Title 23. Electronics and Communication Systems.pdf ✅

Electronics and Communication Systems Electronics and503 23 Communication Systems QUICK LOOK Thermionic valves or vacuum tubes come in many forms including the diode, triode, tetrode, pentode, heptode and many more. These tubes have been manufactured by the millions in years gone by and even today the basic technology finds applications in today's electronics scene. It was the vacuum tube that first opened the way to what we know as electronics today, enabling first rectifiers and then active devices to be made and used. The simplest form of vacuum tube is the diode, was invented by Fleming while triode valve by Le-de Forst It consists of two electrodes: a cathode and anode held within an evacuated glass bulb, connections being made to them through the glass envelope. The direction of current in the valve is from plate to cathode. The plate resistance of a diode valve at saturation current is infinite. A diode value can be used as a rectifier, detector and modulator while it cannot be used as an amplifier. For an ideal diode, forward resistance is zero; while its reverse resistance is infinity-Potential barrier 0, VB = forward resistance Rf = 0 and reverse resistance Rf = ∞ Figure: 23.1 Richardson’s formula for saturation current Ip in a diode value is 2 / W RT s I AT e− = where T = absolute temperature, W = work function and k = Boltzmann constant. Child’s law for space charge limited current 3/ 2 p p I kV = where k = constant A triode valve can be used as an amplifier and oscillator. The phase difference between input and output voltage in a triode amplifier is π. Triode Valve constants: Amplification factor constant p p g I V V μ =   ∆ = −    ∆  Plate resistance, constant g p p p V V R I   ∆ =     ∆  Mutual conductance, constant p p m g V V g V =   ∆ = −    ∆  Relation betweenμ, Rp and gm is μ = ⋅ R g p m Child’s law for triode value is ( ) 3 2 p p g I K V V = + μ or 3 2 ' p p g V I K V μ   = +     Minimum grid potential for plate current to be zero, p p V V μ = − Voltage gain in a triode amplifier voltageacrossload RL v g P L A e R R μ = = − + Maximum Voltage gain, max ( ) ; Av = μ for RL → ∞ In practice Av < A When several stages of amplifier are cascaded, then the net gain Av = A1 × A2 × A2 × .... Band Theory of Solids: To visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. An important parameter in the band theory is the Fermi level, the top of the available electron energy levels at low temperatures. The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties. Table 23.1: Band Theory of Insulator, Semiconductor and Conductor Insulator Semiconductor Conductor Energy of electrons Conduction Band Valence Band Overlap region Fermi level Energy of electrons Conduction Band Valence Band Fermi level Energy of electrons Conduction Band Valence Band Large energy gap between valence and conduction bands A Electron flow A No current
504 Quick Revision NCERT-PHYSICS Most solid substances are insulators, and in terms of the band theory of solids this implies that there is a large forbidden gap between the energies of the valence electrons and the energy at which the electrons can move freely through the material (the conduction band). Glass is an insul- ating material which may be transparent to visible light for reasons closely correlated with its nature as an electrical insulator. The visible pro- perties of glass can also give some insight into the effects of "doping" on the properties of solids. A very small percentage of imp- urity atoms in the glass can give it color by providing specific available energy levels which absorb certain colors of visible light. The ruby mineral (corun- dum) is aluminum oxide with a small amount (about 0.05%) of chromium which gives it its characteristic pink or red color by absorbing green and blue light. For intrinsic semiconduc- tors like silicon and germanium, the Fermi level is essentially halfway between the valence and conduction bands. Although no conduction occurs at 0 K, at higher temperatures a finite number of electrons can reach the conduction band and provide some current. In doped semiconductors, extra energy levels are added. The doping of semiconductors has a much more dramatic effect on their electrical conductivity. Silicon and Germanium Energy Bands: The bonding among electrons of germanium and silicon is covalent. Energy band gap in a semi-conductor is of the order of 1 eV. At finite temperatures, the number of electrons which reach the conduction band and contribute to current can be modeled by the Fermi function. That current is small compared to that in doped semiconductors under the same conditions. In terms of the band theory of solids, metals are unique as good conductors of electricity. This can be seen to be a result of their valence electrons being essentially free. In the band theory, this is depicted as an overlap of the valence band and the conduction band so that at least a fraction of the valence electrons can move through the material. Types of Semiconductor (Intrinsic and Extrinsic Semiconductors) Intrinsic (Pure) Semiconductors: These are the substances without any impurity. Their conductivity is lower than extrinsic semiconductors. They have 4 