← MES Jr. Grade
Past Questions — 2015
The race around condition exists in J-K FF if
A)
J=0, K=0
B)
J=1, K=0
C)
J=0, K=1
D)
J=1, K=1
In a J-K flip-flop, the race-around condition occurs when both inputs are 1. In this case, if the clock pulse remains high long enough, the output keeps toggling repeatedly instead of changing once. Hence, race around exists for J=1 and K=1. The other input combinations do not produce this toggling behavior.
Which of the following is not a sequential circuit?
A)
flip-flop
B)
counter
C)
register
D)
decoder
A sequential circuit depends on present inputs as well as past history (memory). Flip-flops, counters, and registers all store state, so they are sequential circuits. A decoder is a combinational circuit because its output depends only on the current input combination, not on any stored previous state.
A buffer amplifier has a gain of
A)
zero
B)
unity
C)
very large
D)
infinity
A buffer amplifier is used to isolate stages and provide impedance matching without changing the signal voltage. Therefore, its voltage gain is ideally 1, i.e., unity. Zero gain would mean no output, while very large or infinite gain is not the behavior of a buffer.
Indicate which of the following transfer function represents phase lead compensator?
A)
(s+1)/(s+5)
B)
(s+8)/(s+3)
C)
s+2
D)
s+2
A phase lead compensator has a transfer function of the form (s+z)/(s+p) with the zero closer to the origin than the pole, i.e., z < p. This gives positive phase lead. In option (b), the zero is at s = -8 and the pole is at s = -3, so the zero is farther from the origin than the pole, which is actually the form of a lag compensator, not lead. Option (a), (s+1)/(s+5), has zero at -1 and pole at -5, so the zero is closer to the origin than the pole, making it a phase lead compensator. Options (c) and (d) are incomplete and do not represent standard compensator forms. Therefore, the correct answer is option (a).
A voltage divider biased amplifier has its Q point at the middle of the DC load line. What is the maximum unclipped peak to peak output voltage?
A)
VCC
B)
IcqRe
C)
2IcqRe
D)
2Vcc
For a voltage-divider biased amplifier with the Q-point at the middle of the DC load line, the signal can swing equally toward cutoff and saturation. The maximum unclipped output swing is therefore the full supply-limited peak-to-peak value, which is VCC. The other options are not correct because IcqRe relates to emitter voltage drop, 2IcqRe is not the maximum output swing here, and 2Vcc is larger than the available supply swing.
For a reflection coefficient of 0.5, the value of SWR is
A)
-1
B)
1
C)
2
D)
3
For a transmission line, the standing wave ratio (SWR) is given by: SWR = (1 + |Γ|) / (1 - |Γ|), where Γ is the reflection coefficient. Here, |Γ| = 0.5, so SWR = (1 + 0.5) / (1 - 0.5) = 1.5 / 0.5 = 3. Therefore the correct value is 3, which corresponds to option (d).
In a push pull circuit
A)
each transistor conducts for 180°
B)
each transistor conducts for more than 180° but less than 360°
C)
each transistor conducts for less than 180°
D)
the period of conduction of each transistor depends on circuit configuration
In a push-pull amplifier, the two transistors share the input signal so that one conducts during the positive half-cycle and the other during the negative half-cycle. Therefore, each transistor conducts for 180° of the input cycle. Option (b) describes class AB operation, option (c) describes class C operation, and option (d) is too general for the standard push-pull circuit definition.
Consider the following statements : A clamper circuit 1. adds or subtracts a de voltage to a waveform 2. does not change the waveform 3. amplifies the waveform Which are correct?
A)
1,2
B)
1,3
C)
1,2,3
D)
2,3
A clamper circuit shifts the entire waveform up or down by adding a DC level, so statement 1 is correct. It does not change the shape of the waveform, only its DC reference level, so statement 2 is also correct. It does not amplify the signal, so statement 3 is incorrect. Therefore, the correct option is (a) 1,2.
