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Past Questions
The most important use of soft magnetic materials is in:
A)
meters
B)
transducers
C)
transformer and machine cores
D)
transformer winding wire
Soft magnetic materials are easy to magnetize and demagnetize, so they are used where the magnetic field must change repeatedly with low hysteresis loss. The most important application is in transformer and machine cores. Meters and transducers may use magnetic materials in some designs, but they are not the primary use. Transformer winding wire is made of conducting material like copper or aluminum, not magnetic material.
In a series RLC circuit, at resonant frequency:
A)
current is minimum
B)
voltage across C is maximum
C)
impedance is maximum
D)
current is maximum
In a series RLC circuit at resonance, the inductive reactance equals the capacitive reactance (XL = XC), so they cancel each other. The circuit impedance becomes minimum and equal to only the resistance (Z = R). Since current is I = V/Z, the current is therefore maximum at resonance. Hence, (d) is correct. Option (a) is wrong because current is not minimum; option (c) is wrong because impedance is minimum, not maximum. Option (b) can be true in the sense that capacitor voltage may be high at resonance, but the key standard answer for a series RLC circuit is that current is maximum.
Mobility of an electron in a semiconductor:
A)
decreases as the temperature increases
B)
increases as the temperature increases
C)
has no effect on conductivity
D)
is independent of temperature
Electron mobility in a semiconductor generally decreases as temperature increases because lattice vibrations (phonons) become stronger at higher temperatures. These vibrations scatter electrons more frequently, reducing their drift mobility. Therefore, option (a) is correct. Option (b) is opposite to the usual behavior, option (c) is wrong because mobility directly affects conductivity, and option (d) is incorrect because mobility is temperature-dependent.
Which of the following is a stable system?
A)
y(t) = t x(t)
B)
y(t) = t^2 x(t)
C)
y(t) = e^t x(t)
D)
y(t) = e^-t x(t)
A system is BIBO stable if every bounded input produces a bounded output. If x(t) is bounded, then multiplying it by a factor that remains bounded for all t gives a bounded output. Among the options, e^-t is the only factor that does not grow without bound in the stable direction typically intended in such MCQs. The terms t, t^2, and e^t grow without bound, so (a), (b), and (c) are unstable. Hence the intended answer is (d).
The number of branches incident at the node of a graph is called:
A)
degree of the node
B)
order of the node
C)
status of the node
D)
number of the node
In graph theory, the number of branches (edges) incident at a node (vertex) is called the degree of that node. The other options are not standard graph-theory terms: order usually refers to the number of vertices in a graph, and status/number of the node are not used for this definition.
An SCR can be turned OFF by:
A)
interrupting its anode current
B)
reversing polarity of its anode-cathode voltage
C)
low current dropout
D)
All of the above
An SCR (Silicon Controlled Rectifier) can be turned OFF only by reducing its anode current below the holding current, which can be achieved by interrupting the anode current. It can also be turned off by applying reverse bias across the anode-cathode terminals, i.e., reversing the polarity of the anode-cathode voltage. The phrase 'low current dropout' here refers to the current falling below the holding/latching level, which also turns it OFF. Therefore, all the given methods point to turn-off conditions, so the correct answer is 'All of the above'.
After VDS reaches pinch-off value VP in a JFET, drain current ID becomes:
A)
zero
B)
low
C)
saturated
D)
reversed
In a JFET, when the drain-source voltage VDS reaches the pinch-off voltage VP, the channel narrows enough that the drain current ID stops increasing significantly with further increase in VDS. It enters the saturation region and remains nearly constant. Hence, the drain current becomes saturated. Option (a) is wrong because current does not become zero, and (d) is wrong because current does not reverse.
An active device is:
A)
Resistor
B)
Capacitor
C)
Transistor
D)
Inductor
An active device is one that can control current and provide amplification or switching action, usually requiring an external power source. A transistor is an active semiconductor device used for amplification and switching. Resistor, capacitor, and inductor are passive devices because they do not provide power gain or amplification.
The hybrid parameter h11 is called:
A)
short circuit input impedance
B)
short circuit forward current gain
C)
open circuit reverse voltage gain
D)
open circuit output admittance
In the h-parameter (hybrid) model of a two-port network, h11 = V1/I1 with the output port short-circuited, so h11 represents the short-circuit input impedance. By definition, h12 is open-circuit reverse voltage gain, h21 is short-circuit forward current gain, and h22 is open-circuit output admittance. Therefore, the correct answer is (a).
Relative permeability of a paramagnetic material is:
A)
infinite
B)
one
C)
little more than one
D)
less than one
For a paramagnetic material, the magnetic susceptibility is small and positive. Since relative permeability \(\mu_r = 1 + \chi_m\), its value is slightly greater than 1. Hence the correct choice is (c) little more than one.
Why others are incorrect: (a) infinite is for an ideal superconductor, not paramagnetic material; (b) one is approximately true for non-magnetic materials like vacuum/air, but paramagnetic materials are slightly above 1; (d) less than one is characteristic of diamagnetic materials.
A transistor is in active region when:
A)
IC = βIB
B)
IC = βIE
C)
IC = IC
D)
IC = IB
In the active region of a BJT, the collector current is controlled by the base current and is approximately equal to β times the base current: IC = βIB. This is the standard current relation used for transistor amplification. Option (b) is incorrect because IC is not βIE; emitter current is related by IE = IC + IB. Option (c) is a meaningless identity, and option (d) is not the active-region relation.
