← MES Jr. Grade
Past Questions
In a 3-phase semi-converter, the three SCRs are triggered an interval of
A)
60°
B)
90°
C)
120°
D)
180°
In a 3-phase semi-converter, the three SCRs are fired sequentially with a phase displacement of 60° electrical between successive triggering pulses. This is because the three-phase supply has 120° phase separation, and the semi-converter uses one SCR from each phase in a 6-pulse sequence. Hence, the triggering interval between the SCRs is 60°. The other options are incorrect because 90° and 180° do not match the 3-phase firing sequence, and 120° is the phase displacement of the supply phases, not the SCR triggering interval.
Which of the following semiconductor power device is not a current triggered device?
A)
Thyristor
B)
G.T.O.
C)
Triac
D)
MOSFET
Thyristor, GTO, and Triac are current-triggered devices because they require gate current for turn-on (and in the case of GTO, gate current for turn-off as well). A MOSFET is a voltage-controlled device, not a current-triggered device, since its gate is insulated and needs gate voltage rather than gate current.
Turn-on time for an SCR is 10 µsec. If an inductance is inserted in the anode circuit, then the turn-on time will be
A)
10 µsec
B)
more than 10 µsec
C)
less than 10 µsec
D)
depends on the value of inductance
Inserting an inductance in the anode circuit opposes the rise of anode current (di/dt). Since SCR turn-on requires the current to build up from zero to the conducting state, a slower current rise increases the turn-on time. Therefore, the turn-on time becomes more than 10 µsec. Option (a) is wrong because the inductance changes the switching dynamics. Option (c) is wrong because inductance does not speed up turn-on. Option (d) is not the best answer here because the effect is definitely to increase turn-on time.
In a JFET, the amplification factor µ, transconductance gm and the dynamic drain resistance rd are related as
A)
µ = gm /rd
B)
µ = rd /gm
C)
µ = gm .rd
D)
µ = gm . rd^2
For a JFET, the small-signal amplification factor is defined as the ratio of change in drain-source voltage to change in gate-source voltage at constant drain current. The standard relation among the three parameters is: µ = gm × rd, where gm is transconductance and rd is dynamic drain resistance. Hence option (c) is correct. The other options have incorrect dimensional/formula relationships.
A thyristor chopper is preferred over other choppers because it
A)
provides static switching
B)
is economical
C)
can be manufactured easily
D)
none of these
A thyristor chopper is preferred mainly because it is economical compared with other chopper implementations. Thyristors are rugged, reliable, and relatively low-cost devices for high-power switching. Option (a) is not the best reason because all choppers are static electronic switches by nature. Option (c) is not the key advantage, since ease of manufacture is not the primary basis for preference. Therefore, the correct answer is (b) is economical.
The numbers of thyristors required for 1-phase to 1-phase cyloconverter of the mid-point type and for 3-phase to 3-phase 3-pulse type are respectively
A)
4 and 6
B)
4 and 4
C)
4 and 9
D)
4 and 18
For a 1-phase to 1-phase mid-point cycloconverter, two full-wave converters are used, and each needs 2 thyristors, so total = 4 thyristors. For a 3-phase to 3-phase 3-pulse cycloconverter, each 3-pulse converter uses 3 thyristors, and there are 6 such converters in the standard arrangement, so total = 18 thyristors. Hence the correct answer is 4 and 18.
The interface chip used for data transmission between 8086 and a 16-bit ADC is
A)
8259
B)
8255
C)
8253
D)
8251
The 8255 Programmable Peripheral Interface (PPI) is used to interface 8086 with peripheral devices, including ADCs, for parallel data transmission. A 16-bit ADC needs parallel input/output control, which is provided by 8255. 8259 is an interrupt controller, 8253 is a timer/counter, and 8251 is a serial communication interface, so they are not suitable here.
An amplifier has a power gain of 50. This gain in dB will be
A)
17 dB
B)
31 dB
C)
34 dB
D)
68 dB
Power gain in decibels is given by 10 log10(P2/P1). Here the power gain is 50, so dB gain = 10 log10(50) = 10 × 1.699 ≈ 16.99 dB ≈ 17 dB. Hence, option (a) is correct. The other options do not match the standard decibel conversion for power gain.
