Presentation on semiconductor electronic circuits.ppt
- 1. VAGDEVI P U SCIENCE COLLEGE BAGALKOT Seminar on SEMICONDUCTOR ELECTRONIC CIRCUITS by L.K.Kama Director of vagdevi p u science college
- 2. 1. Introduction:- Semiconductor electronic devices have almost replaced the vacuum tubes. The semiconductor devices are small in size, Consume less power, operate at low voltages, have high reliability and long life. The concepts of semi-conductor electronics were floated way back in 1930. The cathode, Ray Tubes (CRT) used in television & computer monitors which work on the principle of vacuum tubes are being replaced by liquid crystal Display (LCD) monitors with supporting solid state electronics.
- 3. CLASSIFICATION OF METALS 1.Conductors: - High conductivity or low resistivily. Metals have a large number of free electrons. Conductivity lies between 102 & 108 S/m. Resistivity lies between 10-2 & 10-8 m. Examples: glass, rubber, plastic etc. 2. Semiconductors:- Conductivity and resistivity in between conductors & insulators. Very few free electrons at room temp. Conductivity lies between 10-5 & 100 S/m. Resistivity lies between 10-4 & 0.5 m. Examples: Germanium, Silicon, Carbon etc.
- 4. CLASSIFICATION OF SEMICONDUCTORS 1. Elemental semi-conductors:- These are the semi-conductors consisting of a single element Ex:- Ge, Si 2. Compound semiconductors:- These are the semi-conductor consisting of two or more elements. Ex:- Cadmium sulphate (Cds). Indium phosphate (Inp). Gallium arsenide (GaAs) etc Ex:- Organic – Anthracene Organic polymers Polyaniline After 1990 organic semiconductors and semi conducting polymers have been developed. Band Theory of Solids The range of energies possessed by an electron in a solid is known as energy band. 1. Valence band:- The range of energies possessed by valence electrons is known as V.B. 2. Conduction band:- The range of energies possessed by conduction electrons is known as C.B. 3. Forbidden energy gap:- The separation between conduction band & valence band is known as forbidden energy gap (Eg)
- 7. Intrinsic semi conductor:- A pure semiconductor is called intrinsic semi conductor They have crystalline structure. They are formed by covalent bonds. The number of free electrons in the conduction band is equal to the number of holes in the valence band. Its electrical conductivity is low. Its electrical conductivity depends on temperature alone. It is of no practical use. Current Conduction in intrinsic semi conductor: I = Ie + Ih
- 8. Extrinsic Semiconductor:- It is an impure semi conductor that is a controlled pentavelent or trivalent impurity added to a pure semi conductor. In an n-type semiconductor, free electrons for exceed the holes. In a p-type semi conductor, it is the reverse. Its electrical conductivity is high. Its electrical conductivity depends on temperature & the amount of doping. It is used in electronic devices. Doping The process of deliberate addition of controlled amount of suitable impurity to a pure semi conductor so as to increases its conductivity is called doping.
- 9. ESSENTIAL REQUIREMENTS FOR THE DOPING PROCESS The concentration of impurity atoms should be proper that is for about 108 atoms of semi conductor, 1 impurity atom should be added. The do pant atom should take the position of semi conductor atom in the crystal The size of the do pant atom should be almost the same as that of the semi conductor atom. For this, atoms of third & fifth group of the periodic table are most suitable. The presence of the do pant atoms should no disturb the crystal lattice. Pentavalent dopants Trivalent dopants The pentavalent dopants have Trivalent dopants have 5 valence electrons 3 valence electrons. Ex: arsenic (As), antimony (Sb) Ex: Indium (In), Aluminium (Al) Born (B) Dopants
- 10. FUNDAMENTAL CONCEPTS . Diodes and transistors are the electronic devices, made up of semiconductors. Diodes is an electronic device which us used to convert AC into DC. AC means alternating current. In India, frequency if AC is 50 Hz that means, this current alternates 50 times in one second. Frequency of DC is zero i.e. the DC us a constant current it is called a direct current. Voltage stabilization is possible using special type of diode e.g. zener diode. Solar cells are the semiconductor devices converts light energy into electrical energy. In computers only two digits i.e. Zero (0) one (1) are used to represent the data. Resistivity of good conductor is of the order of 10-8 m. Resistivity of an insulator is of the order of 104 m. Resistivity of semiconductor is of the order of 10-1 m.
