders work entirely in the digital domain, so they do not convert digital to analog signals.
52. (C): The 8051 microcontroller is a widely used microcontroller in embedded systems and has specific behaviours with respect to its interrupts and pins.
- (b) This is true. The Timer 1 Interrupt Flag gets auto-cleared after it's been triggered.
- (d) Timer 0 Vector location is indeed 000BH in the 8051 microcontroller.
53. (B): The 8051 microcontroller has a variety of interrupt options, and their behaviour can be modified using specific registers.
- (a) This is correct. By programming the TCON (Timer Control) register, the external interrupts INT0 and INT1 can be configured as either edge-triggered or level-triggered.
- (d) The 8051 microcontroller has a dedicated interrupt for serial communication, which is correct.
54. (B): The 8086 microprocessor, a 16-bit microprocessor by Intel, has specific operations and instructions that dictate its behaviour.
- (b) JMP (Jump) instruction in the 8086 microprocessor can indeed have a segment override prefix if the jump is to a different segment.
- (c) Both SAHF (Store AH into Flags) and POPF (Pop Flags) instructions do affect the flag register. SAHF copies the lower byte of the AH register into the lower byte of the flag register, and POPF pops the top of stack into the flag register.
Power is measured in Watts (W) in the International System of Units.
Gain, especially in the context of antennas, is often measured in decibels referenced to an isotropic antenna (dBi).
The unit of electrical resistance is the ohm (Ω).
Inductance is measured in henrys (H) in the International System of Units.
In direct memory addressing mode, the operand's address is explicitly given in the instruction itself, as seen in MOV BX, [2000 H].
In immediate addressing mode, the operand's value is directly given in the instruction, such as MOV [SI], 2500 H where 2500 H is the immediate data.
PUSHF is an instruction used to push the flags onto the stack, which is directly related to flag manipulation.
RET is a control transfer instruction which pops the return address from the stack and transfers control to that address.
Ga (c) → 31, In (b) → 49, As (a) → 33, and Sb (d) → 51.
Semiconductors are vital for modern electronics, and the atomic number is a fundamental property which dictates how an element behaves in electronic applications. Doping semiconductors with specific elements can change their conductive properties.
VLSI has revolutionized electronics by allowing millions of transistors to be integrated into a single chip, like microprocessors.
Op-Amps are versatile electronic components used in a wide range of applications from audio systems to control systems.
(a) Mark the input connections of AND Matrix to generate the required product terms: The AND matrix identifies the products.
(d) Mark the input connections of OR matrix to generate the required output (sum) terms: Following the AND matrix, the OR matrix produces the final sum output.
(b) Mark the connections required for invert/non-invert matrix for setting an active high to active low output: This ensures the output is correctly polarized.
PLA provides a way to implement logical functions and can be reprogrammed to implement various logical operations.
81. (B): The typical priority of polling interrupts in microcontrollers (such as the 8051) is as follows:
(a) External 0 Interrupt: Usually has the highest priority.
(b) External 1 Interrupt: The next in the line after External 0.
(c) Timer 1 Interrupt: After checking external interrupts, the Timer interrupts are usually checked.
(d) Serial Interrupt: Often has the lowest priority among these.
Polling is a method where the CPU checks for interrupt conditions continuously. While it ensures that no interrupts are missed, it can be less efficient than hardware-driven interrupt handling.
82. (A): The energy gap, often used as a measure of how much energy is required to excite an electron from its ground state to a conduction state, is different for various semiconductor materials. For the materials listed:
(d) GaP: ~2.26 eV
(c) GaAs: ~1.43 eV
(a) Si: ~1.12 eV
(b) Ge: ~0.66 eV
Energy gaps are fundamental in determining the electrical properties of semiconductors, and also affect the color of light emitted by LED devices made from these materials.
83. (*): Either question or options is wrong.
84. (B): The dielectric constant, also called relative permittivity, is a measure of a material's ability to store energy when placed in an electric field. In ascending order:
(c) Water (dehydrated): ~80
(b) Nylon: Ranges from ~3 to 4
(a) Alcohol (like Ethanol): ~24.5
(d) Water (Fresh): ~80 (But it's worth noting that pure water's dielectric constant can be even higher, while impurities in real-world samples can decrease it.)
Materials with higher dielectric constants can store more electrical energy, which is crucial for applications like capacitors.
85. (D): For a spectrum analyzer, which is used to measure the magnitude of input signals versus frequency:
(c) Attenuator (Adjustable): This block can reduce the amplitude of the input signal to protect the analyzer and can be adjusted depending on the signal's strength.
(d) Mixer & Tunable Detector: This mixes the input signal with a local oscillator to shift the signal's frequency.
(b) Fixed Frequency Active Filter: It can filter out unwanted frequencies, focusing on the desired frequency range.
(a) Detector & Video Amplifier: Detects and amplifies the filtered signal for display.
Spectrum analyzers are instrumental in studying the frequency spectrum of radio frequency, audio, and other signals.
86. (A): Integrated circuits require precision engineering to work properly. In monolithic ICs, isolation is essential to prevent unintended electrical interaction between different components. The region between the isolation islands often has a p+ type material, which is essentially a p-type material but with a higher concentration of acceptor atoms. Statement I correctly notes this.
heightened concentration compared to a regular p-type substrate. Statement II provides the rationale: the higher density is there to ensure the reverse-biased isolation to substrate junction's depletion region doesn't extend into the p+ material. If it did, it would jeopardize the isolation, leading to faulty operation.
