← MES Jr. Grade
Past Questions — 2012
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.
The main advantage claimed for ECL family of logic gates is its
A)
very large fan-in
B)
use of negative power supply voltage
C)
extremely low propagation time
D)
least power dissipation
ECL (Emitter-Coupled Logic) is mainly known for its very high switching speed, i.e., extremely low propagation delay. It achieves this by avoiding transistor saturation, which makes transitions fast. The other options are not the main advantage: ECL does not have the least power dissipation, and the use of a negative supply voltage is a circuit requirement/feature, not its primary advantage. Very large fan-in is also not its chief claimed advantage.
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.
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.
An SCR is operated
A)
only in reverse-biased condition
B)
only in forward biased condition
C)
both forward and reverse biased conditions
D)
none of these
An SCR (Silicon Controlled Rectifier) is a unidirectional controlled device. It is normally operated in the forward-biased condition, where it can be triggered into conduction by a gate pulse. In reverse bias, it blocks current and is not used for normal operation. Hence, the correct option is (b). Option (a) is wrong because reverse bias is for blocking, not operation. Option (c) is wrong because an SCR does not conduct in both directions like a bidirectional device.
Which of the following logic Gates are universal Gates?
A)
OR and AND Gates
B)
NOR and NAND Gates
C)
OR and NOR Gates
D)
AND and NAND Gates
Universal gates are those using which any Boolean function can be implemented. NAND and NOR are universal because each can be used alone to realize NOT, AND, and OR operations. OR and AND are not universal by themselves, and the other pairs include at least one non-universal gate.
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.
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).
For intelligible voice communication, minimum bandwidth required is
A)
900 Hz
B)
1 KHz
C)
3 KHz
D)
6 KHz
For intelligible voice communication, the essential speech frequencies are roughly from 300 Hz to 3400 Hz, so the required bandwidth is about 3 kHz. This is why standard telephone voice channels are designed with a bandwidth of around 3 kHz. 900 Hz and 1 kHz are too narrow for clear speech, while 6 kHz is wider than the minimum required.
In RC phase shift oscillator circuits
A)
there is no need for feedback
B)
feedback factor is less than unity
C)
pure sine wave output is possible
D)
transistor parameters determine oscillation frequency
An RC phase shift oscillator uses an RC feedback network to provide the required phase shift and positive feedback. It is designed to generate a sinusoidal waveform, so a pure sine wave output is possible. Option (a) is wrong because feedback is essential for oscillation. Option (b) is not the defining feature; the feedback network must satisfy the Barkhausen criterion, not simply be less than unity. Option (d) is wrong because the oscillation frequency is mainly determined by the RC network, not transistor parameters.
In frequency modulation and phase modulation
A)
frequency is constant
B)
phase of the carrier is constant
C)
carrier amplitude is constant
D)
all of these
In both frequency modulation (FM) and phase modulation (PM), the amplitude of the carrier remains constant while its frequency or phase varies according to the modulating signal. Hence, option (c) is correct. Option (a) is incorrect because in FM the frequency changes; option (b) is incorrect because in PM the phase changes; therefore option (d) is also incorrect.
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 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.
Which of the followings is an operational state of a thyristor?
A)
triggering
B)
firing
C)
regulating
D)
reverse blocking
A thyristor has standard operating states such as forward blocking, forward conduction, and reverse blocking. Among the given options, reverse blocking is a recognized operational state. Triggering and firing refer to the process of turning the device ON, not a state. Regulating is not an operational state of a thyristor.
The Fourier transform of the function f(t) = 1, is
A)
unity
B)
(ω)
C)
2 (ω)
D)
(ω)
Using the standard Fourier transform convention
F(ω) = ∫_{-∞}^{∞} f(t)e^{-jωt}dt,
for f(t)=1 we get
F(ω) = ∫_{-∞}^{∞} e^{-jωt}dt = 2πδ(ω).
So the transform of a constant signal is an impulse at zero frequency with strength 2π. The other options are incorrect because unity is not a transform here, and πδ(ω) or δ(ω) miss the correct scaling factor.
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