- 11. SUBJECTIVE MATTER: FOR SELF STUDY INTRODUCTION In case of a single isolated atom, the electrons any orbit posses a definite energy. In solids a very large number of atoms are very closely packed together. So electrons in any orbit can have a range of energies rather than a single energy level. This is known as energy band. Group of discrete but closely spaced energy levels for the orbital electrons in the atom is called energy band. The range of energies possessed by an electron in a solid is called energy band. Energy of different orbits of an isolated atom is shown by energy level diagram. Energy bands in solids:- VALENCE BAND : The range of energies (band) possessed by valence electrons is called valence band. The electrons in the outermost orbit of an atom are known as valence electrons. Valance band has the electrons of highest energy. This band may be completely filled or partially filed with electrons.
- 12. CONDUCTION BAND The lowest unfilled energy band formed just above the valence band is called conduction band. The band (or range of energies) possessed by conduction electrons is known as conduction band. All electrons in the conduction band are free electrons. These electrons are called conduction electrons because they are responsible for the conduction of current in a conductor. If in some substance conduction band is empty means current conduction is not possible. FORBIDDEN ENERGY GAP The separation between conduction band and valence band in the energy level diagram is known as forbidden energy gap. Energy gap Eg = Ec – Ev No electron of a solid can stay in a forbidden energy gap. If this gap is greater, than valence electrons are more tightly bound to the nucleus. The electrical conductivity of solids can be explained in terms of energy bands.
- 13. CONDUCTORS:- The substance which are easily allow the passage of electric current through them are called conductors. When the two energy bands i.e. Valence band conduction band overlap, the material is called conductor. Due to the overlapping, a slight potential difference across a conductor causes the free electrons to constitute electric current. When the temperature of conductor increases, its resistance increases and conductivity decreases. The conductors have a positive temperature coefficient e.g. copper, Aluminium, gold etc. INSULATORS :- Insulators are those substances which do not allow the passage of electric current through them. The substances which do not contain free electron are called insulators e.g. glass, plastic etc. In insulators the conduction band and the valence band are separated by large energy gap. In insulators the valence band is full while the conduction band is empty. The forbidden gap is very large and it is from 5 eV to 15 eV. The forbidden energy gap for diamond is about 6 eV, which means that 6 eV energy is required to electron move from valence to conduction band. Insulator have negative temperature coefficient.
- 14. SEMICONDUCTORS The semiconductors are crystalline materials whose electrical conductivity lies in between conductors and insulators e.g. Ge, Si etc. OR – The materials which acts as insulator at absolute zero temperature and acts as conductor at high temperature is called semiconductor. The valence band is almost filled and conduction band is almost empty. The width of the forbidden band is about 1 eV. For Germanium it is 0.72 eV and fir silicon 1.1 eV. Conductivity of semiconductor increases as its temperature increases i.e. it has negative temperature coefficient. As the temperature rises, many electrons from the valence band jump to the conduction band and a vacancy is created in the valence band. This vacancy is called hole. The flow of electricity in a semiconductor is constituted by the drift of electrons and holes. . INTRINSIC AND EXTRINSIC SEMICONDUCTOR A semiconductor in an extremely pure form is known as intrinsic semiconductor. Ge and Si are widely used as semiconductor. They posses the same crystal structure and similar characteristics. A silicon atom has 14 electrons (K-2, L-8, M-4), while a germanium atom has 32 electrons (K-2, L-8, M-18, N-4). Both have four electrons in outermost orbit i.e. they have four valence electrons. All these four electrons are used to form covalent bonds with electrons of neighboring atom hence no free electrons are left.
