The Zener Diode Regulator

Transcription

1 1. How Zener diode can stabilize the voltage across load. The device consists of a reverse biased, highly doped, p-n junction diode operating in the breakdown region.the Zener Diode or Breakdown Diode as they are sometimes called, are basically the same as the standard PN junction diode but are specially designed to have a low pre-determined Reverse Breakdown Voltage that takes advantage of this high reverse voltage. The zener diode is the simplest types of voltage regulator and the point at which a zener diode breaks down or conducts is called the Zener Voltage ( Vz ). The Zener Diode Regulator Characteristics of a Zener Diode The Zener Diode is used in its reverse bias or reverse breakdown mode, i.e. the diodes anode connects to the negative supply. Application of Zener Diode: The main application of this type of diodes are as voltage regulator. Over voltage protector, as voltage reference. The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the changes in the supply voltage or the variation in the load current and the zener diode will continue to regulate the voltage until the diodes current falls below the minimum I Z(min) value in the reverse breakdown region. Some common Zener diode breakdown voltages are: 1.8, 3.3, 5.1, 7.5, and 12.6, making them ideal for use in many small circuits. The resistor, R S is connected in series with the zener diode to limit the current flow through the diode with the voltage source, V S being connected across the combination. The stabilised output voltage V out is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive of the DC supply so it is reverse biased and will be operating in its breakdown condition. If there is no load resistance, shunt regulators can be used to dissipate total power through the series resistance and the Zener diode. As long as input voltage is greater than zener voltage, the zener operates in the breakdown region and maintains constant voltage across the load. The series limiting resistance limits the input current. Basically there are two type of regulations such as: a) Line Regulation :- Line regulation is a measure of the ability of the power supply to maintain a steady output voltage when the input line voltage changes. 1

2 Percentage of line regulation can be calculated by = ΔV 0 is the change in output voltage for a particular change in input voltage ΔV IN. Variations in Input Voltage If the voltage source is greater than Vz and I=(Vin Vz) / Rs and I= Iz+ I L If Vin increases --- Is increses --- but I L is constant so Iz increases as Iz = Is-I L If Vin decreses Is decrese --- But I L is constant so Iz decreses. Iz< Izmax then zener voltage remain constant. Only by changing zener current voltage Vz remain constant. b) Load Regulation :- Load regulation is a measure of the ability of the output voltage to remain constant when the output current changes due to a change in the load. Percentage of load regulation = where is the null load resistor voltage (ie. remove the load resistance and measure the voltage across the Zener Diode) and is the full load resistor voltage The load current may vary : The circuit is arranged so that the total supply current I S is made up of the output load current I OUT plus the current in the zener diode I Z : I S = I Z + I L Is = (Vin Vz)/ Rs If R L increases I L decreases --- but Is -- constant so Iz increases. If R L decreases I L increases but Is --constant so Iz decreases. Provided that the zener diode is working within its allowable range of current, the voltage V Z will remain practically constant. Limitations This simple shunt regulator is only suitable for relatively small currents and a fixed range of voltages. There are a number of limitations to the use of this circuit: 2

3 Output voltage: The output voltage is equal to the zener voltage of the diode and so is fixed at one of the available voltage levels and not adjustable. Output current:if the output current falls to zero for any reason (the load may become open circuit due to a fault, or be disconnected from the power supply), all of the load current must be passed by the zener diode. Therefore the maximum current available for the load must be no greater than the maximum safe current for the zener diode alone. Input voltage: The input voltage must be higher (usually about 30% higher) than the output voltage to allow regulation to take place. It should not be too high however, as this will result in more power being dissipated by the diode. Power dissipation:the power dissipated in the diode must be kept within the safe working limits for the device chosen. Maximum power will be dissipated if the load is allowed to go open circuit while the input voltage is at its maximum value. This worst case should not exceed the diode s maximum power rating. 2. Explain LED with application. The electrons dissipate energy in the form of heat for silicon and germanium diodes. But in Galium- Arsenide-phosphorous (GaAsP) and Galium-phosphorous (GaP) semiconductors, the electrons dissipate energy by emitting photons. If the semiconductor is translucent, the junction becomes the source of light as it is emitted, thus becoming a light emitting diode (LED). The actual colour of the visible light emitted by an LED, ranging from blue to red to orange, is decided by the spectral wavelength of the emitted light which itself is dependent upon the mixture of the various impurities added to the semiconductor materials used to produce it. The visible lights that an LED emits are usually orange, red, yellow, or green. The invisible light includes the infrared light. The biggest advantage of this device is its high power to light conversion efficiency. That is, the efficiency is almost 50 times greater than a simple tungsten lamp. The response time of the LED is also known to be very fast in the range of 0.1 microseconds when compared with 100 milliseconds for a tungsten lamp. How LED Works : The light-emitting section of an LED is made by joining n-type and p-type semiconductors together to form a pn junction. When the pn junction is forward-biased, electrons in the n side are excited across the pn junction and into the p side, where they combine with holes. As the electrons combine with the holes, photons are emitted. The pn-junction section of an LED is encased in an epoxy shell that is doped with light scattering particles to diffuse light and make the LED appear brighter. Often a reflector placed beneath the semiconductor is used to direct the light upward. 3