In the given figure shows a 4 bit serial in parallel out right shift register. The initial contents as shown are 0110, After 3 clock pulses the contents will be
A)
0000
B)
0101
C)
1010
D)
1111
A 4-bit serial-in parallel-out right shift register shifts all bits one place to the right on each clock pulse, with 0 entering at the leftmost end unless otherwise specified. Starting from 0110:
- After 1st clock: 0011
- After 2nd clock: 0001
- After 3rd clock: 0000
So the contents after 3 clock pulses are 0000. Hence, option (a) is correct.
A basic S-R flip-flop can be constructed by cross-coupling which basic logic gates?
A)
AND or OR gates
B)
XOR or XNOR gates
C)
NOR or NAND gates
D)
AND or NOR gates
A basic S-R flip-flop (or latch) is formed by cross-coupling two identical gates with feedback. The standard implementations use either two NOR gates or two NAND gates. Hence, the correct answer is NOR or NAND gates. The other options are incorrect because AND/OR, XOR/XNOR, or AND/NOR are not the basic cross-coupled gate pairs used to construct an S-R flip-flop.
The maximum theoretical efficiency of a class A amplifier is
A)
12.5%
B)
25%
C)
50%
D)
75%
For a class A amplifier, the transistor conducts for the full 360° of the input cycle. In the ideal theoretical case, the maximum efficiency is obtained with transformer coupling and is 50%. In practice, the efficiency is much lower. Hence, the correct answer is 50%.
Sectioned convolution is performed if one of the sequences is very much larger than the other in order to overcome
A)
long delay in getting output
B)
require larger memory
C)
both (a) & (b)
D)
none of these
Sectioned convolution (block convolution) is used when one sequence is much longer than the other. It breaks the long sequence into smaller sections so that convolution can be done piecewise. This helps overcome two main problems of direct convolution: (1) a long delay in obtaining the output, since the entire long sequence need not be processed at once, and (2) the need for larger memory, because only a section/block needs to be stored and processed at a time. Hence, the correct answer is both (a) and (b).
What is the stability of system in A(Z) = (Z-0.4)(Z-2)
A)
system is stable
B)
unstable
C)
stable at 0.4
D)
can't say
For a discrete-time system, stability depends on the locations of the poles in the z-plane. The given characteristic equation is A(z) = (z - 0.4)(z - 2), so the poles are z = 0.4 and z = 2. A discrete-time system is stable only if all poles lie inside the unit circle, i.e., |z| < 1. Here, one pole is at z = 2, which lies outside the unit circle, so the system is unstable. Option (a) is incorrect because not all poles are inside the unit circle. Option (c) is incorrect because stability is not determined by a single pole. Option (d) is incorrect because the stability can be determined clearly.
Which of the following responses of an LTI system does not depend on initial conditions?
A)
natural response
B)
free response
C)
forced response
D)
total response
In an LTI system, the **forced response** is the part of the output caused only by the external input, so it does **not depend on initial conditions**. The **natural response** and **free response** are due to the system’s internal dynamics and do depend on initial conditions. The **total response** is the sum of natural and forced responses, so it depends on initial conditions through the natural part.
if the diameter of a wire is doubled, its current carrying capacity becomes
A)
One fourth
B)
Half
C)
Twice
D)
Four times
Current carrying capacity of a wire is proportional to its cross-sectional area. Since area is proportional to the square of diameter, doubling the diameter makes the area 2^2 = 4 times. Therefore, the current carrying capacity becomes four times. Option (a), (b), and (c) are incorrect because they do not account for the square relation between diameter and area.
A band pass signal extends from 1 KHz to 2 KHz. The minimum sampling frequency needed to retain all information in the sampled signal is
A)
1KHz
B)
2KHz
C)
3 KHz
D)
4 KHz
For a band-pass signal, the minimum sampling frequency is given by the band-pass sampling theorem. Here the signal occupies the band from 1 kHz to 2 kHz, so the bandwidth is 1 kHz and the highest frequency is 2 kHz. The minimum sampling frequency must be at least twice the bandwidth plus the lower edge condition satisfied; among the given options, 3 kHz is the minimum that can retain all information without aliasing. Hence, the correct answer is 3 kHz. The other options are too low to sample the band-pass signal properly.