- 15. p & n TYPE SEMICONDUCTOR 1. P-type Semiconductor When a small amount of trivalent impurity such as aluminium, indium, boron, gallium etc, is added to an intrinsic semiconductor (Ge) the resulting crystal is called p-type (positive type) semiconductor. It will form three covalent bonds and fourth one is unbounded. There is a deficiency of electron to form fourth covalent bond which is called as hole. Holes are majority carriers and electrons are minority carriers. These are called acceptor impurity because impurities like Al, In, Ga etc accepts electrons from the pure semiconductor. Since one aluminum atom provides one hole, three is a large number of holes provided by a large number of aluminum atoms in the crystal. The flow of electricity is due to the motion of holes (positive charge) hence the semiconductor is called as p-type of semiconductor.
- 16. 2. n-type semiconductor :- When a small amount of pentavalent impurity such as antimony, arsenic, bismuth etc is added to an intrinsic semiconductor (Ge), the resulting crystal is called n-type semiconductor. Out of the five valence electrons, four electrons form four covalent bonds. The remaining fifth electron is free which is useful for conduction. Electrons are majority carriers and holes are minority carriers. In n-type semiconductor the impurity donates the free electrons, therefore the impurity (pentavalent) is called as donor impurity. The flow of electricity in this type is due to the electrons (negative charge), hence the doped semiconductor is called as n type of semiconductor. The charge carried by the free electrons per second across the semiconductor is called electron current The charge carried by the holes per second across the semiconductor is called hole current. The hole current exists only within the semiconductor. Outside the semiconductor, the current is due to the drifting of electrons. It is found that the drift velocity of the electrons is more than that of the holes.
- 17. p-n JUNCTION DIODE :- p-n junction diode is a two terminal solid state electronic device having rectifying property. Thin crystal of semiconductor material doped from one side by donor impurity and other side by acceptor impurity then a p-n junction is formed. When a p type semiconductor is suitably joined to n type semiconductor, the boundary between them is called p-n junction and the arrangement is called p-n junction diode. The free electrons diffuse from n-region to p region, forming + ve ions near boundary in n region, hence n type acquires a net + ve charge. The holes from p-region diffuse in n-region, forming negative ions near the boundary in p-region, hence p type acquires a net –ve charge. Thus a small portion of p-region near the junction is negatively charged and a small portion of n-region near the junction becomes positively charged. A thin layer on both sides of the p-n junction which is free from the majority charge carriers is called depletion layer. The width of depletion layer is about 10-4 cm around the junction. The potential barrier stops further diffusion of holes and electrons.
- 18. Advantages of semiconductor devices :- A vacuum diode needs a high degree of vacuum and there is a danger of leakage of air in its enclosure. For a semiconductor diode, no vacuum is needed. As semiconductor devices have no filament, hence no power is needed to heat them to cause the emission of electrons. Semiconductor diode operates at a low voltage. Vacuum tube devices take certain time, to start their functioning, but semiconductor devices start almost instantaneously. A semiconductor diode is lighter and smaller than a vacuum diode. During operation semiconductor devices do not produce noise. Semiconductor devices are shock proof. A semiconductor diode is cheaper than a vacuum diode. A semiconductor diode has a much longer life than that of a vacuum diode.
- 19. RECTIFIERS A device which converts an alternating current (AC) into a direct current (DC) is called rectifier. The process in which alternating current is converted into a direct current is called rectification. In p-n junction diode current flows only in one direction i.e. from p region to n-region. Due to this property p-n junction diode can be used as a rectifier. HALF WAVE RECTIFIER:- The circuit containing single diode and transformer behaves as a half wave rectifier. The rectifier that produces DC during half cycle of AC is half wave rectifier. During positive half cycle of input AC voltage, the diode is in forwards bias and hence it conducts sending current through R1. During negative half cycle of input AC voltage, the diode is in reverse bias and hence it dies not conduct, no current flows through load resistance R1. The output waveform of a half wave rectifier is unidirectional but not a perfect DC. The average voltage is called the dc voltage because it is what dc voltmeter connected across R1 would read. How much direct current the diode can handle is called as current rating of diode (I0). During the negative half cycle of the input, all secondary voltage appears across the diode. To avoid breakdown, the PIV rating of the diode must be more than the peak inverse voltage. PIV = Vm and ripple frequency is equal to input frequency. Maximum efficiency of half wave rectifier is 40.6% and the ripple factor is 1.21. Disadvantages:- The pulsating current in the load contains alternating component. Filtering is required to produce steady (direct) current and it is not so easy to filter. The a.c. supply delivers poser only half the time.
