CHAPTER 15 COMMUNICATION SYSTEMS

• Communication is the act of transmission of information. Electronic communication refers to the faithful transfer of information in the form of electrical voltage and current from one point to another point.

• Transmitter, transmission channel and receiver are three basic units of a communication system.

• Two forms of communication system:
  • Analog: Information transmitted in continuous waveform.
  • Digital: Information transmitted has only discrete or quantized levels.

• Every message signal occupies a range of frequencies. The bandwidth of a message signal refers to the band of frequencies which are necessary for satisfactory transmission of the information contained in the signal.

• Low frequencies cannot be transmitted to long distances. Therefore, they are superimposed on a high frequency carrier signal by a process known as modulation.

• In modulation, some characteristics of the carrier signal like amplitude, frequency or phase varies in accordance with the modulating or message signal. Correspondingly, they are called Amplitude Modulated (AM), Frequency Modulated (FM) or Phase Modulated (PM).

• For transmission over long distances, signals are radiated into space using devices called antennas. The radiated signals propagate as electromagnetic waves and the mode of propagation is influenced by the presence of the earth and its atmosphere. Near the surface of the earth, electromagnetic waves propagate as surface waves. Surface wave propagation is useful up to a few MHz frequencies.

• Long distance communication between two points on the earth is achieved through reflection of electromagnetic waves by the ionosphere. Such waves are called sky waves. Sky wave propagation takes place up to frequency of about 30 MHz.

• If an antenna radiates electromagnetic waves from a height h_T , then the range d_T is given by√(2Rh_T ) where R is the radius of the earth.

CHAPTER 14 SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND 

                                                      SIMPLE CIRCUITS

• Semiconductors are the basic materials used in the present solid state electronic devices like diode, transistor, ICs, etc.

• Lattice structure and the atomic structure of constituent elements decide whether a particular material will be insulator, metal or semiconductor.

• Metals have low resistivity. Insulators have a very high resistivity while semiconductors have intermediate values of resistivity.

• Pure semiconductors are called ‘intrinsic semiconductors’. The presence of charge carriers (electrons and holes) is an ‘intrinsic property’ of the material and these are obtained as a result of thermal excitation. The number of electrons (n_e) is equal to the number of holes (n_h) in the intrinsic conductors. Holes are essentially electron valencies with an effective positive charge.

• The number of charge carriers can be changed by ‘doping’ of a suitable impurity in pure semiconductor. Such semiconductors are called extrinsic semiconductors. These are of two types:
  • n-type: n_e >> n_h
  • p-type: n_h >> n_e

• n-type semiconducting Si or Ge is obtained by doping with pentavalent atoms (donors) like As, Sb, P, etc.

• p-type semiconducting Si or Ge can be obtained by doping with trivalent atom (acceptors) like B, Al, In, etc.

• There are two distinct band of energies (called valence band and conduction band) in which the electrons in a material lie. Valence band energies are low as compared to conduction band energies. All energy levels in the valence band are filled while energy levels in the conduction band may be fully empty or partially filled. The electrons in the conduction band are free to move in a solid and are responsible for the conductivity. The extent of the conductivity depends upon the energy gap (E_g) between the top of the valence band (E_V) and the bottom of the conduction band (E_C).

• The electrons from valence band can be excited by heat, light or electrical energy to the conduction band and thus produce a change in the current flowing in the semiconductor.

• p-n junction is the ‘key’ to all the semiconductor devices. When such a junction is made, a depletion layer is formed consisting of immobile ion-cores devoid of their electrons or holes. This is responsible for a junction potential barrier.
• Diodes can be used for rectifying an ac voltage (restricting the ac voltage to one direction). With the help of a capacitor or a suitable filter, a dc voltage can be obtained.

• Transistor is an n-p-n or p-n-p junction device. The central block (thin and lightly doped) is called the ‘Base’ while the other electrodes are ‘Emitter’ and ‘Collectors’. The emitter-base junction is forward biased while collector-base junction is reverse biased.

• When a transistor is used in the cutoff or saturation state, it acts as a switch.

• In the modern day circuit, many logical gates or circuits are integrated in one single ‘Chip’. These are known as Integrated circuits (ICs).

CHAPTER 13 NUCLEI

• An atom has a nucleus. The nucleus is positively charged. The radius of the nucleus is smaller than the radius of an atom by a factor of 10⁴. More than 99.9% mass of the atom is concentrated in the nucleus.

• On the atomic scale, mass is measured in atomic mass units (u). by definition, 1 atomic mass unit (1 u) is 1/12^th mass of one atom of ¹²C i.e. 1 u = 1.660563 x 10^-27

• A nucleus contains a neutral particle called neutron. Its mass is almost the same as that of a proton.

• The atomic number Z is the number of protons in the atomic nucleus of an element. The mass number A is the total number of protons and neutrons in the atomic nucleus. A=Z+N where N denotes the number of neutrons in the nucleus.

• A nuclear species is also known as nuclide.

• Isotopes: Nuclides with the same atomic number Z but different neutron number N.

• Isotones: Nuclides with the same number of neutrons N.

• Isobars: Nuclides with the same mass number A.

• Neutrons and protons are bound in a nucleus by the short-range strong nuclear force.

• Radioactivity is the phenomenon in which nuclei of a given species transform by giving out α or β or γ
  • α- rays are helium nuclei
  • β- rays are electrons
  • γ- rays are electromagnetic radiation of wavelengths shorter than X-rays.

• Law of radioactive decay: N (t)= N(0) e^-λt where λ is the decay constant or disintegration constant.

• Nuclear fission occurs when a heavy nucleus breaks into two smaller fragments. Nuclear fusion occurs when lighter nuclei combine to form a larger nucleus.

