Electronics and Communication

Objectives

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• Energy band diagram in semiconductors • Types of semiconductors • Formation of p-n junction diode and its V-I characteristics • Rectification process • Special purpose diodes • Transistors and their immediate applications • Digital and analog signals • Logic gates, Boolean algebra and De Morgan’s theorem • Modulation and its types • Basic elements of communication system • Propagation of electromagnetic waves through space • Some important communication systems

Summary

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„ Energy bands in solids are used to classify them into metals, insulators, and semiconductors „ In a N-type semiconductor, electrons are the majority charge carriers and holes are the minority carriers. „ In P-type semiconductor, holes are the majority charge carriers and electrons are the minority charge carriers. „ The thin region near the junction which is free from charge carriers (free electrons and holes) is called depletion region. „ When a PN junction diode is forward biased, the width of the depletion region decreases and the diode conducts. „ A PN junction diode in reverse biased condition functions as a open switch as it does not conduct. The width of the depletion region increases in this case. „ A forward biased PN junction diode functions as a rectifier. The process in which alternating voltage or alternating current is converted into direct voltage or direct current is known as rectification. „ The half wave rectifier rectifies one half of the input signal and produces a pulsating DC output. „ Full wave rectifier rectifies both halves of the input signal. „ Zener breakdown happens in a heavily doped PN junction diode when a strong electric field is applied. „ Avalanche breakdown occurs in lightly doped junctions which have wide depletion layers. It is due to the breaking of covalent bonds by the thermally generated minority charge carriers. „ Zener diode is a heavily doped PN junction diode which works in the reverse biased condition. „ Light emitting diode is a forward biased semiconductor device that emits emits visible or invisible light when energized. The recombination of minority charge carriers with the majority charge carriers in the respective regions release energy in the form of photons. „ A PN junction diode which converts an optical signal into electric signal is called a photodiode. „ When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. These electrons and holes are swept across the p-n junction by the electric field created by reverse voltage before recombination takes place and in turn generates photo current. „ A solar cell, also known as photovoltaic cell, works on the principle of photovoltaic effect. Accordingly, the p-n junction of the solar cell generates emf when solar radiation falls on it. . „ A bipolar junction transistor (BJT) is a semiconductor device and there are two types in that: NPN and PNP. „ A BJT can be operated in three different configurations: Common base, common emitter, common collector. „ The forward current gain in common base configuration α gives the ratio of the collector current to emitter current. „ The forward current gain in common emitter configuration β gives the ratio of the collector current to the base current „ The BJT connected in common emitter configuration can be used as an amplifier. There exists a phase reversal of 1800 between the input signal and the amplified output signal. „ A transistor amplifier combined with a tank circuit and positive feedback acts as an oscillator „ The logic gates are logical circuits providing output only for a combination of inputs. „ De Morgan’s First theorem states that the complement of the sum of two inputs is equal to the product of its complements. „ The second theorem states that the complement of the product of two inputs is equal to the sum of its complements. „ The basic elements required for the transmission and reception of a signal through long distance communication using electromagnetic waves are transducer, amplifier, carrier signal, modulator, power amplifier, medium of transmission, transmitting and receiving antenna, demodulator, detector. „ For long-distance transmission, the baseband signal is modulated with the carrier wave. „ If the amplitude of the carrier signal is modified with the instantaneous amplitude of the baseband signal then it is called amplitude modulation. „ The frequency of the carrier signal is modified with the instantaneous amplitude of the baseband signal in frequency modulation. „ The instantaneous amplitude of the baseband signal modifies the phase of the carrier signal keeping the amplitude and frequency constant in phase modulation „ If the EM waves transmitted by the transmitter glide over the surface of the earth to reach the receiver, then the propagation of EM waves is called ground wave propagation. „ The mode of propagation in which electromagnetic waves radiated from an antenna, directed upwards at large angles gets reflected by the ionosphere back to earth is called sky wave propagation. „ The process of sending and receiving information signal through space is called space wave communication. „ The satellite communication is a mode of communication of signal between transmitter and receiver via satellite. „ Fiber-optic communication is a method of transmitting information by sending pulses of light through an optical fiber. „ Radar basically stands for Radio Detection and Ranging System. It is one of the important applications of communication systems for remote sensing. „ Mobile Communication is used to communicate with others in different locations without the use of any physical connection like wires or cables.