Q22 of 47 Page 1

(a) Three photo diodes D1, D2 and D3 are made of semiconductors having band gapsof 2.5 eV, 2 eV and 3 eV respectively. Which of them will not be able to detectlight of wavelength 600 nm ?

(b) Why photodiodes are required to operate in reverse bias ? Explain.


(a)


Formulas Used:


Energy of a photon:


The energy of a photon of wavelength is given by:



where is the Planck’s constant and is the speed of light in vacuum


The energy of a photon of light of the given wavelength is:







The diodes with band gaps larger than the energy of the photon won’t detect the photon. The answer is D1 and D3 .


(b)


It is easier to observe change in current with change in light intensity in reverse bias.


More from this chapter

All 47 →
20

(a) State Gauss’s law for magnetism. Explain its significance.

(b) Write the four important properties of the magnetic field lines due to a bar magnet.


OR


Write three points of differences between para-, dia- and ferro- magnetic materials, giving one example for each.


 21

Define the term ‘decay constant’ of a radioactive sample. The rate of disintegration of a given radioactive nucleus is 10000 disintegrations/s and 5,000 disintegrations/s after 20 hr. and 30 hr. respectively from start. Calculate the half life and initial number of nuclei at t = 0.

 23

(a) Describe briefly the functions of the three segments of n-p-n transistor.

(b) Draw the circuit arrangement for studying the output characteristics of n-p-n transistor in CE configuration. Explain how the output characteristics is obtained.


OR


Draw the circuit diagram of a full wave rectifier and explain its working. Also, give the input and output waveforms.


 24

(a) If A and B represent the maximum and minimum amplitudes of an amplitude modulated wave, write the expression for the modulation index in terms of A & B.

(b) A message signal of frequency 20 kHz and peak voltage 10 V is used to modulate a carrier of frequency 2 MHz and peak voltage of 15 V. Calculate the modulation index. Why the modulation index is generally kept less than one?