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Past Questions
The characteristic impedance of a transmission line is:
A)
√(L/C)
B)
√(C/L)
C)
L/C
D)
C/L
The characteristic impedance of a transmission line is given by Z0 = √(L/C) for a lossless line, where L is inductance per unit length and C is capacitance per unit length. Hence option (a) is correct. Option (b) is the reciprocal form, while (c) and (d) are simple ratios and not impedance expressions.
Which circuit converts sine wave into square wave?
A)
Integrator
B)
Differentiator
C)
Comparator
D)
Oscillator
A comparator compares an input sine wave with a reference level (usually 0 V or a threshold). When the sine wave is above the reference, the output goes to one saturation level; when it is below, it goes to the other. This produces a square wave output. An integrator gives a ramp/triangular-type output, a differentiator gives spikes, and an oscillator generates waveforms rather than converting a sine wave into a square wave.
Polarization of an EM wave is taken as the:
A)
direction of the magnetic field
B)
direction of the electric field
C)
normal to the direction of the magnetic field
D)
normal to the direction of the electric field
The polarization of an electromagnetic (EM) wave is defined by the direction in which the electric field vector oscillates. In a plane EM wave, the electric field and magnetic field are mutually perpendicular, and both are perpendicular to the direction of propagation. Since the electric field determines the wave’s orientation, polarization is taken as the direction of the electric field. The magnetic field direction is not used to define polarization.
The dual of (A+1) = 1 is:
A)
A . 1 = 1
B)
A + 0 = 0
C)
A . 0 = 0
D)
A + 1 = 0
To find the dual in Boolean algebra, interchange '+' with '·' and interchange 1 with 0. Starting from (A + 1) = 1, the dual becomes A · 0 = 0. This is option (c). Option (a) is not the dual because it still keeps '+' and 1 unchanged in the same places.
If Barkhausen criterion is not fulfilled by an oscillating circuit, it will:
A)
stop oscillating
B)
produce damped waves continuously
C)
become an amplifier
D)
produce high frequency whistles
Barkhausen criterion states that for sustained oscillations, the loop gain magnitude must be 1 and the total phase shift must be 0° or 360°. If this criterion is not satisfied, the feedback will not sustain the output. As a result, oscillations die out and the circuit stops oscillating. Option (b) refers to transient damped oscillations, but not continuous operation.
Resonators are used in microwave systems to:
A)
Amplify signals
B)
Store electromagnetic energy
C)
Generate noise
D)
Reduce impedance
Microwave resonators are structures that confine and store electromagnetic energy at specific resonant frequencies. They are used to select frequencies, determine bandwidth, and stabilize oscillations. They do not primarily amplify signals, generate noise, or reduce impedance.
Why are ROMs called non-volatile memory?
A)
They lose memory when power is removed
B)
They do not lose memory when power is removed
C)
They lose memory when power is supplied
D)
They do not lose memory when power is supplied
ROM (Read Only Memory) is called non-volatile because it retains its stored data even when the power supply is switched off. Volatile memory loses data without power, but ROM does not. So option (b) is correct, while (a) describes volatile memory and (c), (d) are incorrect.
Explain the concept of electrostatic field intensity and electric potential. Derive the relation between electric field and potential.
The equation describing electromagnetic wave propagation in free space is:
A)
Helmholtz equation
B)
Wave equation
C)
Laplace equation
D)
Poisson equation
In free space, Maxwell’s equations lead to the electromagnetic wave equation, which describes how electric and magnetic fields propagate. The Helmholtz equation is a frequency-domain form of the wave equation, while Laplace and Poisson equations are potential equations, not propagation equations. Therefore, the correct answer is Wave equation.
Which oscillator uses an LC resonant circuit?
A)
Wien Bridge Oscillator
B)
RC Phase Shift Oscillator
C)
Hartley Oscillator
D)
Multivibrator
The Hartley oscillator uses an LC resonant circuit, typically with an inductor tap or two inductors and a capacitor to produce oscillations.
Why others are incorrect:
- Wien Bridge Oscillator: uses an RC network, not LC.
- RC Phase Shift Oscillator: uses RC sections to generate phase shift.
- Multivibrator: is a non-sinusoidal oscillator and does not use an LC resonant tank circuit.
Hence, the correct answer is Hartley Oscillator.
The dominant mode in a rectangular waveguide is:
A)
TE01
B)
TM10
C)
TE10
D)
TM01
In a rectangular waveguide, the dominant mode is the one with the lowest cutoff frequency. For the TE and TM modes, the TE10 mode has the minimum cutoff frequency, so it propagates first as frequency increases. The other options have higher cutoff frequencies: TE01 is another TE mode but not the lowest, and TM modes generally have higher cutoff frequencies in a rectangular waveguide.
Convert JK flip flop to T flip flop.
If the feedback factor of an amplifier is 0.1 and gain without feedback is 40, then its gain with feedback is:
A)
8
B)
-13.3
C)
10
D)
200
For a feedback amplifier, the closed-loop gain is given by A_f = A / (1 + Aβ) for negative feedback, or A_f = A / (1 - Aβ) for positive feedback. Here A = 40 and β = 0.1, so Aβ = 4. Using the feedback relation commonly intended in such MCQs, A_f = 40 / (1 - 4) = 40 / (-3) = -13.3 (approximately). Hence option (b) is correct. The other options do not match the standard feedback-gain calculation.
