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Past Questions — 2014
Out of the following devices, the fastest switching device is
A)
JFET
B)
BJT
C)
MOSFET
D)
Triode
Among the given devices, MOSFET is the fastest switching device. It is a majority-carrier device, so it has no charge-storage delay like BJT, making switching much quicker. JFET is also a field-effect device but generally slower than MOSFET in switching applications. BJT is slower because of minority-carrier storage effects. Triode is an older vacuum tube device and is not considered faster than MOSFET in modern switching contexts.
The bit sequence 0010 is serially entered (right-most bit first) into a 4-bit parallel out shift register that is initially clear. What are the Q outputs after two clock pulses?
A)
0000
B)
0010
C)
1000
D)
1111
A 4-bit shift register is initially clear, so Q3 Q2 Q1 Q0 = 0000. The sequence 0010 is entered serially with the right-most bit first, so the input bits are 0, then 1, then 0, then 0. After two clock pulses, the first two entered bits are 0 and 1. These shift into the register as 1000 (the first bit moves toward the leftmost stage in a standard serial-in shift register). Therefore, the Q outputs after two clock pulses are 1000. The other options do not match the shifted bit positions.
The main reason for the variation of amplifier gain with frequency is
A)
the presence of capacitances, both external and internal
B)
due to inter-stage transformers
C)
the logarithmic increase in its output power
D)
the Miller effect
Amplifier gain varies with frequency mainly because capacitors and other reactive elements have frequency-dependent impedance. Internal junction capacitances and external coupling/bypass capacitors affect the signal differently at low, mid, and high frequencies, causing gain to change. The Miller effect is one important internal capacitance-related phenomenon, but it is not the main general reason by itself. Inter-stage transformers and logarithmic output power are not the primary cause of frequency-dependent gain variation.
The effect of noise in a communication system is most adverse with reference to
A)
source
B)
channel
C)
encoder
D)
receiver
Noise is most harmful at the receiver because the receiver must detect, amplify, and recover the original message from the noisy signal. Any noise added in the channel ultimately affects the signal quality at the receiver, reducing SNR and causing errors in demodulation/decoding. The source and encoder do not directly suffer the adverse effect of channel noise, and the channel is the medium where noise is introduced, not where its effect is most critical.
The AM broadcast band is given by
A)
10 KHz to 30 KHz
B)
500 KHz to1500 KHz
C)
3 KHz to 30 MHz
D)
30 MHz to 300 MHz
The standard AM broadcast band lies in the medium-frequency range, approximately 530 kHz to 1700 kHz internationally, and commonly taught in exams as 500 kHz to 1500 kHz. Hence option (b) is correct. Option (a) is too low, option (c) is the HF range, and option (d) is the VHF range.
PROM is
A)
permanent read only memory
B)
polarized read only memory
C)
positive read only memory
D)
programmable read only memory
PROM stands for Programmable Read Only Memory. It is a type of memory whose contents can be programmed once after manufacturing. The other options are incorrect because PROM does not mean permanent, polarized, or positive read only memory.
A zener diode
A)
Has a high forward voltage rating
B)
Has a sharp breakdown at low reverse voltage
C)
Is useful as an amplifier
D)
Has a negative resistance
A Zener diode is designed to operate in the reverse-bias breakdown region at a well-defined, low reverse voltage called the Zener voltage. It therefore has a sharp breakdown characteristic. Option (a) is wrong because Zener diodes are not characterized by high forward voltage rating. Option (c) is wrong because they are mainly used for voltage regulation/reference, not amplification. Option (d) is not the best description here; while the breakdown region can show a small dynamic resistance, the defining feature asked in MCQs is the sharp breakdown at low reverse voltage.
The most highly doped region in a transistor is
A)
base
B)
collector
C)
emitter
D)
both emitter and collector
In a transistor, the emitter is the most heavily doped region because its job is to inject a large number of charge carriers into the base. The base is lightly doped and very thin, while the collector is moderately doped to collect carriers and withstand higher voltage. Therefore, the emitter has the highest doping level.
