Science (English Medium) इयत्ता १२ - CBSE Important Questions

A semiconductor device is connected in series with a battery, an ammeter and a resistor. A current flows in the circuit. If. the polarity of the battery is reversed, the current in the circuit almost becomes zero. The device is a/an ______.

With the help of a circuit diagram, explain how a full wave rectifier gives output rectified voltage corresponding to both halves of the input ac voltage.

• Assertion (A): The resistance of an intrinsic semiconductor decreases with an increase in its temperature.
• Reason (R): The number of conduction electrons as well as hole increase in an intrinsic semiconductor with the rise in its temperature.

• Assertion (A): In insulators, the forbidden gap is very large.
• Reason (R): The valence electrons in an atom of an insulator are very tightly bound to the nucleus.

The energy required by an electron to jump the forbidden band in silicon at room temperature is about ______.

Which one of the following elements will require the highest energy to take out an electron from them?

Pb, Ge, C and Si

• Assertion (A): Putting the p-type semiconductor slab directly in physical contact with the n-type semiconductor slab cannot form the pn junction.
• Reason (R): The roughness at contact will be much more than inter atomic crystal spacing and continuous flow of charge carriers is not possible.

Name the device which utilizes the unilateral action of a pn diode to convert ac into dc.

Read the following paragraph and answer the questions that follow.

1. In the given figure, a diode D is connected to an external resistance R = 100 Ω and an emf of 3.5 V. If the barrier potential developed across the diode is 0.5 V, the current in the circuit will be:
   
   (a) 40 mA
   (b) 20 mA
   (c) 35 mA
   (d) 30 mA
2. In which of the following figures, the pn diode is reverse biased?
   (a)
   
   (b)
   
   (c)
   
   (d)
   
3. Based on the V-I characteristics of the diode, we can classify the diode as:
   (a) bilateral device
   (b) ohmic device
   (c) non-ohmic device
   (d) passive element
   OR
   Two identical PN junctions can be connected in series by three different methods as shown in the figure. If the potential difference in the junctions is the same, then the correct connections will be:
   (a) in the circuits (1) and (2)
   (b) in the circuits (2) and (3)
   (c) in the circuits (1) and (3)
   (d) only in the circuit (1)
4. 
   
   The V-I characteristic of a diode is shown in the figure. The ratio of the resistance of the diode at I = 15 mA to the resistance at V = -10 V is
   (a) 100
   (b) 10⁶
   (c) 10
   (d) 10^-6

A signal of 5 kHz frequency is amplitude modulated on a carrier wave of frequency 2 MHz. What are the frequencies of the side bands produced?

Why is base band signal not transmitted directly? Give any two reasons

State three important factors showing the need for translating a low frequency signal into a high frequency wave before transmission

Draw a sketch of a sinusoidal carrier wave along with a modulating signal and show how these are superimposed to obtain the resultant amplitude modulated wave

The carrier wave is given by

C(t) = 2sin(8πt) volt.

The modulating signal is a square wave as shown. Find modulation index.

Write the functions of the following in communication systems:

Modulator

Write the functions of the following in communication systems:

Transducer

Write the functions of the following in communication systems:

Demodulator

Why is Modulation index kept low?

What is the role of a bandpass filter?

Distinguish between broadcast mode and point-to-point mode of communication and give one example for each.