← MES Jr. Grade
Past Questions
For proper functioning of a clamper, time constant of the circuit should be
A)
large
B)
small
C)
equal to signal time period
D)
greater than five times the signal time period
A clamper must hold the capacitor charge almost constant during the non-conducting interval, so the RC time constant should be much larger than the signal period. A common rule is RC ≥ 5T, where T is the signal time period. If the time constant is small, the capacitor discharges too quickly and proper clamping will not occur. Hence, the correct choice is greater than five times the signal time period.
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.
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.
In single pulse modulation of PWM inverter, third harmonic can be eliminated if pulse width is equal to corresponding channel capacity will be nearly
A)
30°
B)
60°
C)
120°
D)
150°
In single-pulse PWM, the harmonic content depends on the pulse width. Third harmonic elimination occurs when the pulse width is adjusted so that the 3rd harmonic component becomes zero. For a symmetrical single pulse, this happens when the pulse width is about 120°. Hence the correct option is (c). The other values do not correspond to the standard third-harmonic elimination condition in single-pulse modulation.
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.
The main part of an inverter is
A)
oscillator circuit
B)
d.c. source
C)
step up transformer
D)
filter
An inverter converts DC into AC, and its key controlling part is the oscillator circuit, which generates the switching pulses needed to produce alternating output. The DC source only supplies input power, the step-up transformer is used in some inverter designs but is not the main part, and the filter is used to smooth the output waveform.
A step-up chopper has V as the source voltage and δ as the duty cycle. The output voltage for this chopper is given by
A)
V(1 + δ)
B)
V/(1 - δ)
C)
V(1 – δ)
D)
none of these
For a step-up chopper (boost chopper), the output voltage is related to the source voltage by the standard relation Vo = V / (1 - δ), where δ is the duty cycle. This comes from the fact that the inductor stores energy during the ON period and releases it to the load during the OFF period, raising the output above the input. Hence option (b) is correct. Option (a) is not the standard boost-chopper formula, and option (c) corresponds to a step-down type relation, not step-up.
The extremely high input impedance of a MOSFET is primarily due to the
A)
absence of its channel
B)
negative gate-source voltage
C)
depletion of current carriers
D)
extremely small leakage current of its gate capacitor
A MOSFET has an insulated gate separated from the channel by a thin oxide layer. Because of this insulation, the gate draws almost no current, so the input impedance is extremely high. Hence the high input impedance is primarily due to the extremely small leakage current of its gate capacitor. The other options do not explain the input impedance: absence of channel is not generally true for all operating conditions, negative gate-source voltage is not the main reason, and depletion of current carriers relates to device operation, not input impedance.
Out of the following three memories, which are of the combinational logic type? 1. Read only memory 2. Random access memory 3. Shift register memory
A)
1 and 2
B)
2 and 3
C)
1 and 3
D)
1, 2 and 3
Combinational logic circuits produce outputs based only on present inputs, without memory of past states. In digital electronics, ROM is treated as a combinational memory because its output depends on the addressed input lines. Shift register memory is also based on combinational logic with storage elements arranged to shift data. RAM, however, is a sequential memory because it uses storage cells and retains state. Therefore, the correct combination is 1 and 3.
In microprocessor architecture, flag indicates
A)
the number of the microprocessor
B)
the name of the manufacturer
C)
the internal status of the CPU
D)
the bit size of the microprocessor
In a microprocessor, the flag register contains status flags such as zero, carry, sign, overflow, and parity. These flags indicate the internal status of the CPU after arithmetic or logical operations. Therefore, the correct answer is the internal status of the CPU. The other options are incorrect because flags do not represent the processor number, manufacturer name, or bit size.
Which stage of a D.C. power supply uses a Zener as the main component?
A)
rectifier
B)
voltage divider
C)
regulator
D)
filter
A Zener diode is commonly used in the regulation stage of a D.C. power supply because it maintains an almost constant output voltage despite changes in input voltage or load current. A rectifier converts AC to DC, a filter smooths the pulsations, and a voltage divider only splits voltage—it does not provide regulation.
The output of each gate is connected to the input of the other and this feedback combination is called
A)
Transistor
B)
MOSFETs
C)
flip-flops
D)
FETs
When the output of each gate is fed back to the input of the other gate, the circuit forms a bistable feedback arrangement. Such a circuit is called a flip-flop. The other options are types of transistors/devices, not feedback memory circuits.
An ideal OP-AMP has
A)
infinite open-loop gain
B)
infinite input resistance
C)
Zero output resistance
D)
all of these
An ideal op-amp is defined by several ideal characteristics: infinite open-loop gain, infinite input resistance, and zero output resistance. Since options (a), (b), and (c) are all true, the correct answer is (d) all of these.