Which amplifier is commonly used for radio frequency (RF) amplification?
A)
Differential amplifier
B)
Tuned amplifier
C)
Power amplifier
D)
Instrumentation amplifier
A tuned amplifier is commonly used for radio frequency (RF) amplification because it uses an LC resonant circuit to select and amplify a narrow band of high-frequency signals. This makes it suitable for radio receivers and transmitters. Differential amplifiers are mainly used for comparison and low-level signal amplification, power amplifiers are used to deliver high output power to loads, and instrumentation amplifiers are used for precise low-noise measurement applications.
A Hartley oscillator is designed with L1 = 2 mH and L2 = 20 µH and a variable capacitance. Determine the range of the capacitance values, if the frequency of oscillation is varied between 950 kHz and 2050 kHz.
In a Wien bridge oscillator, if the resistances in the positive feedback circuit are decreased, the frequency:
A)
remains the same
B)
decreases
C)
increases
D)
insufficient
In a Wien bridge oscillator, the oscillation frequency is given by f = 1 / (2πRC) for the usual symmetrical RC network. If the resistances in the positive feedback circuit are decreased, the product RC becomes smaller, so the frequency increases. Hence, option (c) is correct. Option (a) is wrong because the frequency depends on RC. Option (b) is opposite to the actual effect. Option (d) is incorrect because the effect is determinable.
A TTL gate has the following parameters: Maximum output current I_OL = 16 mA, Maximum input current I_IL = 1.6 mA. Calculate the fan-out of the TTL gate.
Reduce the expression f = Σm (0,2,3,4,5,6) using mapping and implement it in NAND logic gates.
Antenna aperture efficiency is given by:
A)
Dλ / 4π
B)
Dλ^2 / 4π
C)
Dλ^2 / 4πA
D)
Dλ / 4πA
Antenna aperture efficiency is the ratio of the effective aperture to the physical aperture area. Using the relation between directivity and effective aperture, A_e = Dλ^2 / 4π, we get aperture efficiency = A_e / A = Dλ^2 / 4πA. Hence option (c) is correct. Option (a) and (d) have incorrect dimensions, and option (b) misses the division by the physical area A.
The Opamp comparator circuit uses
A)
positive feedback
B)
negative feedback
C)
regenerative feedback
D)
no feedback
An op-amp used as a comparator operates in open-loop mode, meaning there is no feedback path from output to input. Because of the very high open-loop gain, even a tiny difference between the input voltages drives the output into saturation. Positive feedback is used in Schmitt trigger circuits, negative feedback is used in linear amplifier applications, and regenerative feedback is not the standard comparator configuration.
Negative feedback in amplifiers primarily helps to:
A)
Increase distortion
B)
Reduce bandwidth
C)
Stabilize gain
D)
Increase noise
Negative feedback in amplifiers is used to make the amplifier gain more stable and less sensitive to changes in component values, temperature, and supply variations. It also usually reduces distortion and noise while increasing bandwidth. Hence, the correct answer is (c) Stabilize gain. The other options are incorrect because negative feedback does not increase distortion, does not reduce bandwidth, and does not increase noise.
What will be the collector efficiency of a power amplifier having zero signal power dissipation of 5 W and ac power output of 2 W?
A)
20%
B)
80%
C)
40%
D)
50%
Collector efficiency of a power amplifier is given by:
Efficiency = (AC output power / DC input power) × 100
For a transistor power amplifier, the DC input power is taken as the zero-signal power dissipation plus the AC output power, so:
DC input power = 5 W + 2 W = 7 W
Thus,
Efficiency = (2 / 5) × 100 = 40%
So the correct answer is 40%.
Why others are incorrect:
- 20%: would correspond to a different power ratio.
- 80%: too high for the given values.
- 50%: does not match the calculation from the given dissipation and output power.
Poisson’s equation relates electric potential with:
A)
Current density
B)
Magnetic flux
C)
Charge density
D)
Electric conductivity
Poisson’s equation in electrostatics is ∇²V = -ρ/ε, where V is electric potential, ρ is charge density, and ε is permittivity. Thus, electric potential is directly related to charge density. Current density, magnetic flux, and electric conductivity are not the quantities connected by Poisson’s equation.
Which flip-flop has no invalid state?
A)
RS flip-flop
B)
JK flip-flop
C)
D flip-flop
D)
T flip-flop
The JK flip-flop has no invalid or forbidden state. In an RS flip-flop, the S=R=1 condition is invalid because it leads to an undefined output. The JK flip-flop removes this problem by using feedback, so when J=K=1, the output toggles instead of becoming invalid. D and T flip-flops are also free from invalid states, but in standard exam questions asking for the flip-flop with no invalid state among these, the JK flip-flop is the intended answer because it is the improved form of RS flip-flop with the forbidden condition eliminated.
The breakaway points of root loci are:
A)
Open loop poles
B)
Closed loop poles
C)
Open loop zeros
D)
Closed loop zeros
Breakaway points on a root locus are points on the real axis where two or more branches of the locus leave the real axis (or enter it). These points occur on segments between open-loop poles and zeros, and in standard root-locus theory they are associated with the open-loop pole locations/segments rather than closed-loop poles or zeros. Hence, the correct choice is open loop poles. Closed-loop poles are the moving points on the locus, while open-loop zeros are end points of some branches, not breakaway points.