4 The forward bias Voltage-Current (V-I) curve and the output characteristics curve is shown in the figure above. Forward bias of approximately 1 volt is needed to give significant forward current. The second figure is used to represent a radiant power-forward current curve. The output power produced is very small and thus the efficiency in electrical-to-radiant energy conversion is very less. Typical LED Characteristics Semiconductor Material Wavelength Colour V 20mA GaAs nm Infra-Red 1.2v GaAsP nm Red 1.8v GaAsP nm Amber 2.0v GaAsP:N nm Yellow 2.2v AlGaP nm Green 3.5v SiC nm Blue 3.6v GaInN 450nm White 4.0v Application of LED or Light Emitting Diode(LED As a Seven segment Display) The main advantage of light emitting diodes is that because of their small size, several of them can be connected together within one small and compact package producing what is generally called a 7-segment Display.. In motorcycle and bicycle lights. In traffic lights and signals. In message displaying boards. In light bulbs and many more. Now, practically if we sit to list all the applications it will be a non-ending list.. i) Indicators and signs:- these are mainly used in traffic signals, exit signs, light weight message, displaying box etc ii) Lighting:- Light Emitting Diode lamps have become highly popular and as the energy consumption is very low for them, they are also being made by LED s. In television and computer/laptop displaying, LEDs are used. iii) Non visual application:- Communication, sensor are the main area of non visual application of LEDs. Advantages of LED s 4

5 Very low voltage and current are enough to drive the LED. Voltage range 1 to 2 volts. Current 5 to 20 milliamperes. Total power output will be less than 150 milliwatts. The response time is very less only about 10 nanoseconds. The device does not need any heating and warm up time. Miniature in size and hence light weight. Have a rugged construction and hence can withstand shock and vibrations. An LED has a life span of more than 20 years. Disadvantages A slight excess in voltage or current can damage the device. The device is known to have a much wider bandwidth compared to the laser. The temperature depends on the radiant output power and wavelength. LED as seven segment display --- In order to produce the required numbers or characters from 0 to 9 and A to F respectively, on the display the correct combination of LED segments need to be illuminated. A standard seven segment LED display generally has eight input connections, one for each LED segment and one that acts as a common terminal or connection for all the internal segments. A Typical Seven Segment LED Display The displays common pin is generally used to identify which type of 7-segment display it is. As each LED has two connecting pins, one called the Anode and the other called the Cathode, there are therefore two types of LED 7-segment display called: Common Cathode (CC) and Common Anode (CA). The Common Cathode (CC) In the common cathode display, all the cathode connections of the LED segments are joined together to logic 0 or ground. The individual segments are illuminated by application 5

6 of a HIGH, or logic 1 signal via a current limiting resistor to forward bias the individual Anode terminals (a-g). The Common Anode (CA) In the common anode display, all the anode connections of the LED segments are joined together to logic 1. The individual segments are illuminated by applying a ground, logic 0 or LOW signal via a suitable current limiting resistor to the Cathode of the particular segment (a-g). 7-Segment Display Segments for all Numbers. In general, common anode displays are more popular as many logic circuits can sink more current than they can source. Depending upon the decimal digit to be displayed, the particular set of LEDs is forward biased. For instance, to display the numerical digit 0, we will need to light up six of the LED segments corresponding to a, b, c, d, e and f. Then the various digits from 0 through 9 can be displayed using a 7-segment display as shown. Then for a 7-segment display, we can produce a truth table giving the individual segments that need to be illuminated in order to produce the required decimal digit from 0 through 9 as shown below. 7-segment Display Truth Table Decimal Individual Segments Illuminated Digit a b c d e f g Explain photodiode with application. 6