The number of diodes required in a bridge rectifier circuit is
A)
one
B)
two
C)
three
D)
four
A bridge rectifier uses four diodes arranged in a bridge configuration. During each half-cycle of AC input, two diodes conduct and two are reverse-biased, but the circuit as a whole requires 4 diodes. Hence, the correct answer is four.
The ASCII code is a
A)
5-bit code
B)
7-bit code
C)
9-bit code
D)
11-bit code
ASCII stands for American Standard Code for Information Interchange. The original ASCII standard uses 7 bits, allowing 128 characters (0 to 127). Therefore, the correct answer is 7-bit code. The other options are incorrect because ASCII is not a 5-bit, 9-bit, or 11-bit code.
The main factor which makes a MOSFET likely to break down during normal handling is its
A)
very low gate capacitance
B)
high leakage current
C)
high input resistance
D)
both (a) and (c)
A MOSFET has an insulated gate, so its gate capacitance is very low and its input resistance is extremely high. Because of this, even a small static charge or improper handling can create a large gate-to-source voltage and damage the thin gate oxide. Hence the correct choice is both very low gate capacitance and high input resistance. High leakage current is not the main reason for breakdown during handling.
The potential barrier existing across a PN junction
A)
facilitates recombination of electrons and holes
B)
prevents flow of minority carriers
C)
prevents total recombination of holes and minority carriers
D)
prevents neutralization of acceptor and donor ions
In a PN junction, diffusion of electrons and holes near the junction leaves behind fixed ionized donor ions on the N-side and ionized acceptor ions on the P-side. This creates the depletion region and the built-in potential barrier. The barrier opposes further diffusion and thus prevents the neutralization of these immobile donor and acceptor ions. Option (a) is wrong because the barrier actually opposes carrier movement rather than facilitating recombination. Option (b) is incorrect because minority carriers are mainly affected by the junction field in reverse bias, not by the definition of the barrier itself. Option (c) is vague and not the correct physical description.
In FM, when frequency deviation is doubled
A)
modulation is doubled
B)
modulation is halved
C)
carrier swing is halved
D)
modulation index is decreased
In frequency modulation (FM), the modulation index is directly proportional to the frequency deviation. If the frequency deviation is doubled, the modulation index also doubles. Therefore, the correct option is (a) modulation is doubled. The other options are incorrect because halving the modulation or carrier swing would happen if deviation decreased, and the modulation index does not decrease when deviation increases.
The output of a 2-input OR gate is zero only when
A)
both inputs are 0
B)
either input is 0
C)
both inputs are 1
D)
either input is 1
For a 2-input OR gate, the output is 1 if at least one input is 1. Therefore, the output becomes 0 only when both inputs are 0. If either input is 1, the output is 1, so options (b), (c), and (d) are incorrect.
In amplitude modulation
A)
carrier frequency is changed
B)
carrier amplitude is changed
C)
three sidebands are produced
D)
frequency is improved
In amplitude modulation (AM), the amplitude of the carrier wave is varied in accordance with the message signal, while the carrier frequency and phase remain essentially constant. Hence, option (b) is correct. Option (a) describes frequency modulation, option (c) is incorrect because standard AM produces two sidebands, and option (d) is not a property of AM.
The stack pointer in the 8085 microprocessor is a
A)
16-bit register that point to stack memory locations
B)
a 16-bit accumulator
C)
memory location in the stack
D)
flag register used for the stack
In the 8085 microprocessor, the stack pointer (SP) is a 16-bit register. It stores the address of the top of the stack in memory, so it points to stack memory locations. Option (b) is wrong because the accumulator is an 8-bit register used for arithmetic and logic operations. Option (c) is wrong because the stack pointer is not a memory location itself; it is a register. Option (d) is wrong because flags are stored in the flag register, not the stack pointer.
