Lecture notes for Analog Electronics pdf

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CONTENT GENERATION UNDER EDUSAT PROGRAMME ANALOG ELECTRONICS ANALOG ELECTRONICS AND AND OPAMP OPAMP ETT 321 ETT 321 RD 3 SEM ELECTRICAL ENGG. Under SCTE&VT, Odisha PREPARED BY: - PREPARED BY: - 1. Er. DEBI PRASAD PATNAIK Sr. Lecture, Dept of ETC, UCP ENGG. SCHOOL, Berhampur 2. Er. PARAMANANDA GOUDA Lecturer (PT), Dept of ETC, UCP ENGG. SCHOOL, Berhampur PAGE – 1. 1 CHAPTER - 1 P-N JUNCTION DIODE  DEFINITION:-  When a p-type semiconductor is suitably joined to n-type semiconductor, the contact surface is called p-n Junction.  FORMATION OF PN JUNCTION  In actual practice, the characteristic properties of PN junction will not be apparent if a p- type block is just brought in contact with n-type block.  It is fabricated by special techniques and one common method of making PN junction is called Alloying.  In this method, a small block of indium (trivalent impurity) is placed on an n-type germanium slab as shown in Fig (i). The system is then heated to a temperature of about 500ºC. The indium and some of the germanium melt to form a small puddle of molten germanium-indium mixture as shown in Fig (ii). The temperature is then lowered and puddle begins to solidify.  Under proper conditions, the atoms of indium impurity will be suitably adjusted in the germanium slab to form a single crystal.  The addition of indium overcomes the excess of electrons in the n-type germanium to such an extent that it creates a p-type region.  As the process goes on, the remaining molten mixture becomes increasingly rich in indium. When all germanium has been re-deposited, the remaining material appears as indium but- ton which is frozen on to the outer surface of the crystallized portion as shown in Fig (iii).  PROPERTIES OF PN JUNCTION  To explain PN junction, consider two types of materials: - 1) P-Type-P-type semiconductor having –ive acceptor ions and +ive charged holes. 2) N-Type -N-type semiconductor having +ive donor ions and –ive free electrons.  P-type has high concentration of holes & N-type has high concentration of electrons.  The tendency for the free electron to diffuse over p-side and holes to n-side process is called Diffusion. PAGE – 1. 2  When a free electron move across the junction from n-type to p-type, positive donor ions are removed by the force of electrons. Hence positive charge is built on the n-side of the junction. Similarly negative charge establish on p-side of the junction.  When sufficient no of donor and accepter ions gathered at the junction, further diffusion is prevented.  Since +ive charge on n-side repel holes to cross from p-side to n-side, similarly –ive charge on p-side repel free electrons to cross from n-type to p-type.  Thus a barrier is set up against further movement of charge carriers is hole or electrons.  This barrier is called as Potential Barrier/ Junction Barrier (V0) and is of the order 0.1 to 0.3 volt. This prevents the respective majority carriers for crossing the barrier region. This region is known as Depletion Layer.  The term depletion is due to the fact that near the junction, the region is depleted (i.e. emptied) of charge carries (free electrons and holes) due to diffusion across the junction. It may be noted that depletion layer is formed very quickly and is very thin compared to the n region and the p-region.  Once pn junction is formed and depletion layer created, the diffusion of free electrons stops. In other words, the depletion region acts as a barrier to the further movement of free electrons across the junction.  The positive and negative charges set up an electric field as shown in fig below.  The electric field is a barrier to the free electrons in the n-region.  There exists a potential difference across the depletion layer and is called barrier potential (V0). The barrier potential of a p-n junction depends upon several factors including the type of semiconductor material, the amount of doping and temperature.  The typical barrier potential is approximately: - For Si, V = 0.7 V, For Ge, V = 0.3 V. 0 0  PN JUNCTION UNDER FORWARD BIASING  When external D.C. voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, it is called Forward Biasing.  To apply forward bias, connect positive terminal of the battery to p-type and negative terminal to n-type as shown in fig below. PAGE – 1. 3  The applied forward potential establishes an electric field which acts against the field due to potential barrier. Therefore, the resultant field is weakened and the barrier height is reduced at the junction.  As potential barrier voltage is very small (0.1 to 0.3 V), therefore, a small forward voltage is sufficient to completely eliminate the barrier.  Once the potential barrier is eliminated by the forward voltage, junction resistance becomes almost zero and a low resistance path is established for the entire circuit.  Therefore, current flows in the circuit. This is called Forward Current.  