87. (D): For Statement I, when a reverse-biased photodiode is exposed to light, the photons hitting the diode generate electron-hole pairs, leading to a photocurrent. This photocurrent flows in the opposite direction of the reverse saturation current, and as a result, the net current flowing through the diode will be reduced, not equivalent to the reverse saturation current. Thus, Statement I is incorrect. Statement II explains the photovoltaic effect seen in photodiodes. When a voltage equal in magnitude but opposite in polarity to the photovoltaic emf is applied, the net current through the device becomes zero. Hence, Statement II is accurate.
88. (C): Assertion (A) indicates a behaviour observed in certain microcontrollers. Some instructions do read the status of an internal latch rather than the actual pin state. This can be useful for ensuring that the reading is stable and not subject to transient changes. Reason (R) is misleading. In many cases, there's a distinct difference between reading the status of an input pin (which can change rapidly) and the more stable internal latch of an output port. Therefore, the assertion is correct, but the reason is not.
89. (A): M-ary modulation schemes are indeed preferable in situations where bandwidth conservation is a priority, as indicated by Assertion (A). These modulation schemes, such as M-ary QAM or M-ary PSK, use symbols that represent log2(M) bits. As a result, for each symbol transmitted, multiple bits of information are conveyed, leading to more efficient use of bandwidth. Reason (R) captures this essence by comparing it to BPSK
(Binary Phase Shift Keying), which is a 2-level scheme. For M-ary schemes, the bandwidth requirement is indeed reduced by factors of log2M compared to BPSK, making the reason correct and also the correct explanation for the assertion.
90. (D): Potentiometric accelerometers and LVDT (Linear Variable Differential Transformer) accelerometers have distinct operating principles and advantages. Assertion (A) suggests that potentiometric accelerometers have higher resolution. However, this isn't intrinsically accurate. The resolution of an accelerometer depends on various factors, including its design, noise characteristics, and the resolution of the associated electronics. It's not inherently true that potentiometric accelerometers always outperform LVDTs in terms of resolution. On the other hand, Reason (R) suggests that LVDT accelerometers offer less resistance to motion than potentiometric accelerometers. LVDTs, being transformer-based devices, don't have the wiper contacts like potentiometric devices, which can introduce resistance. Thus, the resistance to motion, or friction, can indeed be less in LVDT accelerometers.
91. (A): For radar, the distance to a target is half the round-trip time multiplied by the speed of the radar signal (which is the speed of light in a vacuum). This is because the signal has to go to the target and come back. So,
Given the speed of electromagnetic waves (c) is 3 × 108 m/s, the formula simplifies to R = 150 × 106 × t. The specific distance depends on the round-trip time t provided. Without a specific value for t, it is not possible to calculate the exact distance. However, the relationship between R and t is captured by this formula.
92. (C): Radar frequency bands are designated by specific names. The frequency of 10 GHz corresponds to the X-Band, which typically
covers the range from 8 GHz to 12 GHz. This band is widely used for radar applications, especially in missile guidance, weather observations, and airborne radars. It provides a good trade-off between resolution and atmospheric absorption, making it ideal for many radar tasks.
93. (*): Either question or options is wrong.
94. (A): The minimum detectable signal Pmin of -90 dBm can be converted to watts using the formula:
Here, PdBm is the power level in dBm. For a power level of -90 dBm:
Thus, the minimum detectable signal in watts is 10−12 W.
95. (A) 96. (B)
97. (A): In a two-quadrant single-phase SCR Drive, the armature current becomes continuous when the conducting angle (or conduction interval) of the SCR is equal to or greater than 180°. The relationship β − α denotes the conduction interval, where α is the firing angle and β is the turn-off angle. For the armature current to be continuous, β − α should be equal to or greater than 180°, ensuring that there's no period within the cycle where the current drops to zero.
98. (A): In one-quadrant converters such as a half-controlled bridge circuit or a single-phase
circuit with flywheel diodes for DC motor, the average output voltage is always positive. This is because the converter operates in only one quadrant of the voltage-current plane, specifically the first quadrant, which signifies positive voltage and positive current. The flywheel diodes maintain the continuity of current and prevent its reversal, ensuring that the voltage remains positive throughout the operation.
99. (C): For a chopper's filter in a DC Drive, it should be designed such that the chopper operating frequency is significantly higher than the resonant frequency to avoid resonant conditions. Operating at a frequency close to or at the resonant frequency can cause large current or voltage spikes and may damage components. By selecting a chopper operating frequency at least 4 times the resonant frequency, we ensure that the system remains stable and far from resonance, minimizing the risk of overcurrents or overvoltages.
100. (A): In a single-quadrant chopper, the direction of the DC motor can be reversed by changing the field polarity. The chopper controls the voltage (and therefore speed) applied to the motor. To reverse the motor's direction, we don't typically modify the chopper frequency or use filters. Instead, reversing the field polarity changes the direction of the generated torque, and hence, the rotation direction of the motor. It's one of the basic principles of DC motor operation where the direction is governed by the interaction between the armature and field magnetomotive forces.