Download Electronics Engineering ELEX Board Ultimate Exam and more Exams Technology in PDF only on Docsity!
Exam
Question 1. Which of the following best describes the energy band that allows electrons to move freely in a semiconductor at room temperature? A) Valence band B) Conduction band C) Forbidden band D) Impurity band Answer: B Explanation: The conduction band contains energy levels that are partially filled, permitting electrons to move under an electric field, which is essential for conductivity. Question 2. In a common-emitter BJT configuration, the small-signal voltage gain is approximately equal to: A) –β B) –gm·RC C) –RC/RE D) –RC/ (re′) Answer: D Explanation: For a common-emitter amplifier, Av ≈ – RC / (re′), where re′ is the intrinsic emitter resistance (≈ 26 mV/IE). Question 3. The Laplace transform of the function f(t)=te⁻³ᵗ (t ≥ 0) is: A) 1/(s+3)² B) 1/(s-3)² C) 2/(s+3)² D) 2/(s-3)² Answer: A Explanation: L{t e⁻³ᵗ}=1/(s+3)² using the standard transform L{t e^{-at}}=1/(s+a)².
Exam
Question 4. Which Boolean expression is equivalent to the NAND gate output? A) ABG B) (A·B)′ C) A′+B′ D) (A+B)′ Answer: B Explanation: NAND implements the complement of the AND operation, i.e., (A·B)′. Question 5. The ideal op-amp has an infinite input impedance. In a practical op-amp, the input bias current is usually: A) Zero B) In the pico-ampere range C) In the milli-ampere range D) Equal to the output current Answer: B Explanation: Real op-amps exhibit very small bias currents, typically pico-ampere to nano-ampere, due to input transistor leakage. Question 6. A series RLC circuit has R=10 Ω, L=100 mH, and C=10 μF. Its resonant frequency (in rad/s) is: A) 1000 B) 3162 C) 5000 D) 10 000 Answer: B Explanation: ω₀ = 1/√(LC) = 1/√(100 mH·10 μF)=1/√(10⁻³)=≈3162 rad/s. Question 7. In a 4-bit binary-coded decimal (BCD) representation, the decimal number 9 is coded as: A) 1001 B) 0110
Exam
Explanation: Phase modulation (PM) directly changes the carrier phase according to the instantaneous amplitude of the modulating signal. Question 11. The determinant of a 2×2 matrix [[a, b],[c, d]] equals: A) ad-bc B) a+b-c-d C) a·c-b·d D) a·d+ b·c Answer: A Explanation: For a 2×2 matrix, det = a·d – b·c. Question 12. In a MOSFET operating in saturation, the drain current ID is proportional to: A) (VGS-VTH)² B) VDS² C) (VDS-VGS)² D) VGS·VDS Answer: A Explanation: In saturation, ID ≈ (½)·μ·Cox·(W/L)·(VGS-VTH)², independent of VDS. Question 13. The Nyquist stability criterion states that a closed-loop system is stable if the Nyquist plot of the open-loop transfer function does not encircle the point: A) (0,0) B) (-1,0) C) (1,0) D) (0,-1) Answer: B Explanation: Encirclement of the critical point (-1,0) indicates potential instability.
Exam
Question 14. Which of the following is a non-linear op-amp application? A) Inverting amplifier B) Summing amplifier C) Schmitt trigger D) Voltage follower Answer: C Explanation: A Schmitt trigger uses positive feedback to create hysteresis, a non-linear behavior. Question 15. The thermal noise voltage generated across a resistor R in bandwidth B is given by: A) √(4kTRB) B) 4kTRB C) √(kTB/R) D) kTB/R Answer: A Explanation: Johnson-Nyquist noise: v_n = √(4kTRB). Question 16. In a 3-phase, Y-connected system, the line voltage is: A) √3 times the phase voltage B) 1/√3 times the phase voltage C) Equal to the phase voltage D) Twice the phase voltage Answer: A Explanation: For Y-connection, V_line = √ 3 ·V_phase. Question 17. The Fourier series of a periodic square wave with amplitude A and period T contains only: A) Even harmonics B) Odd harmonics
Exam
Answer: A Explanation: Standard formula for a solid cylinder about its symmetry axis. Question 21. In a digital system, the Gray code is preferred over binary for: A) Faster arithmetic B) Minimizing transition errors C) Reducing power consumption D) Simplifying logic design Answer: B Explanation: Adjacent Gray code words differ by only one bit, reducing errors in analog-to-digital conversion. Question 22. The gain-bandwidth product of an op-amp is: A) Constant for a given op-amp B) Variable with temperature C) Dependent on feedback network D) Zero for ideal op-amps Answer: A Explanation: For a given op-amp, the product of closed-loop gain and bandwidth remains approximately constant. Question 23. The time constant τ of an RC low-pass filter is: A) R·C B) R/C C) C/R D) √(RC) Answer: A Explanation: τ = RC defines the exponential response of the filter. Question 24. In a three-phase induction motor, the slip s is defined as:
Exam
A) (Ns-Nr)/Ns B) (Nr-Ns)/Nr C) (Ns-Nr)/Nr D) (Nr-Ns)/Ns Answer: C Explanation: Slip = (Ns-Nr)/Ns, where Ns is synchronous speed and Nr is rotor speed. Question 25. The Smith chart is primarily used for: A) Plotting Bode diagrams B) Solving differential equations C) Impedance matching on transmission lines D) Designing digital filters Answer: C Explanation: The Smith chart graphically represents complex impedances for matching. Question 26. Which of the following statements about a diode’s I-V characteristic is true? A) It is linear for all voltages. B) It conducts equally in forward and reverse bias. C) It exhibits exponential increase in forward bias. D) It has zero reverse leakage current. Answer: C Explanation: The diode equation shows exponential current increase with forward voltage. Question 27. The eigenvalues of a 2×2 matrix [[2,1],[1,2]] are: A) 1 and 3 B) 2 and 2 C) 0 and 4
Exam
Question 31. In a 555 timer configured as an astable multivibrator, the frequency of oscillation is inversely proportional to: A) R1·C B) (R1+R2)·C C) R2·C D) √(R1·R2·C1) Answer: B Explanation: f ≈ 1.44/[(R1+2R2)·C] for standard astable configuration; frequency ∝ 1/[(R1+2R2)·C]. Question 32. The effective resistance seen by a source driving a parallel network of R1=100 Ω and R2=200 Ω is: A) 66.7 Ω B) 150 Ω C) 300 Ω D) 50 Ω Answer: A Explanation: 1/R_eq = 1/100 + 1/200 = 0.015 → R_eq ≈ 66.7 Ω. Question 33. In a digital communication system, the bit error rate (BER) for BPSK in AWGN is given by: A) Q(√(2Eb/N0)) B) Q(√(Eb/N0)) C) ½·erfc(√(Eb/N0)) D) 1-Q(√(Eb/N0)) Answer: A Explanation: BER for BPSK = Q(√(2Eb/N0)), where Q is the tail probability of the normal distribution. Question 34. Which of the following is a first-order linear differential equation?
Exam
A) d²y/dt² + 3dy/dt + 2y = 0 B) dy/dt + 5y = sin t C) y · dy/dt = t D) d³y/dt³ – y = 0 Answer: B Explanation: The highest derivative is first order, and the equation is linear in y. Question 35. In a three-phase, delta-connected load, the line current is: A) √3 times the phase current B) 1/√3 times the phase current C) Equal to the phase current D) Twice the phase current Answer: A Explanation: For delta, I_line = √3·I_phase. Question 36. Which of the following statements about MOSFET threshold voltage (VTH) is correct? A) It is independent of temperature. B) It increases with increasing substrate doping. C) It is the same for NMOS and PMOS devices. D) It is zero for enhancement-mode devices. Answer: B Explanation: Higher substrate doping raises the surface potential, increasing VTH. Question 37. In a Fourier Transform, the duality property states that: A) Time scaling corresponds to frequency scaling. B) Multiplication in time corresponds to convolution in frequency. C) A time-domain function can be expressed as a frequency-domain impulse. D) The transform of a transform returns the original function. Answer: D
Exam
Question 41. In a three-wire phase-shift transformer, the phase shift between primary and secondary is achieved by: A) Winding polarity reversal B) Adding a capacitor in series C) Using a delta-wye connection D) Changing the number of turns only Answer: C Explanation: Delta-wye (or wye-delta) connections produce a 30° phase shift. **Question 42. Which of the following is a characteristic of an ideal current source? ** A) Infinite output voltage B) Zero internal resistance C) Infinite internal resistance D) Constant voltage regardless of load Answer: C Explanation: An ideal current source maintains constant current irrespective of load, implying infinite internal resistance. Question 43. The probability of obtaining exactly 3 heads in 5 tosses of a fair coin follows which distribution? A) Binomial with n=5, p=0. B) Poisson with λ=2. C) Normal with μ=2.5, σ=1. D) Geometric with p=0. Answer: A Explanation: Number of heads in a fixed number of independent Bernoulli trials follows a binomial distribution.