With forward bias to PN junction, the following points are worth noting : (i) The potential barrier is reduced and at some forward voltage (0.1 to 0.3 V), it is eliminated altogether. (ii) The junction offers low resistance (called forward resistance, R ) to current flow. f (iii) Current flows in the circuit due to the establishment of low resistance path. The magnitude of current depends upon the applied forward voltage.  CURRENT FLOW IN A FORWARD BIASED PN JUNCTION:-  It is concluded that in n-type region, current is carried by free electrons whereas in p-type region, it is carried by holes. However, in the external connecting wires, the current is carried by free electrons.  PN JUNCTION UNDER REVERSE BIASING  When the external D.C. voltage applied to the junction is in such a direction that potential barrier is increased, it is called Reverse Biasing.  To apply reverse bias, connect negative terminal of the battery to p-type and positive terminal to n-type. PAGE – 1. 4  It is clear that applied reverse voltage establishes an electric field which acts in the same direction as the field due to potential barrier.  Therefore, the resultant field at the junction is strengthened and the barrier height is increased as shown in fig below .  The increased potential barrier prevents the flow of charge carriers across the junction.  Thus, a high resistance path is established for the entire circuit and hence the current does not flow.  With reverse bias to PN junction, the following points are worth noting: (i) The potential barrier is increased. (ii)The junction offers very high resistance (Reverse Resistance R ) to current flow. r (iii)No current flows in the circuit due to the establishment of high resistance path.  VOLT-AMPERE CHARACTERISTICS OF PN JUNCTION:-  Volt-ampere or V-I characteristic of a pn junction (also called a crystal or semiconductor diode) is the curve between voltage across the junction and the circuit current.  Usually, voltage is taken along x-axis and current along y-axis. Fig. shows the circuit arrangement for determining the V-I characteristics of a pn junction.  The characteristics can be studied under three heads namely: 1) Zero external voltage 2) Forward Bias 3) Reverse Bias.  ZERO EXTERNAL VOLTAGE: -  When the external voltage is zero, i.e. circuit is open at K; the potential barrier at the junction does not permit current flow. Therefore, the circuit current is zero as indicated by point O in Fig. PAGE – 1. 5 (II) FORWARD BIAS: -  With forward bias to the pn junction i.e. p-type connected to positive terminal and n-type connected to negative terminal, the potential barrier is reduced.  At some forward voltage (0.7 V for Si and 0.3 V for Ge), the potential barrier is altogether eliminated and current starts flowing in the circuit.  From now onwards, the current increases with the increase in forward voltage.  Thus, a rising curve OB is obtained with forward bias as shown in Fig. From the forward characteristic, it is seen that at first (region OA), the current increases very slowly and the curve is non-linear.  It is because the external applied voltage is used up in overcoming the potential barrier.  However, once the external voltage exceeds the potential barrier voltage, the pn junction behaves like an ordinary conductor.  Therefore, the current rises very sharply with increase in external voltage (region AB on the curve). Here the curve is almost linear. (III) REVERSE BIAS:-  With reverse bias to the pn junction i.e. p-type connected to negative terminal and n-type connected to positive terminal, potential barrier at the junction is increased.  Therefore, the junction resistance becomes very high and practically no current flows through the circuit.  However, in practice, a very small current (of the order of μA) flows in the circuit with reverse bias as shown in the reverse characteristic.  This is called Reverse Saturation Current (I ) and is due to the minority carriers. s  It may be recalled that there are a few free electrons in p-type material and a few holes in n-type material.  These undesirable free electrons in p-type and holes in n-type are called minority carriers. Therefore, a small current flows in the reverse direction. PAGE – 1. 6  If reverse voltage is increased continuously, the kinetic energy of electrons (minority carriers) may become high enough to knock out electrons from the semiconductor atoms.  At this stage breakdown of the junction occurs, characterized by a sudden rise of reverse current and a sudden fall of the resistance of barrier region. This may destroy the junction permanently.  Note: -The forward current through a p-n junction is due to the majority carriers produced by the impurity.  However, reverse current is due to the minority carriers produced due to breaking of some covalent bonds at room temperature.  