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Past Questions — 2015
Which 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, also called an emitter follower, is known for high input impedance, low output impedance, and high current gain. Its voltage gain is approximately unity (slightly less than 1), not high. Therefore, 'High voltage gain' is the incorrect specification.
Which of the following has the longest wavelength?
A)
HF
B)
VHF
C)
UHF
D)
SHF
In the radio-frequency spectrum, wavelength is inversely proportional to frequency. Among the given bands, HF (High Frequency) has the lowest frequency range, so it has the longest wavelength. VHF, UHF, and SHF have progressively higher frequencies and therefore shorter wavelengths.
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 carry, zero, sign, overflow, etc. These flags represent 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 indicate the processor number, manufacturer name, or bit size.
In AM, the carrier is changed by a modulating signal. What parameter of the carrier is changed?
A)
amplitude
B)
frequency
C)
pulse width
D)
phase
In Amplitude Modulation (AM), the amplitude of the carrier wave is varied in accordance with the modulating signal, while its frequency and phase remain essentially constant. Therefore, the carrier parameter changed in AM is amplitude. The other options correspond to other modulation types: frequency in FM, phase in PM, and pulse width in PWM.
A thyristor controlled reactor is used to get
A)
variable resistance
B)
variable capacitance
C)
variable inductance
D)
improved reactive power factor
A thyristor controlled reactor (TCR) is a shunt-connected reactor whose effective inductive reactance is varied by controlling the firing angle of the thyristors. By changing the conduction period, the fundamental current through the reactor is adjusted, so the reactor behaves like a variable inductance. It is not used to provide variable resistance or capacitance. The improved reactive power factor is a result of its application in reactive power compensation, but the direct effect of a TCR is variable inductance.
Transistor is a
A)
current controlled current device
B)
current controlled voltage device
C)
voltage controlled voltage device
D)
voltage controlled current device
A transistor (specifically BJT) works by a small base current controlling a much larger collector current, so it is called a current controlled current device. Option (d) describes a voltage-controlled current device, which is the behavior of a FET, not a BJT. The other options do not match standard transistor operation.
Which logic gates are designated as universal gates?
A)
NOT, OR and AND gates
B)
XNOR, NOR and NAND gates
C)
NOR and NAND gates
D)
XOR, NOR and NAND gates
Universal gates are those using which any Boolean function can be implemented. NOR and NAND are called universal because each of them alone can be used to build NOT, OR, AND, and all other basic logic operations. The other options include gates like XOR, XNOR, NOT, OR, and AND, but these are not designated as universal gates.
The TIMER of PPI 8155 is of
A)
16 bits
B)
14 bits
C)
12 bits
D)
8 bits
The Intel 8155 contains a built-in programmable timer, and its timer is 14-bit wide. Therefore, the correct answer is 14 bits. The other options are incorrect because 16-bit, 12-bit, and 8-bit do not match the standard specification of the 8155 timer.
The anode current through a conducting SCR is 10A. If its gate current is made one-fourth, then what will be the anode current?
A)
0
B)
5A
C)
10 A
D)
20 A
In an SCR, once it is turned ON and is conducting, the anode current is determined by the external circuit, not by the gate current. The gate current is only needed to trigger the SCR; after conduction starts, reducing the gate current does not reduce the anode current (as long as it remains above the holding current conditions). Therefore, the anode current remains 10 A.
A broadcast AM radio transmitter radiates 125 kW when the modulation percentage is 70. How much of this is carrier power?
A)
≈25 kW
B)
50 kW
C)
75 kW
D)
100 kW
For standard AM, total transmitted power is P_t = P_c(1 + m^2/2), where P_c is carrier power and m is modulation index. Given P_t = 125 kW and m = 0.7, we get P_t = P_c(1 + 0.49/2) = P_c(1.245). So P_c = 125/1.245 ≈ 100.4 kW. However, the options suggest the intended simplified relation may be based on power in sidebands or a textbook approximation. For the usual AM power formula, the carrier power is about 100 kW, which matches option (d), not (c).
An FM wave uses a 2-5 V, 500 Hz modulating frequency and has a modulation index of 50. The deviation is
A)
500 Hz
B)
100 Hz
C)
1250 Hz
D)
25000 Hz
For FM, modulation index β = frequency deviation / modulating frequency. Here β = 50 and modulating frequency f_m = 500 Hz. So deviation Δf = β × f_m = 50 × 500 = 25000 Hz. However, the options suggest the intended modulating frequency may be 25 Hz or the intended calculation may involve the 2–5 V signal range. Using the given MCQ structure, the only option matching the standard FM relation for a 50 index with 500 Hz would actually be 25000 Hz, which is option (d).
An EX-OR gate produces an output only when its two inputs are
A)
Same
B)
Different
C)
High
D)
low
An EX-OR (XOR) gate gives output 1 only when its two inputs are different, i.e., one is high and the other is low. If both inputs are the same, the output is 0. So option (b) is correct. Options (a), (c), and (d) are incorrect because XOR does not produce output for identical inputs or for just any high/low input individually.
After firing an SCR, the gate pulse is removed. The current in the SCR will
A)
remain the same
B)
immediately fall to zero
C)
rise up
D)
rise a little and then fall to zero
An SCR, once triggered, latches into conduction. After firing, the gate pulse can be removed and the SCR will continue to conduct as long as the anode current stays above the holding current. Therefore, the current does not immediately fall to zero just because the gate signal is removed. Options (b), (c), and (d) are incorrect because gate removal does not control the conducting current directly after turn-on.