       CHAPTER 11 DUAL NATURE OF RADIATION AND MATTER

• Work function: The minimum energy needed by an electron to come out from a metal surface is called the work function of the metal. Energy greater than the work function required for electron emission from the metal can be supplied by suitable heating or applying strong electric field or irradiating it by light of suitable frequency.

• Photoelectric effect: Photoelectric effect is the phenomenon of emission of electrons by metals when illuminated by light of suitable frequency. Certain metals respond to ultraviolet light while others are sensitive even to the visible light.

• Photoelectric effect involves the conversion of light energy into electrical energy. It follows the law of conservation of energy. The photoelectric emission is an instantaneous process and possesses certain special features.

• Photoelectric effect depends on:
  • the intensity of incident light
  • the potential difference applied between the two electrodes
  • the nature of the emitter material

• The stopping potential (V₀) depends on:
  • the frequency of incident light
  • the nature of the emitter material

• Below a certain frequency (threshold frequency), characteristic of the metal, no photoelectric emission takes place, no matter how large the intensity may be.

• The classical wave theory could not explain the main features of the photoelectric effect. Einstein explained these features on the basis of photon picture of light. According to this, light is composed of discrete packets of energy called quanta or photons. Each photon carries an energy E= h ν and momentum p = h/λ which depend on the frequency of the incident light and not on its intensity.

• Einstein’s photoelectric equation is in accordance with the energy conservation law as applied to the photon absorption by an electron in the metal.

                                                 0.5  m v²_max = V₀ e

• Radiation has dual nature: wave and particle. The nature of the experiment determines whether a wave or particle description is best suited for understanding the experimental result.

• The waves associated with the moving particles are called matter waves or de Broglie waves.

• The de Broglie wavelength (λ) associated with a moving particle is related to its momentum as λ = h/p. the de Broglie wavelength is independent of the charge and the nature of the material particle.

• Electron diffraction experiments by Davisson and Germer and by G.P Thomson have verified and confirmed the wave nature of electrons. The de-Broglie hypothesis of matter waves supports the Bohr’s concept of stationary orbits.

CHAPTER 9 RAY OPTIC AND OPTICAL INSTRUMENTS

• Reflection is governed by the equation ∠i = ∠r’ and refraction by the Snell’s Law, sin i/sin r = n, where the incident ray , reflected ray, refracted ray and normal lie in the same plane. i, r and r’ are the angle of incidence, reflection and refraction respectively.

• The critical angle of incidence i_c for a ray incident from a denser to rarer medium, is that angle for which the angle for refraction is 90^o. For i > i_c , total internal reflection occurs.

• Optical fibres: Optical fibres consist of glass fibres coated with a thin layer of material of lower refractive index. Light incident at one angle at one end comes out at the other, after multiple internal reflections, even if thee fibre is bent.

• Cartesian sign convention: Distances measured in the same direction as the incident light are positive while those measured in the opposite direction are negative. The heights measured upwards above x-axis are taken as positive and the heights measured downwards are taken as negative.

• Mirror equation: 1/v + 1/u = 1/f , where u and v are object and image distances respectively and f is the focal length of the mirror.

• Dispersion: Dispersion is the splitting of light into its constituent colours.

• The eye: The eye has a convex lens of focal length about 2.5 cm. This focal length can be varied somewhat so that the image is always formed on the retina. This ability of the eye is called accommodation. In a defective eye, if the image is focussed before the retina (myopia), a diverging corrective lens is needed to correct the defect. If the image is focussed beyond the retina (hypermetropia), a converging corrective lens is needed. Astigmatism is corrected by using cylindrical lenses.

• Magnifying power (m) of a simple microscope is given by m= 1 + (D + f) where D=25 cm is the least distance of distinct vision and f is the focal length of the convex lens.
• Magnifying power (m) of a telescope is the ratio of the angle subtended at the eye by the image to the angle subtended at the eye by the object.

  CHAPTER 7 ALTERNATING CURRENT

• An alternating voltage v = v_m sin ωt applied to a resistor R drives a current i = i_m sinωt in the resistor, i_m=v_m/R . The current is in phase with the applied voltage.

• For an alternating current i = i_m sin ωt passing through a resistor R, the average power loss P due to joule heating is (1/2) i²_m

• The phase relationship between current and voltage in an ac circuit can be shown by representing voltage and current by rotating vectors called phasors.

• A phasor is a vector which rotates about the origin with angular speed ω.

• The magnitude of a phasor represents the amplitude or peak value of the quantity (voltage or current) represented by the phasor.

• Transformer: A transformer consists of an iron core on which are bound a primary coil of N_p turns and a secondary coil of N_s

• If the secondary coil has greater turns that the primary, the voltage is stepped up and the transformer is called a step-up transformer.

• If the secondary coil has less number of turns than the primary coil, the voltage is stepped down and the transformer is called a step-down transformer.

       CHAPTER 6 ELECTROMAGNETIC INDUCTION

• The magnetic flux through a surface of area A is placed in a uniform magnetic field B is defined as: ϕ_B = B.A= BA cos θ where θ is the angle between B and A.

• Faraday’s laws of induction imply that the emf induced in a col of N turns is directly related to the rate of change of flux through it.

• Lenz’s law states that the polarity of the induced emf is such that it tends to produce a current which opposed the change in magnetic flux that produces it.

• Changing magnetic fields can set up current loops in nearby metal bodies. They dissipate electrical energy as heat. Such currents are called eddy currents.

• Inductance: It is the ratio of the flux-linkage to current.

• A changing current in a coil can induce an emf in a nearby coil.

• When the current in a coil changes, it induces a back emf in the same coil.

• AC generator: In an ac generator, mechanical energy is converted to electrical energy by virtue of electromagnetic induction.