The system y(n) = sin[x(n)] is:
A)
stable
B)
BIBO stable
C)
unstable
D)
none of the above
A system is BIBO stable if every bounded input produces a bounded output. Here, the output is y(n) = sin[x(n)]. Since the sine function always stays within the range [-1, 1] for any real input, the output remains bounded whenever the input is bounded. Therefore, the system is BIBO stable. Option (a) is too vague, (c) is incorrect because the output cannot blow up, and (d) is incorrect.
Hall coefficient is positive for:
A)
Metals
B)
n-type semiconductor
C)
p-type semiconductor
D)
Insulators
The Hall coefficient is positive when the dominant charge carriers are holes. In a p-type semiconductor, holes are the majority carriers, so the Hall voltage and Hall coefficient come out positive. In contrast, metals and n-type semiconductors generally have electrons as the dominant carriers, giving a negative Hall coefficient. Insulators do not conduct well enough for a meaningful Hall effect in the usual sense.
For the circuit shown in the figure below, for R =300 kΩ, calculate IB, IC and VCE.
Norton equivalent current is:
A)
Open circuit current
B)
Short circuit current
C)
Peak current
D)
RMS current
Norton’s theorem states that any linear two-terminal network can be replaced by an equivalent current source in parallel with a resistance. The Norton equivalent current is the current that flows when the output terminals are short-circuited, so it is the short-circuit current. Open-circuit current is not the Norton current, and peak/RMS current are unrelated general AC quantities.
The circuit which satisfies Reciprocity Theorem is called?
A)
Short circuit
B)
Open circuit
C)
Linear circuit
D)
Non-linear circuit
The Reciprocity Theorem applies to **linear, bilateral networks**. Such a circuit is commonly called a **linear circuit**. In these circuits, interchanging the positions of source and response does not change the current/voltage relationship.
- **(a) Short circuit**: This is only a circuit condition, not the type required for reciprocity.
- **(b) Open circuit**: Also just a circuit condition, not the required category.
- **(c) Linear circuit**: Correct, because reciprocity holds for linear networks.
- **(d) Non-linear circuit**: Incorrect, since reciprocity generally does not apply to non-linear circuits.
At absolute zero temperature, all the valence electrons in an intrinsic semiconductor are:
A)
in the conduction band
B)
in the forbidden gap
C)
in the valance band
D)
free electrons
In an intrinsic semiconductor at absolute zero (0 K), there is no thermal energy available to excite electrons across the band gap. So all valence electrons remain bound in the valence band, and the conduction band is empty. They are not in the forbidden gap and not free electrons.
A full wave bridge rectifier is supplied voltage at 50 Hz. The lowest ripple frequency will be:
A)
400 Hz
B)
200 Hz
C)
100 Hz
D)
50 Hz
In a full-wave bridge rectifier, both half-cycles of the AC input are converted into output pulses. So the ripple frequency becomes twice the supply frequency. Given supply frequency = 50 Hz, ripple frequency = 2 × 50 = 100 Hz. Hence the lowest ripple frequency is 100 Hz. Option (a) 400 Hz is too high, (b) 200 Hz is too high, and (d) 50 Hz is the original supply frequency, not the full-wave ripple frequency.
In a full wave bridge rectifier, the transformer’s secondary voltage is 100 sin vt. The forward resistance of each diode is 25Ω and load resistance is 950 Ω. Calculate D.C. output voltage, ripple factor, efficiency of rectification and PIV of the non-conducting diode.
Analyze the working of a Zener diode and explain its application in voltage regulation with a suitable circuit.
Superconductors exhibit Meissner effect, meaning:
A)
High resistance
B)
Expulsion of magnetic field
C)
Heat generation
D)
Voltage drop
The Meissner effect is the complete expulsion of magnetic flux from the interior of a superconductor when it is cooled below its critical temperature. This is a defining property of superconductivity. Option (a) is wrong because superconductors have zero electrical resistance, not high resistance. Option (c) is wrong because superconductors do not primarily indicate heat generation. Option (d) is wrong because voltage drop is not the meaning of Meissner effect.
Quartz crystal oscillator works on:
A)
Magnetic effect
B)
Thermal effect
C)
Piezoelectric effect
D)
Optical effect
A quartz crystal oscillator uses the piezoelectric effect of quartz. When an electric field is applied, the crystal mechanically vibrates, and these vibrations are used to generate a stable oscillation frequency. The other options are incorrect because quartz oscillators do not operate based on magnetic, thermal, or optical effects.
Explain the operation of a Bipolar Junction Transistor (BJT) in common emitter configuration and discuss its input and output characteristics.
Hard superconductors observe:
A)
breakdown by Silsbee’s rule
B)
incomplete Meissner effect
C)
high critical field and transition temperature
D)
all of these
Hard superconductors are type-II superconductors. They are characterized by: (1) very high critical magnetic fields, (2) relatively high transition temperature in many practical materials, and (3) incomplete Meissner effect because magnetic flux can partially penetrate in the mixed state. Silsbee’s rule is associated with the limiting current/field behavior in superconductors and is relevant in describing breakdown conditions. Hence, all given statements are taken as features observed in hard superconductors.
In a linear network, when the ac input is doubled, the ac output becomes:
A)
Two times
B)
Four times
C)
Half
D)
One-fourth
In a linear network, output is directly proportional to input. So if the AC input is doubled, the AC output also doubles. Hence the output becomes two times. The other options are incorrect because linearity does not produce squaring, halving, or quartering of the output for a doubled input.