- 21. In the circuit diagram of diode as full wave rectifier, AC source, centre tap transformer, two diodes and load resistance are used. During the positive half cycle, diode D1 is forward biased and D2 is in reverse biased. The current is through D1, R1 and the upper half of the secondary winding. During negative half cycle, D2 is forward biased and D1 is in reverse biased. The current flows through D2, R1 and lower half of the secondary winding. The current through the load resistor is in the same direction in both half cycles. Therefore this circuit provides full wave rectified output. Neglecting the diode drop, the average or dc value is = 0.636 VP. VP is the peak value of voltage across half the secondary winding. Each diode conducts for only half a cycle. Thus the current rating (I0) of diode need only be greater than half the dc load current (0.5 Idc). Maximum efficiency of full wave rectifier is 81.2% and ripple factor is 0.482. Disadvantages:- It is difficult to locate the centre tap on the secondary winding. The d.c. output is small as each diode utilizes only one half of the transformer secondary voltage. The diodes used must have high peak inverse voltage.
- 22. ZENER DIODE AS A VOLTAGE REGUATOR ZENER DIODE p-n junction diode conducts in forward bias and it does not conduct in reverse bias. If we connect the p-n junction diode in reverse bias and potential is increased, then at a specific potential diode conductors for a small time. It is called as break down of diode. The applied potential is called as break down potential. But after few second, the diode gets damaged. Zener diode is a special purpose diode which is manufactured to use in reverse bias. In may electronic applications it is desired that the output voltage should remain constant regardless of the variations in the input voltage or load, for this a zener diode is used. The critical value of the reverse voltage at which p-n junction breaks down with sudden rise in reverse current is called as breakdown voltage. The satisfactory explanation of breakdown of the junction was first given by American scientist Clarence Melvin Zener in 1934, hence break down voltage is called zener voltage and sudden increases in current is called zener current. The specially designed junction diode, which can operate in the reverse breakdown voltage region continuously without being damaged is called Zener diode. A Zener diode is like an ordinary diode except that it is properly doped so as to have s sharp breakdown voltage.
- 23. A Zener diode is always reverse connected i.e. it is always used in reverse biased. A zener diode has sharp breakdown voltage, called Zener voltage VZ. When Zener diode is in forward biased, its characteristics are similar to ordinary diode. When the reverse voltage across a Zener diode exceeds the breakdown voltage VZ, the current increases very sharply. In this region, the curve is almost vertical. It means that voltage across Zener diode is constant at VZ even though the current through it changes. Therefore in the breakdown region, an ideal Zener may be represented by a battery of voltage VZ. When doping is high, width of depletion layer becomes very small and it is about 10-7 m. The breakdown voltage depends upon the amount of doping. A lightly doped diode has a higher breakdown voltage and vice-versa. Since silicon can sustain to higher temperature and handle higher current, it is preferred as compared to germanium. A large current flows through zener diode at breakdown because of two effects called as zener effect and avalanche effect. Zener effect: When applied reverse voltage is breakdown voltage or more, large number of electron hole pairs are generated because they are pulled from covalent bonds. Therefore current suddenly increases, this is called a zener effect. Avalanche effect: It occurs with lightly doped p-n junctions due to the breakdown of covalent bonds as a result of collision of electrons and holes with the valence electrons. This generates new electrons which are again accelerated. So more atoms gets ionised and thus a bunch of electrons or avalanche of electrons is produced which increases the reverse current through zener. This is called a avalanche effect.
- 24. Zener diode as a voltage Regulator :-
- 25. A rectifier with an appropriate filter serves as a good source of DC output called power supply. The major disadvantage of such power supply is that the output voltage changes with the variation in the input voltage or load. In many electronic devices such as T.V., freeze, computer etc. requires a constant voltage. To obtain a voltage stabilisation a zener diode is used. A zener diode can be used as voltage regulator to provide a constant voltage from a source whose voltage may vary over sufficient range. Zener regulator is also called as zener diode shunt regulator. The load regulation (load effect) is defined as the change in regulated output voltage when the load current changes from minimum to maximum value. The line regulation (source effect) is the change in regulated load voltage for the specified range of line voltage. Change in supply voltage (line Regulation): If there is increase in the input voltage Vin, current through zener diode Iz increases and the load current IL remains constant. The extra voltage is dropped across RS and output voltage across load remains constant.