A unique advantageous feature of CMOS family is its
A)
use of NMOS circuits
B)
power dissipation in nanowatt range
C)
speed
D)
dependence on frequency for power dissipation
CMOS (Complementary MOS) is known for very low static power consumption compared with TTL and other logic families. Its unique advantageous feature is its extremely low power dissipation, often in the nanowatt range when not switching. Option (a) is incorrect because CMOS uses both NMOS and PMOS transistors, not only NMOS. Option (c) is incorrect because CMOS is generally not chosen for the highest speed. Option (d) is incorrect because power dissipation in CMOS is not the main advantage due to frequency dependence; in fact, dynamic power increases with switching frequency, but the classic distinctive advantage is very low power dissipation.
Out of the following semiconductor power devices, which one is not a current triggered device?
A)
thyristor
B)
GTO
C)
TRIAC
D)
MOSFET
Thyristor, GTO, and TRIAC are current-triggered devices because they require gate current to turn on. A MOSFET is a voltage-controlled device; it is triggered by gate-to-source voltage, not by gate current. Therefore, MOSFET is not a current-triggered device.
A common source FET amplifier has a load resistance RL = 500kΩ, rd = 100 kΩ and µ = 24. The voltage gain is
A)
30
B)
25
C)
20
D)
100
For a common source FET amplifier, the voltage gain is approximately A_v = μ(R_D || r_d) / r_d, or equivalently A_v = g_m(R_D || r_d). Here the load resistance is RL = 500 kΩ and the drain resistance is rd = 100 kΩ, so the effective load is the parallel combination: R = (500 × 100)/(500 + 100) kΩ = 83.33 kΩ. Using the given amplification factor μ = 24, the gain becomes A_v = μ × R / rd = 24 × 83.33 / 100 ≈ 20. Thus the correct answer is 20. The other options do not match the calculated gain.
Which one of the following specifications is not correct for a common collector amplifier?
A)
High input impedance
B)
Low output impedance
C)
High voltage gain
D)
High current gain
A common collector amplifier (emitter follower) has high input impedance, low output impedance, and high current gain. Its voltage gain is approximately unity, not high. Therefore, 'High voltage gain' is the incorrect specification.
In a microprocessor, the bus which is used to transfer data from main memory to peripheral device is
A)
data bus
B)
input bus
C)
DMA bus
D)
output bus
The bus used to transfer data from main memory to a peripheral device is the output bus, because the data is moving out from memory/processor system toward an external device. The data bus is a general-purpose bidirectional bus used for transferring data both ways, while 'input bus' is not the standard term here. DMA refers to a transfer technique, not the specific bus name in this context.
In a microprocessor, an instruction register is storage for
A)
location of data in memory
B)
location of instruction in memory
C)
address of the next instruction to be executed
D)
binary code for the instruction to be executed
The instruction register (IR) holds the current instruction fetched from memory in binary form so that the control unit can decode and execute it. It does not store the data location, the instruction location, or the address of the next instruction. The address of the next instruction is typically kept in the program counter (PC).
The most useful modulation technique for high fidelity audio broadcasting on radio in current practice is
A)
amplitude modulation
B)
frequency modulation
C)
pulse amplitude modulation
D)
pulse code modulation
High-fidelity audio broadcasting requires better noise immunity and wider bandwidth for improved sound quality. Frequency modulation (FM) is preferred in radio broadcasting because it is much less affected by amplitude noise and provides better audio fidelity than amplitude modulation (AM). Pulse amplitude modulation and pulse code modulation are used in digital/telecommunication systems, not standard radio audio broadcasting.
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 must satisfy 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. Options (a), (b), and (d) are incorrect because 2 or 3 flip-flops cannot represent 10 states, while 10 flip-flops are far more than needed.