electrons in their outermost shell. These four electrons forms covalent bond with neighboring atoms. At high temperature some of the covalent bond are broken and electron-hole pairs are created. These electrons are known as free electrons. Free electrons and holes are always equal in number, therefore, semiconductor is always electrically neutral. These free electrons and holes are carriers of electricity in an intrinsic semiconductor. Extrinsic Semiconductors: An extrinsic semiconductor is a semiconductor doped by a specific impurity which is able to deeply modify its electrical properties, making it suitable for electronic applications (diodes, transistors, etc.) or optoelectronic applications (light emitters and detectors). P-type: They are impure semiconductors with P-type (trivalent) of impurities doped in. They have majority of holes as charge carriers. N-type: When N-type (pentavalent) impurity is added to intrinsic semiconductor they become N-type extrinsic semiconductors. Majority carriers are electrons in N-type semiconductors. Forbidden Energy Gap: The separation between conduction band and valence band on the energy level diagram is called Forbidden energy gap (Eg). For a semiconductor Eg depends upon the temperature by following relationship. Table 23.2: Eg at 0°K and room temperature (300°K) 0°K 300°K Ge 0.785 eV 072 eV Si 1.21 eV 1.1 eV T is temperature in °K 300°K Ge ( ) 4 E T T eV g 0.785 2.23 10− = − ×     13 3 2.5 10 / Cm × Si ( ) 4 E T T eV g 1.21 3.6 10− = − ×     10 3 1.5 10 / Cm × Types of Dopings: If trivalent or pentavalent impurities are added to intrinsic semiconductor, extrinsic (impure) semiconductor is formed. Trivalent Impurities → B, Al, Ga, In, Tl Pentavalent Impurities → N, P, As, Sb, Bi Doping can be donor type or acceptor type. Effect of Doping on Properties of Semiconductor: The doping has following effects on semiconductor. Increased number of charge carries (holes or electrons). Conductivity (σ) increases ( ∵ σ ∝ number of charge carriers). Fermi levels shifts. It shifts towards valence band for P-type and towards conduction band for N-type semiconductors. Concentration of electrons in conduction band increases for N-type semiconductor. Valence Band 0 K (No electrons in conduction bond) Fermi level 1.09 eV 300 K Conduction Band Valence Band 0 K (No electrons in conduction bond) 0.22 eV Fermi level 300K Conduction Band

506 Quick Revision NCERT-PHYSICS Figure: 23.2 (c) Load Line Concept: Point or region of operation of devices (i.e., diode, transistor, FET etc.) is decided by applied load. Point of intersection of device characteristics with load line determine the point of operation of the system. The point of operation is usually called the quiescent point (or Q point). Figure: 23.3 (a)) Diode circuit, (b)) V-I characteristics in forward region, (c) load line and (d) combined curve Diode as circuit element and RL is load resistance V E V E IR = − = − L L ⇒ 1 L L E I V R R = − + Where, I and V represents current carried by diode and voltage drop across diode under forward bias. Equation resembles to that of a straight line (like y mx c = − + ) and shown in figure 23.3 (c). Intercepts of load line with vertical axis can be found by putting V = 0 in the equation L V 0 E I R = = Intercept with horizontal axis is calculated by putting I = 0 in equation, therefore, 0 L L V E R R = − + ⇒ V E = The slope of the line is dependent only upon load resistance RL ( ) 1 Slope L m R = − Since vertical intercepts as well as slope of the line is dependent upon load RL , therefore, this line is termed as load line. Variation of Q-Point: Change of load resistance (RL ) and input voltage (E) changes the load line, therefore, point of intersection of two curves (i.e., Q point changes. Let us see how Q point is varied. Resistance dc resistance of diode de V r I = The dynamic (or ac resistance) of diode is dV 26 r k dI I η ∝ = = Ω (for sufficiently forward biased) Rectifier: A rectifier converts ac into dc. Junction diodes are used for rectification and detection. A half wave rectifier uses one diode, while full wave diode used two diodes. In a half wave rectifier dc I I ∝ > while for a full wave . dc I I ∝ < Filter circuits are used to reduce/eliminate ripples (ac components), which are simple inductances and capacitance suitably arranged. Half-wave rectifier Currents 0 0 , 2 dc rms I I I I π = = Power: DC power, 2 P I R dc dc L = AC power 2 ( ) P I R R ac rms L F = + Efficiency of rectification 40.6 100% % 1 dc R ac F L P P R R η = × =     +   Full wave rectifier Currents 0 0 2 , 2 dc rms I I I I π = = Power: 2 2 , ( ) P I R P I R R dc dc L ac rms L F = = + Efficiency of rectification, 81.2 100% % 1 dc R ac F L P P R R η = × =     +   Ripple factor, 0.482 ac dc I I γ = = i.e., ac dc I I < For transistor or Triode: E B C I I I = + Figure: 23.4 (a) p n p Collector Emitter Base n p n Collector Emitter Base (a) V E RL VL (b) 0 I V (c) V E I L E R Load Slope ( ) 1 L m R − = (d) V0 E IQ Q L E R V I point (VQ, + I Forward Current Forward Bias ‘knee’ voltage +V Forward Voltage 0.3v Germanium Reverse Breakdown Voltage –V Reverse Voltage Reverse Current – 50 mA Germaniu ‘Zener’ Breakdown Or Avalanche Region –I (μA) Reverse Bias N P Anode Cathod Conventional current flow