- 26. Ratings of a Zener diode:- Power dissipation (Pz): This is the product of voltage and current Pz = Iz Vz. As long as Pz is less than the power rating, the zener diode can operate in the breakdown region without being destroyed. Power ratings may be from W to more than 50 W. SOLAR CELL:- It is a device which converts solar energy into electrical energy. It is also called as photovoltaic cell or solar energy converter. It is based on the generation of electron hole pairs with incident light photon. When energy of photon hv is greater than then the forbidden energy gap of the semiconductor then photons will excite electrons from the valence band to the conduction band leaving hole in the valence band and hole pairs are generated. For the activation of solar cell no external biasing in required. Solar cells can be constructed by using the difference types of junction like p-n junction, metal oxide semiconductor (MOS) or metal insulator semiconductor (MIS) junction. In solar cell the active junction area is kept large to get more power.
- 27. Applications of solar cell:- Emergency lamps require a battery. In these lamps the battery is charged with help of a small solar panel. A street light in villages use solar energy. Solar panel charges a battery during day time and in the night battery supplies power to the street lights. Solar cells are used for supplying power to different electrical and electronic devices in the satellite. Solar cells are used to produce electrical power in the remote areas, where power from the power generating stations is unavailable. Solar cells are used in power traffic signs (signals). Solar cells are used in water pumping sets for micro irrigation. A calculator requires very small current. Most of the calculators are operated with a solar cell. Solar cell provides energy for weather monitoring equipments. Solar cells are used to charge the storage batteries. Solar cells are used in railway signalling equipment. By mounting solar panels on the building roofs, the homes and offices can be powered. Such as solar heater, solar cooker, solar bulbs, etc. Light emitting diode (LED) LED is a special p-n junction diode which emits light when it is in forward bias. LEDS are prepared from compound semiconductors such as gallium arsenide (GaAs), Gallium phosphide (GaP) or gallium Arsenide Phosphide (GaAsP). The colour of the light emitted by the diode depends upon the material and the doping level. The commonly available LED’s emit red, orange, green, yellow and blue light. When LED is in forward bias it emit light instead of heat generated by normal diode. In case of silicon and germanium diodes, the energy released is in infrared region. In the junction diode made of Gap, GaAs or GaAsP, the energy released in visible region. The band gap energy of GaAsP is 1.9 eV, then the wavelength of emitted photon is: Which is the visible wavelength.
- 28. 1. Advantages :- 1. LED’s require low operational voltage and less power consumption. 2. Fast action and no warm up time required 3. Long life, smaller in size and weight. 4. The bandwidth of emitted light is 100 Ao to 500 Ao 5. Infrared, visible as well as ultraviolet rays are emitted which depends on the material used. 2. Uses of LED’s:- 1. LED’s are used in numeric displays such as seven segment display. 2. In traffic light. 3. In Burglar alarm circuit 4. In photovoltaic cell. 5. In digital watches, calculators, MultiMate. 6. In fibre optical communication systems. 7. In indicator lams and display. 8. LED’s are used as indicators in different equipments like a power supply, transistor, radio, T.V. mobile phone etc. 9. Infrared LED’s are used for remote control for a T.V. set. 10. White color LED’s are used in torch.