7 Photo means light and diode means a device consisting of two electrodes.a photo diode is a light sensitive electronic device capable of converting light into a voltage or current signal. It works on the principle of photo generation. Photo diode has two terminals anode and cathode with the arrows indicating that the light rays falling on photo diode reflecting its significance as a photo detector.light energy can be considered in terms of photons or packets of light. When a photon of sufficient energy enters the depletion region of a semiconductor diode, it may strike an atom with sufficient energy to release the electron from the atomic structure. This creates a free electron and a hole (i.e. an atom with a space for an electron). The VI characteristics of a photo diode is shown in the figure A photo diode is always operated in reverse bias mode. From the photo diode characteristics it is seen clearly that the photo current is almost independent of applied reverse bias voltage. For zero luminance the photo current is almost zero except for small dark current. It is of the order of nano amperes. As optical power increases the photo current also increases linearly. The maximum photo current is limited by the power dissipation of the photo diode. Quantum efficiency, Dark current of photo diode Quantum efficiency is defined as fraction of incident photons contributing to photo current. It is unit less as it is a fraction. P =N E /(N Photons ) Where N E is the number of generated carriers/unit time N Photons is the number of incident photons/unit time Dark current is the current through the diode for zero illumination. It will be non zero due to back ground radiation and thermally excited minority saturation current. The relation between the Responsivity and quantum efficiency is given as Q = R*h*ν/e and R = I S /P in Where R is Responsivity, Q is Quantum efficiency, h is Planck s constant, ν is frequency of incident photon, Energy of h*ν photon = h*ν, I S is photo generated current, P in is incident optical power. 7

8 Hence the current in a photo diode is given as Disadvantages of normal PN junction photo diodes Normal PN junction photo diodes have very high response times. It has very low sensitivity Applications of photo diodes Photo diodes are used as photo detectors Photo diodes are used in providing electric isolation using a special circuitry called as Optocouplers. Optocoupler is an electronic component which is used in coupling optically the two isolated circuits by using light. The two circuits are optically coupled but electrically isolated. It is a combination of light emitting diode and photo diode (or) avalanche diode (or) photo transistor. Optocouplers are faster than the conventional devices. They are used in consumer electronics They are used in cameras as photo sensors, Slotted optical switch, in scintillators e.t.c Optocoupler : A device containing light-emitting and light-sensitive components, used to couple isolated circuits. In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is a component that transfers electrical signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecting the system receiving the signal. In many applications SCR and triac power circuits are under the control of sensitive electronic systems. For example, it is not unusual to have a microprocessor system programmed to turn motors, lights, and heaters on and off. To reduce the possibility of powerline noise being induced into the control electronics, and to protect it in the event of an SCR or triac failure, it is highly desirable to provide isolation. Optocouplers are used in electronics-sensitive applications. For example, you may use this in a mobile robot application to separate the microcontroller circuitry (low voltage/power) from the motor driver circuitry (high voltage/power). The optical coupling method eliminates the need for a relay-controlled contact or an isolating transformer, which are traditional methods of providing electrical isolation between circuits. The optical coupling method is superior in many applications, because it gets rid of some of the less desirable features of relays and transformers. 8

9 Bipolar junction Transistor 5. Explain Biasing of transistor & CE, CB, CC configuration. Also compare between different configurations. Biasing of the bipolar junction transistor (BJT) is the process of applying external voltages to it. In order to use the BJT for any application like amplification, the two junctions of the transistor CB and BE should be properly biased according to the required application. Depending on whether the two junctions of the transistor are forward or reverse biased, a transistor is capable of operating in three different modes.1.cutoff 2. Saturation 3. Active When transistors are placed into circuits, they are in one of three configurations, known as common emitter, common base, and common collector. Common Base Configuration has Voltage Gain but no Current Gain. Common Emitter Configuration has both Current and Voltage Gain. Common Collector Configuration has Current Gain but no Voltage Gain. The Common Base (CB) Configuration As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal and the output signal. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively (I E =I B +Ic) therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of 1 (unity) or less. Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly load resistance ( RL ) to input resistance 9

10 ( Rin ) giving it a value of Resistance Gain. Then the voltage gain ( Av ) for a common base configuration is therefore given as: Common Base Voltage Gain Where: The current gain, alpha ( α ) = Ic Ie and RL/ Rin is the resistance gain. For this configuration very low input impedance, while having a relatively very high output impedance is there. This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-phase. The common base circuit is generally only used in single stage amplifier circuits such as microphone preamplifier or radio frequency ( Rf ) amplifiers due to its very good high frequency response. The Common Emitter (CE) Configuration In the Common Emitter or grounded emitter configuration, the input signal is applied between the base and the emitter, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward biased PN-junction, while the output impedance is HIGH as it is taken from a reverse biased PNjunction. As the load resistance ( RL ) is connected in series with the collector, the current gain is quite large as it is the ratio of Ic Ic/Ib. Beta, ( β ) = Ib ( Ic ). Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current ( Ib ), will result in a much larger change in the collector current 10