IMPORTANT TERMS: - (i)BREAKDOWN VOLTAGE: - It is the minimum reverse voltage at which pn junction breaks down with sudden rise in reverse current. (ii)KNEE VOLTAGE: - It is the forward voltage at which the current through the junction starts to increase rapidly. (iii) PEAK INVERSE VOLTAGE (PIV):- It is the maximum reverse voltage that can be applied to the pn junction without damage to the junction. If the reverse voltage across the junction exceeds its PIV, the junction may be destroyed due to excessive heat. The peak inverse voltage is of particular importance in rectifier service. (iv)MAXIMUM FORWARD CURRENT:- It is the highest instantaneous forward current that a pn junction can conduct without damage to the junction. Manufacturer’s data sheet usually specifies this rating. If the forward current in a pn junction is more than this rating, the junction will be destroyed due to overheating. (v) MAXIMUM POWER RATING: - It is the maximum power that can be dissipated at the junction without damaging it. The power dissipated at the junction is equal to the product of junction current and the voltage across the junction. This is a very important consideration and is invariably specified by the manufacturer in the data sheet.  DC LOAD LINE:-  The line obtained by joining the maximum values of I and V in the output c ce characteristics of a CE configuration transistor is known as the DC Load Line.  PN JUNCTION BREAKDOWN:-  Electrical break down of semiconductor can occur due to two different phenomena. Those two phenomena are 1. Zener breakdown 2. Avalanche breakdown  ZENER BREAKDOWN:-  A properly doped crystal diode which has a sharp breakdown voltage is known as a Zener Diode. PAGE – 1. 7  It has already been discussed that when the reverse bias on a crystal diode is increased, a critical voltage, called Breakdown Voltage is reached where the reverse current increases sharply to a high value.  The breakdown region is the knee of the reverse characteristic as shown in Figure.  The satisfactory explanation of this breakdown of the junction was first given by the American scientist C. Zener.  The breakdown voltage is sometimes called Zener Voltage and the sudden increase in current is known as Zener Current. The breakdown or Zener voltage depends upon the amount of doping. If the diode is heavily doped, depletion layer will be thin and consequently the breakdown of the junction will occur at a lower reverse voltage.  On the other hand, a lightly doped diode has a higher breakdown voltage. Fig. shows the symbol of a Zener diode. It may be seen that it is just like an ordinary diode except that the bar is turned into z-shape.  PROPERTIES OF ZENER DIODE:-  The following points may be noted about the Zener diode:  A Zener diode is like an ordinary diode except that it is properly doped to have a sharp breakdown voltage. A Zener diode is always reverse connected i.e. it is always reverse biased. A Zener diode has sharp breakdown voltage, called Zener voltage V . Z  When forward biased, its characteristics are just those of ordinary diode.  The Zener diode is not immediately burnt just because it has entered the breakdown region. As long as the external circuit connected to the diode limits the diode current to less than burn out value, the diode will not burn out.  Zener diode operated in this region will have a relatively constant voltage across it, regardless of the value of current through the device. This permits the Zener diode to be used as a Voltage Regulator.  WORKING/OPERATION OF ZENER BREAKDOWN:-  When the reverse voltage across the pn junction diode increases, the electric field across the diode junction increases (both internal & external).  This results in a force of attraction on the negatively charged electrons at junction.  This force frees electrons from its covalent bond and moves those free electrons to conduction band. When the electric field increases (with applied voltage), more and more electrons are freed from its covalent bonds.  This results in drifting of electrons across the junction and electron hole recombination occurs. So a net current is developed and it increases rapidly with increase in electric field. Zener breakdown phenomena occurs in a pn junction diode with heavy doping & thin junction (means depletion layer width is very small).  Zener breakdown does not result in damage of diode since current is only due to drifting of electrons, there is a limit to the increase in current as well. PAGE – 1. 8  AVALANCHE BREAKDOWN:-  Avalanche breakdown occurs in a p-n junction diode which is moderately doped and has a thick junction (means its depletion layer width is high).  Avalanche breakdown usually occurs when we apply a high reverse voltage across the diode (obviously higher than the zener breakdown voltage,say V ). z  By increasing the applied reverse voltage, the electric field across junction will keep increasing. If applied reverse voltage is V and the depletion layer width is d, then the a generated electric field can be calculated as E =V /d. a a  This generated electric field exerts a force on the electrons at junction and it frees them from covalent bonds. These free electrons will gain acceleration and it will start moving across the junction with high velocity.  This results in collision with other neighboring atoms. These collisions in high velocity will generate further free electrons. These electrons will start drifting and electron-hole pair recombination occurs across the junction. This results in net current which rapidly increases.  