Given a unit step function u(t), its time-derivative is
A)
a unit impulse
B)
another step function
C)
a unit ramp function
D)
a sine function
The unit step function u(t) changes abruptly from 0 to 1 at t = 0. Its time-derivative is therefore zero everywhere except at t = 0, where it is represented by the Dirac delta function, i.e., a unit impulse. Hence, the correct option is (a). The other options are incorrect because the derivative of a step is not another step, not a ramp, and not a sine function.
Which of the following materials are piezoelectric?
A)
mica & quartz
B)
mica & barium titanate
C)
mica & diamond
D)
quartz & barium titanate
Piezoelectric materials generate an electric charge when mechanical stress is applied. Quartz is a classic natural piezoelectric crystal, and barium titanate is a well-known synthetic ferroelectric ceramic with strong piezoelectric properties. Mica and diamond are not standard piezoelectric materials in this context. Hence, the correct pair is quartz and barium titanate.
The Fourier transform (FT) of a function x (t) is X (f). The FT of dx(t)/dt will be
A)
dX(f) df
B)
2infX(f)
C)
j2πfX(f)
D)
X(f) Gf
Using the Fourier transform differentiation property, if x(t) ↔ X(f), then d x(t)/dt ↔ j2πf X(f). This is because differentiation in the time domain corresponds to multiplication by j2πf in the frequency domain. Hence option (c) is correct. The other options do not match the standard FT property.
Which of these has lowest dielectric strength?
A)
Dry air
B)
Wet air
C)
Cotton
D)
Paper
Dielectric strength is the maximum electric field a material can withstand without breaking down. Among the given options, wet air has the lowest dielectric strength because moisture increases ionization and makes electrical breakdown easier. Dry air has higher dielectric strength than wet air, while cotton and paper are solid insulating materials with comparatively higher dielectric strength than air.
In an intrinsic semiconductor, the number of electrons is equal to the number of holes at which temperature?
A)
0°K
B)
0°c
C)
high temperature
D)
at all temperature
In an intrinsic semiconductor, electron-hole pairs are generated thermally in equal numbers. Therefore, the number of electrons is always equal to the number of holes at any temperature, including room temperature and other temperatures. Option (a) is wrong because at 0 K an intrinsic semiconductor has no free carriers. Options (b) and (c) are not correct because the equality is not limited to only 0°C or high temperature.
An ideal current source has
A)
Infinite source resistance
B)
Zero source resistance
C)
Large value of source resistance
D)
Finite value of source resistance
An ideal current source must deliver a constant current independent of the load. To do this, its internal/source resistance must be infinite, so that no current is diverted internally. A zero or finite resistance would allow the source current to vary with load, which is not ideal.
The color code of 1K Q, resistance is
A)
black, brown, red
B)
red, brown, brown
C)
brown, black, red
D)
black, black, red
For a 3-band resistor code, the first two bands give the significant digits and the third band is the multiplier.
1 kΩ = 1000 Ω = 10 × 10^2.
So the digits are 1 and 0, which are brown and black, and the multiplier 10^2 is red.
Therefore the correct color code is brown, black, red.
Option (a) gives 01 × 10^2 = 100 Ω, (b) gives 11 × 10^1 = 110 Ω, and (d) gives 00 × 10^2 = 0 Ω.
In what state is a silicon diode if the voltage drop across it is about 0.7V?
A)
No bias
B)
Forward bias
C)
Reverse bias
D)
Zener region
A silicon diode typically has a forward voltage drop of about 0.7 V when it is conducting. This means the diode is in forward bias. In no bias, there is no applied voltage; in reverse bias, the diode blocks current and does not show the normal 0.7 V drop; and the Zener region refers to reverse breakdown operation, not the usual silicon diode forward drop.
Which of the following is a passive component?
A)
Semi-conductor device
B)
Vacuum tube device
C)
Capacitors
D)
All of these
A passive component does not generate power; it only stores, dissipates, or releases energy. A capacitor is a passive component because it stores electrical energy in an electric field. Semiconductor devices and vacuum tube devices are active components because they can control current and provide amplification.