Exam
Question 44. The dominant time constant in an RLC series circuit is determined by: A) The smallest resistance B) The largest inductance C) The ratio L/R D) The product RC Answer: C Explanation: The exponential decay rate is governed by τ = L/R for series RLC (underdamped case). Question 45. In a digital encoder, a 4-to-2 priority encoder will output “ 11 ” when the two highest-order inputs are high. What is the output when inputs D3=0, D2=1, D1=1, D0=0? A) 00 B) 01 C) 10 D) 11 Answer: C Explanation: Priority is given to the highest-order ‘ 1 ’; D2 is highest active, corresponding to binary “ 10 ”. Question 46. The main purpose of a snubber circuit across a thyristor is to: A) Increase forward voltage drop B) Reduce dv/dt triggering C) Limit reverse recovery current D) Provide a path for leakage current Answer: B Explanation: Snubbers limit the rate of voltage change (dv/dt), preventing unintended turn-on. Question 47. In a balanced three-phase system, the total instantaneous power is:
Exam
D) RC high-pass Answer: B Explanation: An RLC series circuit exhibits a second-order response with a resonant peak. Question 51. In a digital communication system, the Shannon capacity (C) of a channel of bandwidth B with signal-to-noise ratio S/N is: A) B·log₂(1+S/N) B) B·log₁₀(1+S/N) C) B·(1+S/N) D) B·(S/N) Answer: A Explanation: Shannon’s theorem: C = B·log₂(1+S/N) bits per second. Question 52. The voltage across a capacitor cannot change instantaneously because: A) The capacitor has infinite resistance B) It would require infinite current C) The dielectric breaks down D) The voltage is always zero Answer: B Explanation: dv/dt = i/C; an instantaneous change would need infinite i, which is impossible. Question 53. In a 7-segment display, to display the numeral “ 2 ” the segments a, b, g, e, and d must be: A) On, others off B) Off, others on C) All on D) All off Answer: A
Exam
Explanation: Standard segment labeling shows “2” lit on a, b, g, e, d. Question 54. The root-locus of a system with open-loop transfer function G(s) = K/(s(s+2)) starts at: A) s = 0 and s = – B) s = –K C) s = ∞ D) s = – Answer: A Explanation: Poles of the open-loop transfer function are at s = 0 and s = – 2; loci start there. Question 55. The most common method to reduce the ripple voltage in a full-wave rectifier is to: A) Increase diode forward voltage B) Add a smoothing capacitor C) Use a higher frequency transformer D) Insert a series resistor Answer: B Explanation: A capacitor filters the pulsating DC, reducing ripple. Question 56. In a digital system, the term “metastability” refers to: A) A state where a flip-flop cannot settle to a defined logic level within the clock period B) A permanent logic high C) A glitch caused by power supply noise D) The stable operation of a PLL Answer: A Explanation: Metastability occurs when setup/hold times are violated, leading to uncertain output.
Exam
A) Longitudinal B) Shear C) Flexural D) Torsional Answer: A Explanation: Quartz crystals vibrate in the thickness-shear mode, but the fundamental frequency is due to longitudinal stress; the accepted answer in most textbooks is longitudinal. Question 61. The term “bias stability” in a BJT amplifier refers to: A) The ability of the bias point to remain unchanged with temperature variations B) The maximum gain achievable C) The frequency response bandwidth D) The power dissipation in the collector Answer: A Explanation: Bias stability ensures the Q-point does not drift due to temperature or β variations. Question 62. The most widely used error-detecting code in digital communications is: A) Hamming code B) Cyclic Redundancy Check (CRC) C) Reed-Solomon code D) Parity check Answer: B Explanation: CRC provides strong error detection with modest overhead, making it common in protocols. Question 63. In a DC motor, the back-EMF constant (Kb) relates the: A) Armature current to torque B) Speed to generated voltage
Exam
C) Supply voltage to field flux D) Power to resistance Answer: B Explanation: Back-EMF = Kb·ω, linking angular speed to induced voltage. Question 64. Which of the following is a characteristic of an ideal transformer? A) No core losses and infinite magnetizing inductance B) Finite leakage inductance C) Non-linear magnetization curve D) Frequency-dependent turns ratio Answer: A Explanation: An ideal transformer assumes zero copper/core losses and infinite magnetizing inductance. Question 65. The total harmonic distortion (THD) of a signal is defined as: A) Ratio of RMS of all harmonic components to RMS of fundamental B) Ratio of peak harmonic voltage to fundamental voltage C) Difference between fundamental and harmonic powers D) Sum of all harmonic amplitudes Answer: A Explanation: THD = √(Σ V_h²)/V₁, where V_h are harmonic RMS values. Question 66. In a MOS capacitor, the capacitance in accumulation is approximately: A) Cox (oxide capacitance) B) Zero C) Infinite D) Dependent on depletion width Answer: A
