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 Multiple Choice QuestionsMultiple Choice Questions

1.

The temperature dependence of resistances of Cu and undoped Si in the temperature range 300-400 K is best described by:

  • Linear increase for Cu, linear increase for Si.

  • Linear increase for Cu, exponential increase for Si.

  • Linear increase for Cu, exponential decrease for Si.

  • Linear decrease for Cu, linear decrease for Si.


C.

Linear increase for Cu, exponential decrease for Si.

As we know Cu is a conductor, so increase in temperature, resistance will increase.  Then, Si is a semiconductor, so with the increase in temperature, resistance will decrease.

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2.

A p–n junction (D) shown in the figure can act as a rectifier. An alternating current source (V) is connected in the circuit.


C.

Only +ve current passes though the diode. Given figure is half wave rectifier.

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3.

The I-V characteristic of an LED is


A.

For same value of current, higher value of voltage is required for higher frequency.

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4.

Identify the semiconductor devices whose characteristics are given below, in the order (a), (b), (c), (d):


A.

Zener diode works in breakdown region
 So, simple  diode → (a)
zener diode (b)
solar cell → (c)
Light dependent resistance → (d)

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5.

The combination of gates shown below yields

  • OR gate

  • NOT gate

  • XOR gate

  • NAND gate


A.

OR gate

A B X
0 0 0
0 1 1
1 0 1
1 1 1

This comes out be truth table of OR gate
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6.

In a common emitter amplifier circuit using an n-p-n transistor, the phase difference between the input and the output voltages will be

  • 135°

  • 180°

  • 45°

  • 90°


B.

180°

In common emitter amplifier circuit input and output voltage are out of phase. When the input voltage is increased then ib is increased, ic also increases so the voltage drop across Rc is increased. However, increase in voltage across RC is in opposite sense.

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7.

A charge Q is uniformly distributed over the surface of non conducting disc of radius R. The disc rotates about an axis perpendicular to its plane and passing through its centre with an angular velocity ω. As a result of this rotation, a magnetic field of induction B is obtained at the centre of the disc. If we keep both the amount of charge placed on the disc and its angular velocity to be constant and vary the radius of the disc than the variation of the magnetic induction at the centre of the disc will be represented by the figure


A.


Consider ring like the element of the disc of radius r and thickness dr.
If σ is charge per unit area, then charge on the element
dq = σ(2πr dr)
current ‘i’ associated with rotating charge dq is
straight i space equals space fraction numerator left parenthesis dp right parenthesis straight w over denominator 2 straight pi end fraction space equals space straight sigma space straight w space straight r space dr
Magnetic field dB at center due to element

dB space equals space fraction numerator straight mu subscript 0 straight i over denominator 2 straight r end fraction space equals space fraction numerator straight mu subscript 0 σωdr over denominator 2 end fraction
straight B subscript net space equals space integral dB space equals space fraction numerator straight mu subscript straight o σω squared over denominator 2 end fraction space integral subscript 0 superscript straight R dr space equals space fraction numerator straight mu subscript straight o σωR over denominator 2 end fraction
rightwards double arrow space straight B subscript net space equals fraction numerator straight mu subscript straight o Qω over denominator 2 πR end fraction
So if Q and w are unchanged then
straight B subscript net space proportional to 1 over straight R

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8.

Truth table for system of four NAND gates as shown in figure is


A.

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9.

The forward biased diode connection is


A.

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10.

If a, b, c, d are inputs to a gate and x is its output, then, as per the following time graph, the gate is:


  • NOT

  • AND

  • OR

  • NAND


C.

OR

The output of OR gate is 0 when all inputs are 0 and output is 1 when at least one of the input is 1.

Observing output x it is 0 when all inputs are 0 and it is 1 when at least one of the inputs is 1.

therefore, the gate is OR

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