FET has offset voltage of approximately
A)
0.2 volt
B)
0.6 volt
C)
1.1 volt
D)
zero
A FET (Field Effect Transistor) is a voltage-controlled device and its input gate current is ideally zero. Therefore, the offset voltage is approximately zero. The other values (0.2 V, 0.6 V, 1.1 V) are associated with diode or BJT junction forward voltages, not FET offset voltage.
The modulation index in FM signal depends on
A)
amplitude of the modulating signal
B)
frequency of the modulating signal
C)
both (a) and (b)
D)
none of these
In frequency modulation (FM), the modulation index is defined as β = Δf / fm, where Δf is the frequency deviation and fm is the modulating signal frequency. The frequency deviation Δf is proportional to the amplitude of the modulating signal, so the modulation index depends on the amplitude of the modulating signal as well as the frequency of the modulating signal. Therefore, the correct answer is both (a) and (b).
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'.
The noise figure of an amplifier is 6dB. If the input S/N ratio is 38 dB, then the output S/N ratio
A)
44 dB
B)
40 dB
C)
36 dB
D)
32 dB
Noise figure in dB is the degradation in signal-to-noise ratio from input to output. So,
Output S/N (dB) = Input S/N (dB) - Noise figure (dB) = 38 - 6 = 32 dB.
However, the options given do not include 32 dB as the result of this standard relation. If the question intended noise factor addition in terms of noise figure affecting S/N directly, the correct computed output is 32 dB, which matches option (d), not (c). Therefore the correct option is (d) 32 dB.
The number of flip flops required in a decade counter is
A)
2
B)
3
C)
4
D)
10
A decade counter counts 10 states (0 to 9). The number of flip-flops required is the smallest n such that 2^n >= 10. Since 2^3 = 8 is not enough and 2^4 = 16 is sufficient, 4 flip-flops are required. Hence, option (c) is correct.
100% modulation is produced in AM when carrier
A)
frequency equals signal frequency
B)
frequency exceeds signal frequency
C)
amplitude equals signal amplitude
D)
amplitude exceeds signal amplitude
In amplitude modulation (AM), 100% modulation occurs when the modulating signal amplitude is equal to the carrier amplitude, i.e., modulation index m = 1. At this point, the envelope just reaches zero at its minimum without distortion. If the signal amplitude exceeds the carrier amplitude, overmodulation occurs and distortion results. Frequency is not the deciding factor for modulation percentage.
A push-pull amplifier suppresses
A)
only the second harmonic component
B)
only the odd harmonic components
C)
only the even harmonic components
D)
both odd and even harmonic components
In a push-pull amplifier, the two transistors/valves conduct on alternate half-cycles and their outputs are combined in such a way that the even-order harmonics cancel out. This is because the even harmonics are in phase opposition in the two halves, while the odd harmonics add. Therefore, a push-pull amplifier suppresses only the even harmonic components. Option (a) is too specific, and (b) and (d) are incorrect.
Negative feedback in an amplifier
A)
lowers its lower 3 dB frequency
B)
raises its upper 3 dB frequency
C)
increases its bandwidth
D)
all of these
Negative feedback in an amplifier reduces gain but improves frequency response. It lowers the lower cutoff frequency, raises the upper cutoff frequency, and therefore increases the overall bandwidth. Hence all the given statements are correct.
A PIN photodiode has ultrafast response due to
A)
the presence of middle I-layer
B)
heavy doping of P and N regions
C)
higher electrical conductivity of Si
D)
the wide spectral response
A PIN photodiode has an intrinsic (I) layer between the P and N regions. This wide depletion region reduces junction capacitance and allows faster carrier collection, giving an ultrafast response. Heavy doping would increase recombination, higher conductivity of Si is not the reason for speed, and wide spectral response is unrelated to response time.
Which of the following systems is preferred for chopper drives?
A)
Constant frequency system
B)
Variable frequency system
C)
constant voltage system
D)
none of these
Chopper drives are generally preferred with a constant frequency system because it gives better control, simpler design, and easier filtering of ripple. In chopper control, the output voltage is varied by changing the duty ratio while keeping the switching frequency fixed. Variable frequency systems are more common in some other drive/control schemes, but for chopper drives constant frequency operation is preferred. Hence, option (a) is correct.
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.
A grinding wheel is said to be of _________ if the abrasive grains can be easily dislodged.
A)
soft grade
B)
medium grade
C)
hard grade
D)
none of these
Option A: soft grade. This matches the standard definition or established fact for the concept asked.