From the above fig we can see that avalanche breakdown occurs at a voltage (V ) which a is higher than zener breakdown voltage (V ). z  It is because avalanche phenomena occurs in a diode which is moderately doped and junction width (say d) is high where as zener break down occurs in a diode with heavy doping and thin junction (here d is small).  The electric field that occur due to applied reverse voltage (say V) can be calculated as E = V/d. So in a Zener breakdown, the electric field necessary to break electrons from covalent bond is achieved with lesser voltage than in avalanche breakdown due to thin depletion layer width. PAGE – 1. 9  In avalanche breakdown, the depletion layer width is higher and hence much more reverse voltage has to be applied to develop the same electric field strength (necessary enough to break electrons free).  CLIPPING CIRCUITS  The circuit with which the waveform is shaped by removing (or clipping) a portion of the applied wave is known as a clipping circuit.  Clippers find extensive use in radar, digital and other electronic systems.  Although several clipping circuits have been developed to change the wave shape, we concentrate only on diode clippers.  These clippers can remove signal voltages above or below a specified level.  The important diode clippers are:- 1. Positive clipper and negative clipper 2. Biased positive clipper and biased negative clipper 3. Combination clipper.  POSITIVE CLIPPER  A positive clipper is that which removes the positive half-cycles of the input voltage.  The positive clipper is of two types 1. Positive series clipper 2. Positive shunt clipper  The below Fig. shows the typical circuit of a positive shunt clipper using a diode.  Here the diode is kept in parallel with the load.  During the positive half cycle, the diode ‘D’ is forward biased and the diode acts as a closed switch. This causes the diode to conduct heavily.  This causes the voltage drop across the diode or across the load resistance R to be zero. L Thus output voltage during the positive half cycles is zero.  During the negative half cycles of the input signal voltage, the diode D is reverse biased and behaves as an open switch. Consequently the entire input voltage appears across the diode or across the load resistance R if R is much smaller than R L L  Actually the circuit behaves as a voltage divider with an output voltage of -R / R+ R L L V ≅ -V ( Taking or assuming when R R). max max L PAGE – 1. 10  NEGATIVE CLIPPER  A negative clipper is that which removes the positive half-cycles of the input voltage. The negative clipper is of two types 1. Negative series clipper 2. Negative shunt clipper  The below Fig. shows the typical circuit of a negative shunt clipper using a diode.  During the negative half cycle, the diode ‘D’ is forward biased and the diode acts as a closed switch. This causes the diode to conduct heavily.  This causes the voltage drop across the diode or across the load resistance R to be zero. L Thus output voltage during the negative half cycles is zero.  During the positive half cycles of the input signal voltage, the diode D is reverse biased and behaves as an open switch. Consequently the entire input voltage appears across the diode or across the load resistance R if R is much smaller than R L L  Actually the circuit behaves as a voltage divider with an output voltage of R / R+ R L L V ≅ V ( Taking or assuming when R R). max max L  BIASED POSITIVE CLIPPER  When a small portion of the positive half cycle is to be removed, it is called a biased positive clipper. The circuit diagram and waveform is shown in the figure below.  During negative half cycle, when the input signal voltage is negative, the diode ‘D’ is reverse-biased. This causes it to act as an open-switch. Thus the entire negative half cycle appears across the load, as illustrated by output waveform.  During positive half cycle, when the input signal voltage is positive but does not exceed battery the voltage ‘V’, the diode ‘D’ remains reverse-biased and most of the input voltage appears across the output. PAGE – 1. 11  When during the positive half cycle of input signal, the signal voltage becomes more than the battery voltage V, the diode D is forward biased and so conducts heavily. The output voltage is equal to ‘+ V’ and stays at ‘+ V’ as long as the magnitude of the input signal voltage is greater than the magnitude of the battery voltage, ‘V’.  Thus a biased positive clipper removes input voltage when the input signal voltage becomes greater than the battery voltage.  BIASED NEGATIVE CLIPPER  When a small portion of the negative half cycle is to be removed, it is called a biased negative clipper. The circuit diagram and waveform is shown in the figure below.  During positive half cycle, when the input signal voltage is positive, the diode ‘D’ is reverse-biased. This causes it to act as an open-switch. Thus the entire positive half cycle appears across the load, as illustrated by output waveform.  