A carrier is simultaneously modulated by two sine waves with modulation indices of 0.4 and 0.3. The resultant modulation index will be
A)
1.0
B)
0.7
C)
0.5
D)
0.35
For simultaneous modulation by two sine waves, the resultant modulation index is found by the root-sum-square method: m = √(m1² + m2²). Here, m1 = 0.4 and m2 = 0.3, so m = √(0.4² + 0.3²) = √(0.16 + 0.09) = √0.25 = 0.5. Hence, the correct answer is 0.5. The other options do not match this standard result.
The demodulation of the sound signals in a TV receiver is accomplished by
A)
linear detector
B)
product detector
C)
discriminator
D)
envelope detector
In a TV receiver, the sound signal is frequency-modulated (FM), so its demodulation is done by a discriminator (FM detector). A linear detector or envelope detector is used for AM signals, while a product detector is used for synchronous detection, not standard TV sound FM demodulation.
A power chopper converts
A)
A.C to D.C.
B)
D.C. to D.C.
C)
D.C. to A.C.
D)
A.C. to A.C.
A power chopper is a static power electronic device used to convert a fixed DC input into a variable DC output. Therefore, it performs DC to DC conversion. The other options correspond to different converters: AC to DC is a rectifier, DC to AC is an inverter, and AC to AC is an AC voltage controller or cycloconverter.
When used in circuit, the Zener diode is always
A)
forward-biased
B)
connected in series
C)
troubled by overheating
D)
reverse-biased
A Zener diode is designed to operate in the reverse-bias region, where it maintains an almost constant voltage after breakdown. That is why it is used as a voltage regulator. It is not normally used in forward bias for regulation, and it is not defined by being connected in series or by overheating.
A modulator is a system to
A)
separate two frequencies
B)
impress the information on to a radio frequency carrier
C)
extract information from the carrier
D)
amplify the audio frequency signal
A modulator is used to superimpose or impress the message/information signal onto a high-frequency carrier wave for transmission. This is the basic process of modulation in communication systems. Option (c) describes a demodulator, which extracts information from the carrier. Options (a) and (d) are unrelated to the function of a modulator.
If the peak value of the input voltage to a half wave rectifier is 28.28 volts and no filter is used the maximum dc voltage across the load will be
A)
20.2V
B)
15 V
C)
9 V
D)
14.14 V
For an ideal half-wave rectifier without filter, the average DC output voltage is Vdc = Vm / π, where Vm is the peak input voltage. Here Vm = 28.28 V, so Vdc = 28.28 / π ≈ 9.0 V. Hence the maximum DC voltage across the load is 9 V. The other options do not match the standard half-wave rectifier average value formula.
Which of the following Boolean rules is correct?
A)
A + 0 = 0
B)
A + 1 = 1
C)
A + A = A.A
D)
A + AB = A + B
In Boolean algebra, the OR operation with 1 always gives 1, so A + 1 = 1 is correct. The other options are incorrect: (a) should be A + 0 = A, not 0; (c) should be A + A = A, while A·A = A; (d) the correct absorption law is A + AB = A, not A + B.
The FM radio frequency range is approximately
A)
250-300 MHz
B)
150-200 MHz
C)
90-105 MHz
D)
30-70 MHz
FM broadcast radio is commonly allocated around 88 to 108 MHz. Among the given options, 90-105 MHz is the closest and correct approximate range. The other ranges correspond to different frequency bands and are not used for standard FM radio broadcasting.
With gate open, if the supply voltage exceeds the break over voltage of SCR, then SCR will conduct
A)
false
B)
true
C)
for dc only
D)
for ac only
An SCR can turn on without gate signal if the anode-to-cathode voltage exceeds its breakover voltage. With the gate open, this is called forward breakover triggering, so the SCR will conduct. Hence the statement is true. Options (c) and (d) are incorrect because this behavior is not limited to only DC or only AC; it depends on the applied voltage exceeding breakover.