- 29. Transistor as an amplifier:- 1. Transistor Transistor was first invented in 1948 by J. Bardeen and W. H. Brattain of Bell Telephone, U.S.A. A transistor is a three terminal device consists of two p-n junctions formed by sandwiching either p type or n type semiconductor between a pair of opposite type of semiconductors. Transistor is similar to the vacuum tube triode and is capable of achieving amplification of weak signals. On adding a third doped element to a crystal diode in such a way that two p n junction are formed, we get a device known as transistor. There are two types of transistor 1) p-n-p transistor 2) n-p-n transistor p-n-p transistor: A thin layer of n type semiconductor is sandwiched between two layers of p type semiconductor. n-p-n transistor: A than layer of p type semiconductor is sandwiched between two layers of n type semiconductor. The transistors has three sections. The three sections are named as emitter, collector and base depending upon their functions. Three sandwiched thin layer is base and other two are emitter and collector. The base is very thin and it is very lightly doped, the emitter is heavily doped and collector is moderately doped. The emitter emit electrons or holes (depending on transistor), collector collect them through base and base produce interaction between emitter and collector.
- 30. • In the symbolic representation of transistor, an arrow is always associated with emitter. • If an arrow is directed inward, the transistor is called as p-n-p transistor and if an arrow is directed outward, it is n-p-n transistor. •Transistor has two p-n junctions. •The junction between emitter and base is called EB junction OR EB diode or emitter diode. •The junction between base and collector is called collector base junction or collector base diode. •In a transistor three are three terminals taken from each type of semiconductor. •In normal operation of a transistor, the emitter base junction is always forward bias and collector base junction is in reverse bias. •Suitable P.D. should be applied across the two junctions to operate the transistor, this is called biasing of transistor. •While using the transistor, base is not connected at the output and collector is not connected at the input in any circuit. •The resistance of emitter diode is very small as compared to collector diode. •In order to have input and output circuit, we require four terminals, hence in transistor one of the terminal is made common. •Transistor can be operated in three different modes. 1. Common emitter mode in which base is at the input and collector is at the output. 2.Common base mode in which emitter is at the input and collector is at the output. 3.Common collector mode in which base is at the input and emitter is at the output.
- 32. In p-n-p transistor current is due to motion of holes whole in n-p-n transistor current is due to the motion of electrons. Figure shows action of p-n-p transistor in common base mode. E-B junction is in forward bias, hence a large emitter current IE flows. The base is lightly doped, hence a small base current IB flows. The remaining more than 95% holes enter into the collector region and collector current IC flows. Generally IB IC. This is the action of any transistor. In case of n-p-n transistor, collector current is directed towards the emitter. In this also base current is small and IC IE. The emitter base junction of a transistor is in forward biased and therefore its resistance is low about 50 . The base collector junction is in reverse biased and its resistance is high about 50,000. As we have seen, the input emitter current almost entirely flows in the collector circuit. Therefore, a transistor transfers the input signal current from a low resistance circuit to a high resistance circuit. This is the key factor responsible for the amplifying capability.
- 33. 2. Characteristics of transistor:- Transistor characteristics means the curves which represent relationship between different DC currents and voltage of a transistor. i) Input characteristics: The graph between the input voltage and the input current keeping the output voltage constant is called the input characteristic of transistor. In C E mode, it is the curve between base current IB and base emitter voltage VBE at constant collector emitter voltage VCE. The characteristic curve is similar to the forward bias characteristics of junction diode. The ratio of change in base emitter voltage to change in base current at constant collector emitter voltage is called input resistance in CE mode, constant The value of input resistance for a CE mode circuit is of the order of a few hundred ohms. ii) Output characteristics: The graph between the output voltage and output current keeping the input current constant is called output characteristics of the transistor. In CE mode, it is the curve between collector current IC and collector emitter voltage at constant base current. Collector current increases rapidly and it becomes constant. After a specific value of VCE, it becomes independent on VCE. Up to specific value of collector to emitter potential, collector current increases with base current. The value of VCE up to which collector current IC changes is called the knee voltage Vknee. The transistors are always operated in the region above knee voltage. The ratio of change in collector emitter voltage to the change in collector current at constant base current is called output resistance. constant The value of output resistance is large and it is of the order of 50 K.