During negative half cycle, when the input signal voltage is negative but does not exceed battery the voltage ‘V’, the diode ‘D’ remains reverse-biased and most of the input voltage appears across the output.  When during the negative half cycle of input signal, the signal voltage becomes more than the battery voltage V, the diode D is forward biased and so conducts heavily. The output voltage is equal to ‘-V’ and stays at ‘-V’ as long as the magnitude of the input signal voltage is greater than the magnitude of the battery voltage, ‘V’.  Thus a biased negative clipper removes input voltage when the input signal voltage becomes greater than the battery voltage.  COMBINATION CLIPPER:-  Combination clipper is employed when a portion of both positive and negative of each half cycle of the input voltage is to be clipped (or removed) using a biased positive and negative clipper together. The circuit for such a clipper is given in the figure below. PAGE – 1. 12  For positive input voltage signal when input voltage exceeds battery voltage +V diode 1 D conducts heavily while diode D is reversed biased and so voltage +V appears across 1 2 1 the output. This output voltage +V stays as long as input signal voltage exceeds +V . 1 1  On the other hand for the negative input voltage signal, the diode D remains reverse 1 biased and diode D conducts heavily only when input voltage exceeds battery voltage 2 V in magnitude. 2  Thus during the negative half cycle the output stays at -V so long as the input signal 2 voltage is greater than -V . 2  APPLICATIONS OF CLIPPER:-  There are numerous clipper applications however, in general, clippers are used to perform one of the following two functions: (i) CHANGING THE SHAPE OF WAVEFORM: - Clippers can alter the shape of a waveform. For example, a clipper can be used to convert a sine wave into a rectangular wave, square wave etc. They can limit either the negative or positive alternation or both alternations of an a.c. voltage. (ii) CIRCUIT TRANSIENT PROTECTION:- Transients can cause considerable damage to many types of circuits e.g., a digital circuit. In that case, a clipper diode can be used to prevent the transient form reaching that circuit.  CLAMPER CIRCUITS:-  A clamping circuit is used to place either the positive or negative peak of a signal at a desired level. The dc component is simply added or subtracted to/from the input signal.  The clamper is also referred to as an IC restorer and ac signal level shifter.  A clamp circuit adds the positive or negative dc component to the input signal so as to push it either on the positive side.  The clamper is of two types :- 1. Positive clamper 2. Negative clamper  The circuit will be called a positive clamper, when the signal is pushed upward side by the circuit and the negative peak of the signal coincides with the zero level.  The circuit will be called a negative clamper, when the signal is pushed downward by the circuit and the positive peak of the input signal coincides with the zero level. PAGE – 1. 13  For a clamping circuit at least three components — a diode, a capacitor and a resistor are required. Sometimes an independent dc supply is also required to cause an additional shift. The important points regarding clamping circuits are: 1. The shape of the waveform will be the same, but its level is shifted either upward or downward, 2. There will be no change in the peak-to-peak or r.m.s value of the waveform due to the clamping circuit. Thus, the input waveform and output waveform will have the same peak-to-peak value that is, 2V . This is shown in the figure above. It must also be noted max that same readings will be obtained in the ac voltmeter for the input voltage and the clamped output voltage. 3. There will be a change in the peak and average values of the waveform. In the figure shown above, the input waveform has a peak value of V and average value over a max complete cycle is zero. The clamped output varies from 2 V and 0 (or 0 and -2V ). max max Thus the peak value of the clamped output is 2V and average value is V max max. 4. The values of the resistor R and capacitor C affect the waveform. 5. The values for the resistor R and capacitor C should be determined from the time constant equation of the circuit, t = RC. The values must be large enough to make sure that the voltage across capacitor C does not change significantly during the time interval the diode is non-conducting. In a good clamper circuit, the circuit time constant t = RC should be at least ten times the time period of the input signal voltage. It is advantageous to first consider the condition under which the diode becomes forward biased.  POSITIVE CLAMPER:-  Consider a negative clamping circuit, a circuit that shifts the original signal in a vertical downward direction.  The diode D will be forward biased and the capacitor C is charged with the polarity shown, when an input signal is applied.  During the negative half cycle of input, the output voltage will be equal to the barrier potential of the diode, V and capacitor is charged to (V – V ). 0 0 PAGE – 1. 