If two stages of a cascaded amplifier have decibel gains of 60 and 30, then the overall gain is
A)
90dB
B)
1800dB
C)
2dB
D)
0.5dB
For cascaded amplifier stages, gains in decibels add directly. So the overall gain = 60 dB + 30 dB = 90 dB. The other options are incorrect because 1800 dB is an impossible arithmetic error, and 2 dB or 0.5 dB are far too small for the given stage gains.
When a transistor is used as a switch, its operation is confined in
A)
cut-off region
B)
saturation region
C)
active region
D)
both (a) and (b)
When a transistor is used as a switch, it operates only in the two extreme states: cut-off and saturation. In cut-off, the transistor is OFF and no collector current flows. In saturation, the transistor is fully ON and conducts maximum current. The active region is used for amplification, not switching. Therefore, the correct answer is both cut-off and saturation regions.
In a microprocessor, the address of the next instruction to be executed is stored in
A)
stack pointer
B)
address latch
C)
program counter
D)
general purpose register
The Program Counter (PC) stores the address of the next instruction to be executed in a microprocessor. After each instruction fetch, the PC is automatically updated to point to the following instruction. The stack pointer stores the top of the stack, the address latch temporarily holds address information during bus operations, and general-purpose registers are used for data manipulation, not for tracking the next instruction.
The effect of noise in a communication system is most adverse with reference to
A)
source
B)
encoder
C)
channel
D)
receiver
Noise is most harmful at the receiver because the receiver must detect and reconstruct the original message from the noisy signal. Any noise added in the channel ultimately degrades the signal quality seen at the receiver, affecting demodulation, decoding, and overall intelligibility. The source and encoder generate/process the message, but the receiver is where the adverse effect of noise is most directly observed.
Out of the following devices, the fastest switching device is
A)
JEFT
B)
BJT
C)
MOSFET
D)
triode
MOSFETs are the fastest switching devices among the given options because they are majority-carrier devices and do not suffer from charge storage delay. A JFET is also a field-effect device, but MOSFETs are generally preferred for very fast switching in digital and power applications. A BJT is slower due to minority-carrier storage effects, and a triode vacuum tube is much slower and obsolete for such applications.
For an FM wave of carrier 100 MHz, modulating frequency 10 KHz and the maximum frequency deviation of 1 MHz, the bandwidth required is nearly
A)
20 KHz
B)
1 MHz
C)
2 MHz
D)
5 MHz
For an FM signal, the approximate bandwidth is given by Carson’s rule:
Bandwidth ≈ 2(Δf + fm)
Here, maximum frequency deviation Δf = 1 MHz and modulating frequency fm = 10 kHz = 0.01 MHz.
So,
Bandwidth ≈ 2(1 + 0.01) MHz = 2.02 MHz ≈ 2 MHz.
Hence, the correct answer is (c) 2 MHz.
Why others are incorrect:
- (a) 20 kHz is far too small.
- (b) 1 MHz ignores the doubling in Carson’s rule.
- (d) 5 MHz is larger than the required approximate bandwidth.
A bi-stable multi-vibrator (BMV) is a .............. oscillator
A)
triggered
B)
free-running
C)
sine-wave
D)
sawtooth
A bi-stable multivibrator has two stable states and changes state only when an external trigger pulse is applied, so it is a triggered oscillator. A free-running oscillator operates without external triggering, while sine-wave and sawtooth refer to waveform types, not the operating mode of a bi-stable multivibrator.
According to Boolean algebra, (A + Ā) equals
A)
A
B)
1
C)
0
D)
Ā
By the complement law in Boolean algebra, a variable ORed with its complement equals 1: A + Ā = 1. So the correct option is (b). The other options are incorrect because A + Ā is not A, not 0, and not Ā.
In AM system, full information can be conveyed by transmitting only
A)
the carrier
B)
the upper sideband
C)
the lower sideband
D)
any one sideband
In amplitude modulation (AM), the information is contained in the sidebands, not in the carrier itself. Since the upper sideband and lower sideband are mirror images and each contains the complete message information, transmitting only one sideband is sufficient to convey full information. The carrier alone carries no information, and a single specified sideband (upper or lower) can be used. Therefore, the correct answer is 'any one sideband'.