Output of the assembler in machine code is referred to as
A)
Object program
B)
Source program
C)
Macroinstruction
D)
Symbolic instruction
An assembler translates assembly language source code into machine code. The machine-code output produced by the assembler is called an object program (or object code).
Why others are incorrect:
- (b) Source program: this is the original assembly language input, not the output.
- (c) Macroinstruction: this is a higher-level assembly directive/instruction, not the machine-code output.
- (d) Symbolic instruction: this refers to assembly language mnemonics, not the assembled machine code.
In 8085 name the 16 bit registers
A)
Stack pointer
B)
Register B
C)
Register A
D)
PSW
In the 8085 microprocessor, the Stack Pointer (SP) is a 16-bit register because it stores a memory address. Register B and Register A are 8-bit registers, and PSW (Program Status Word) is treated as a 16-bit pair, but in standard single-register form, the clear 16-bit register among the given options is the Stack Pointer. Hence, the correct answer is Stack pointer.
Eight memory chips of 32×4 bit size have their address buses connected together. What is the size of this memory system?
A)
512×2 bits
B)
256×4 bits
C)
64×16 bits
D)
32×32 bits
Each chip is 32×4 bits, meaning it has 32 memory locations and each location stores 4 bits. If 8 such chips have their address buses connected together, they all respond to the same 32 addresses. Since their data buses are combined, the data width adds up: 8 × 4 = 32 bits. So the overall memory organization is 32 locations of 32 bits each, i.e. 32×32 bits. The other options do not match the combined word size.
A modern power semiconductor device that combines the characteristics of BJT and MOSFET is
A)
GTO
B)
FCT
C)
IGBT
D)
MCT
The IGBT (Insulated Gate Bipolar Transistor) combines the input characteristics of a MOSFET with the output/conduction characteristics of a BJT. It has high input impedance like a MOSFET and low on-state voltage drop like a bipolar device.
Why others are incorrect:
- GTO: A thyristor that can be turned off by gate signal, but it does not combine MOSFET and BJT features.
- FCT: Field Controlled Thyristor, not the standard device described here.
- MCT: MOS-Controlled Thyristor, also a gate-controlled thyristor, but the device specifically known for combining MOSFET and BJT characteristics is IGBT.
The output Y of the logic circuit given in the following figure is [Figure omitted in text]
A)
1
B)
X
C)
0
D)
null
Figure not available in text. Typical elementary logic circuit (e.g., NAND/NOR combination resulting in constant high) often evaluates to 1. As best guess without diagram, output is logic 1. Option A=1.
Instruction RET (Return) of a 8085 microprocessor
A)
Stack Pointer and Program counter
B)
Stack Pointer only
C)
Program counter only
D)
Set Interrupt Mask and Acc
In the 8085, the RET instruction returns from a subroutine by restoring the saved address from the stack into the Program Counter (PC). This operation uses the Stack Pointer (SP) to access the top of the stack and then updates the PC with the return address. Hence, both the Stack Pointer and Program Counter are involved. Option (b) is incomplete, (c) ignores the stack access, and (d) is unrelated.
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 control and convert a fixed d.c. input into a variable d.c. output. Hence, it performs d.c. to d.c. conversion.
Why others are wrong:
- (a) a.c. to d.c. is done by a rectifier.
- (c) d.c. to a.c. is done by an inverter.
- (d) a.c. to a.c. is done by an ac voltage controller or cycloconverter, depending on frequency conversion.
The main purpose of modulation is to
A)
combine two waves of different frequencies
B)
achieve wave-shaping of the carrier wave
C)
transmit low-frequency information over long distances efficiently
D)
produce sidebands
Modulation is used to shift a low-frequency message signal onto a high-frequency carrier so that it can be transmitted efficiently over long distances. This improves antenna size requirements, radiation efficiency, and reduces interference. Option (d) is only a result of modulation, not its main purpose. Options (a) and (b) do not describe the primary objective.
Thyristors connected in series need
A)
static equalizing circuit
B)
dynamic equalizing circuit
C)
both static and dynamic equalizing circuits
D)
none of these
When thyristors are connected in series, the applied voltage may not divide equally because of differences in leakage current and switching characteristics. To ensure proper voltage sharing:
- Static equalizing circuits are used to equalize voltage in the off-state (steady-state condition).
- Dynamic equalizing circuits are used to equalize voltage during switching/transient conditions.
Therefore, both types of equalizing circuits are needed.
A tunnel diode is
A)
A very heavily doped PN junction diode
B)
A high resistivity PN junction diode
C)
A slow switching device
D)
Used with reverse bias
A tunnel diode is made by very heavy doping of the p- and n-regions, which creates a very thin depletion layer. Because of this, electrons can tunnel through the junction, giving its characteristic negative resistance region. It is not a high-resistivity diode, it is known for very fast switching rather than slow switching, and it is not simply classified as a device used with reverse bias.
Out of the following devices, the fastest switching device is
A)
JFET
B)
BJT
C)
MOSFET
D)
triode
MOSFET is the fastest switching device among the given options because it is a majority-carrier device and has no charge-storage delay. JFET is also a field-effect device but is generally slower than MOSFET in switching applications. BJT is slower because it is a minority-carrier device and suffers from storage time during turn-off. A triode vacuum tube is much slower and obsolete for modern high-speed switching compared to semiconductor devices. Hence, the correct answer is MOSFET.
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 place the information signal onto a high-frequency carrier wave for transmission. This process is called modulation. Option (c) describes a demodulator, which extracts the information from the carrier. Option (a) is not the purpose of modulation, and option (d) refers to amplification, not modulation.