14  During the positive half cycle, the diode becomes reverse-biased and acts as an open- circuit. Thus, there will be no effect on the capacitor voltage.  The resistance R, being of very high value, cannot discharge C a lot during the positive portion of the input waveform.  Thus during positive input, the output voltage will be the sum of the input voltage and capacitor voltage = +V + (V — V ) = +(2 V – V ) . 0 0  The value of the peak-to-peak output will be the difference of the negative and positive peak voltage levels is equal to (2V-V ) - V = 2 V. 0 0  NEGATIVE CLAMPER:-  Consider a negative clamping circuit, a circuit that shifts the original signal in a vertical downward direction.  The diode D will be forward biased and the capacitor C is charged with the polarity shown, when an input signal is applied.  During the positive half cycle of input, the output voltage will be equal to the barrier potential of the diode, V and capacitor is charged to (V – V ). 0 0  During the negative half cycle, the diode becomes reverse-biased and acts as an open- circuit. Thus, there will be no effect on the capacitor voltage.  The resistance R, being of very high value, cannot discharge C a lot during the negative portion of the input waveform.  Thus during negative input, the output voltage will be the sum of the input voltage and capacitor voltage = – V – (V — V ) = – (2 V – V ) . 0 0  The value of the peak-to-peak output will be the difference of the negative and positive peak voltage levels is equal to V - -(2V-V ) = 2 V. 0 0  APPLICATIONS OF CLAMPER:-  Clamping circuits are often used in television receivers as dc restorers in the TV receiver They also find applications in storage counters, analog frequency meter, capacitance meter, divider and stair-case waveform generator.  ALL THE BEST -- ALL THE BEST -- PAGE – 1. 15 CHAPTER - 2 - SPECIAL SEMICONDUCTOR DEVICES  THERMISTOR  Thermistor is the contraction of the term Thermal Resistor.  It is generally composed of semiconductor materials. Most thermistors have a negative coefficient of temperature that is their resistance decreases with the increases of temperature.  This high sensitivity to temperature changes makes thermistors extremely useful for precision temperature measurement, control and compensation.  The temperature measurement of thermistor ranges from -60 0C to 150 0C and the resistance of thermistor ranges from 0.5Ω to 0.75MΩ. It exhibits highly non-linear characteristics of resistance versus temperature.  CONSTRUCTION  These thermistors are composed of sintered mixture of metallic oxides such as Manganese, Nickel, Cobalt, Copper, Iron and Uranium.  These may be in the form of beads or rods or discs or probes.  The relation between resistance and absolute temperature of a thermistor can be represented as  R =R expβ(1/T1)-(1/T2) T1 T2  Where RT1=resistance of thermistor at absolute temperature T1 K  R =resistance of thermistor at absolute temperature T2K T2  And β=a constant depending on the material of the thermistor (usually it ranges from 3500 K to 4500 K).  FEATURES  These are compact, rugged and inexpensive and have good stability when properly aged.  Measuring current is maintained at a value as low as possible so that self-heating of thermistors is avoided otherwise errors are introduced on account of changes of resistance caused by self-heating. PAGE – 2. 1 CHAPTER - 2 - SPECIAL SEMICONDUCTOR DEVICES  THERMISTOR  Thermistor is the contraction of the term Thermal Resistor.  It is generally composed of semiconductor materials. Most thermistors have a negative coefficient of temperature that is their resistance decreases with the increases of temperature.  This high sensitivity to temperature changes makes thermistors extremely useful for precision temperature measurement, control and compensation.  The temperature measurement of thermistor ranges from -60 0C to 150 0C and the resistance of thermistor ranges from 0.5Ω to 0.75MΩ. It exhibits highly non-linear characteristics of resistance versus temperature.  CONSTRUCTION  These thermistors are composed of sintered mixture of metallic oxides such as Manganese, Nickel, Cobalt, Copper, Iron and Uranium.  These may be in the form of beads or rods or discs or probes.  The relation between resistance and absolute temperature of a thermistor can be represented as  R =R expβ(1/T1)-(1/T2) T1 T2  Where RT1=resistance of thermistor at absolute temperature T1 K  R =resistance of thermistor at absolute temperature T2K T2  And β=a constant depending on the material of the thermistor (usually it ranges from 3500 K to 4500 K).  FEATURES  These are compact, rugged and inexpensive and have good stability when properly aged.  Measuring current is maintained at a value as low as possible so that self-heating of thermistors is avoided otherwise errors are introduced on account of changes of resistance caused by self-heating. PAGE – 2. 2  The upper operating limit of temperature for thermistor is dependent on physical changes in the material.  For thermistor the Response time can vary from fraction of second to minute depending on the size of detecting mass and thermal capacity of the thermistor.  