- 34. Transfer characteristics: The graph between input current and output current at constant output voltage is called transfer characteristic of the transistor. The graph is straight line i.e. if base current increases then collector current also increases. AMPLIFIER The electronic device which increases the power of signal is called as amplifier. OR An electronic device which increases the strength of weak input signal is called amplifier. The signal is nothing but it is an alternating voltage of small amplitude. Transistor is used as an amplifier. Voltage Gain is the ratio of output voltage to input voltage. CE amplifier is widely used for amplification than other types of configuration because it has much greater current gain, voltage gain and power gain. The input voltage signal VS which is to be amplified is applied across emitter base circuit (Input) and the amplified output is obtained across the emitter and collector. The signal voltage produces variation in the base current which produces large and similar variations in the collector current. This variable collector current produces corresponding large variation in the voltage across the load resistance i.e. amplified output. In the amplifier, as the signal voltage is increasing in the positive half cycle, the output voltage is increasing in the negative sense i.e. output is 180o out of phase with the input.
- 35. For CE amplifier: When the signal voltage VS is not applied then the gain of amplifier is called DC current gain. When the signal voltage VS is applied then the gain is called AC current gain. Voltage gain: The ratio of the change in output voltage to the change in input voltage is called voltage gain. Current gain: The ratio of change in the collector current to the change in the base current is called current gain.
- 36. OSCILLATOR Oscillators are the electronic circuits which produces electrical oscillations (waveforms) of desired frequency. OR oscillator is a device which can provide output of desired frequency, without any input signal. OR An oscillator is defined as the electronic circuit that that converts energy from a D.C. source into a periodically varying electrical output. As the term oscillator indicates the generation of a frequency, but it should be noted that it does not create energy but it acts as an energy converter. An oscillator can produce sinusoidal or non-sinusoidal waves. An oscillator is nothing but it is an amplifier with positive feedback. The function of oscillator is opposite to that of a rectifier. The process of injecting a fraction of output energy of some device bake to its input is known as feedback. When the feedback energy (voltage or current) is 180o out of phase with the input signal and thus opposes it, it is called negative feedback. When the feedback energy is in phase with the input signal and thus aids it, it is called positive feedback. Amplifier and feedback network, both produces phase shift of 180o each. Hence the total phase shift is 360o . An oscillator consists of three main parts. (a) Tank circuit (b) Transistor amplifier (c) Feedback circuit. The parallel combination of inductor of inductance L and capacitor of capacitance C us called as tank circuit.
- 37. An Integrated circuit- • It is one in which circuit components such as transistors, diodes, resistors, capacitors etc, are automatically part of a small semiconductor chip. • An integrated circuit consists of a number of circuit components (e.g. transistors, diodes, resistors etc.) and their interconnections in a single small package to perform a complete electronic function. These components are formed and connected within a small chip of semiconductor material. The following points are worth noting about integrated circuits. 1. In an IC, the various components are automatically part of a small semiconductor chip and the individual components cannot be removed or replaced. This is in contrast to discrete assembly in which individual components can be removed or replaced if necessary. 2. The size of an IC is extremely small. In fact, ICs are so small that you normally need a microscope to see the connections between the components. Fig. 28.28 shows a typical semiconductor chip having dimensions 0.2 mm x 0.2 mm x 0.001 mm. It is possible to produce circuits containing many transistors, diodes, resistors etc. on the surface of this small chip.
- 38. ADVANTAGES AND DISADVANTAGES OF INTEGRATED CIRCUITS ADVANTAGES Increased reliability due to lesser number of connections. Extremely small size due to the fabrication of various circuit elementsin a single chip of semiconductor material. Lesser weight and **space requirement due to miniaturized circuit. Low power requirements. Greater ability to operate at extreme values of temperature. DISADVANTAGES:- If any components in an IC goes out of order, the whole IC has to be replaced by the new one, In an IC, it is neither convenient nor economical to fabricate capacitances exceeding 30pF. Therefore for high values of capacitance, discrete components exterior to IC chip are connected. It is not possible to fabricate inductors and transformers on the semiconductor chip. Therefore, these components are connected exterior to the semiconductor chip. It is not possible to produce high power ICs (greater than 10W). There is a lack of flexibility in an IC i.e. it is generally not possible to modify the parameters within which an integrated circuit will operate. IC PACKINGS:- In order to protect ICs from external environment and to provide mechanical protection, various forms of encapsulation are used for integrated circuits. Just as with semiconductor devices.