Response time varies inversely with dissipation factor.  APPLICATIONS  It is used for measurement and control of temperature and for temperature compensation.  It is used for measurement of power at high frequency. It is also used for thermal conductivity.  Thermistor is used for measurement of level, flow and pressure of liquid, composition of gases and vaccum measurement. It is used for providing time delay.  BARRETERS  Barreters are the short length wires with fine diameters with operating range around 1500C.  SENSORS  A sensor is a device that detects events or changes in quantities and provides a corresponding output, generally as an electrical or optical signal; for example, a thermocouple converts temperature to an output voltage.  Sensors are used in everyday objects such as touch-sensitive elevator buttons and lamps which dim or brighten by touching the base, besides innumerable applications of which most people are never aware.  With advances in micro machinery and easy to use microcontroller platforms, the uses of sensors have expanded beyond the more traditional fields of temperature, pressure or flow measurement.  Moreover, analog sensors such as potentiometers and force-sensing resistors are still widely used. Applications include manufacturing and machinery, airplanes and aerospace, cars, medicine and robotics.  A sensor's sensitivity indicates how much the sensor's output changes when the input quantity being measured changes.  For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is 1 cm/°C .  Sensors need to be designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages.  Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology.  In most cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. PAGE – 2. 3 ZENER DIODE:-  A properly doped crystal diode which has a sharp breakdown voltage is known as a Zener Diode.  It has already been discussed that when the reverse bias on a crystal diode is increased, a critical voltage, called Breakdown Voltage is reached where the reverse current increases sharply to a high value.  The breakdown region is the knee of the reverse characteristic as shown in Fig.  The satisfactory explanation of this breakdown of the junction was first given by the American scientist C. Zener.  The breakdown voltage is sometimes called Zener Voltage and the sudden increase in current is known as Zener Current.  The breakdown or Zener voltage depends upon the amount of doping. If the diode is heavily doped, depletion layer will be thin and consequently the breakdown of the junction will occur at a lower reverse voltage.  On the other hand, a lightly doped diode has a higher breakdown voltage.  The given figure shows the symbol of a Zener diode. It may be seen that it is just like an ordinary diode except that the bar is turned into z-shape. The following points may be noted about the Zener diode: 1. A Zener diode is like an ordinary diode except that it is properly doped to have a sharp breakdown voltage. 2. A Zener diode is always reverse connected i.e. it is always reverse biased. 3. A Zener diode has sharp breakdown voltage, called Zener voltage V . Z 4. When forward biased, its characteristics are just those of ordinary diode. 5. The Zener diode is not immediately burnt just because it has entered the breakdown region.  As long as the external circuit connected to the diode limits the diode current to less than burn out value, the diode will not burn out. PAGE – 2. 4  Zener diode operated in this region will have a relatively constant voltage across it, regardless of the value of current through the device. This permits the Zener diode to be used as a Voltage Regulator.  TUNNEL DIODE:-  Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p–n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the p-n junction.  As voltage increases further these states become more misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage.  As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the p–n junction, and no longer by tunneling through the p–n junction barrier.  The most important operating region for a tunnel diode is the negative resistance region.  When used in the reverse direction, tunnel diodes are called back diodes (or backward diodes) and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction).  Under reverse bias, filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction.  In a conventional semiconductor diode, conduction takes place while the p–n junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the “reverse breakdown voltage” when conduction begins (often accompanied by destruction of the device).  In the tunnel diode, the dopant concentrations in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction.  However, when forward-biased, an odd effect occurs called quantum mechanical tunnelling which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current.  In the current voltage characteristics of tunnel diode, we can find a negative slope region when forward bias is applied.

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