11 This type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit. This means that the resulting output signal is 180 o outof-phase with the input voltage signal. The Common Collector (CC) Configuration In the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The common collector, or emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance. The Common Collector Current Gain This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-phase. It has a voltage gain that is always less than 1 (unity). With the generalised characteristics of the different transistor configurations given in the following table: Characteristics CB CE CC circuit 1) Input reistance (R i ) 2) Output resistance (R o ) low low high high high low 11

12 3) Current amplification factor β α = l + β α β = l - α 1 γ = l - α 4) Total output current 5) Phase relationship between input and output I C = αi E + I CBO I C = βi B +(l + β) I CBO I E = γi B + γi CBO In-phase Out-of phase In-phase 6) Applications For high frequency applications For audio frequency applications For impedance matching 7) Current gain Less than unity Grater than unity Very high 8) Voltage gain Very high Grater than unity Less than unity 4. Explain Input and output characteristics of common base configuration. Input Characteristics :-- These characteristics are obtained by plotting the emitter current versus emitter base potential V EB at constant collector-base potential V CB The observations of I E against V EB are repeated for some other value of V CB Fig. 12

13 (b) represents the input characteristics of common base p-n-p transistor at different collector-base potentials. These characteristics curves show : (i) The emitter current I E increases rapidly with small increase in emitter-base voltage V EB thereby indicating that the input resistance is very small. (ii) The emitter current is almost independent of collector base voltage V CB voltage (V CB ) i.e., Input Resistance : It is the ratio of change in emitter base voltage ΔV EB to the corresponding change in emitter current ΔI E at constant collector-base Physically input resistance is the hindrance offered to the signal current. The input resistance is very small, of the order of a few ohms, because a small change in V EB causes a large change in I E Output Characteristics These characteristics are obtained by plotting the collector current I C versus collector-base voltage V CB at constant emitter current I E The observations are repeated for some other value of I E Fig. (c) represents the output characteristic curves of a common base p-n-p transistor at different emitter current I E These characteristics show: (i) The collector current I C varies with collector-base voltage V CB only at a very low voltage (<IV). This variation is insignificant because the transistor is never operated in this region. (ii) As the collector-base voltage is raised above 1 volt, the collector current I C becomes independent of collector-base voltage V CB but depends only upon the emitter current I E The transistor is always operated in this region. (iii) A very large change in collector-base voltage produces a very small change in collector current; thereby indicating that the output resistance is very high. Output Resistance. It is the ratio of change in collector-base voltage to the corresponding change in collector current at constant emitter current I E 13

14 5. Explain Input output characteristics of common emitter configuration. The output resistance is very high, of the order of several-tens kilo ohm because a large change in collector-base voltage causes a very small change in collector current. It is a curve which shows the relationship between base current IB and the emitter-base voltage, VBE at constant VCE. The method of determining the characteristic is as follows. Fig shows the input characteristic for common emitter configuration. The following points may be noted from the characteristic. The input characteristics of common emitter configuration are obtained between input current I B and input voltage V BE with constant output voltage V CE. Keep the output voltage V CE constant and vary the input voltage V BE for different points, now record the values of input current at each point. Now using these values we need to draw a graph between the values of I B and V BE at constant V CE. The equation to calculate the input resistance R in is given below. R in = V BE /I B (when V CE is at constant) Output Characteristics It is a curve which shows the relationship between the collector IC and the collector- emitter voltage VCE. This method of determining the characteristic is as follows. 14

15 The output characteristics of common emitter configuration are obtained between the output current I C and output voltage V CE with constant input current I B. Keep the base current I B constant and vary the value of output voltage V CE for different points, now note down the value of collector I C for each point. Plot the graph between the parameters I C and V CE in order to get the output characteristics of common emitter configuration. The equation to calculate the output resistance from this graph is given below. R out = V CE /I C (when I B is at constant) Thus, β=(δic / ΔIB)VCE=constant Very large changes of V CE produces a small change in I C i.e output resistance is very high. 11.Explain the three region active region,cut-off region,saturation region for transistor. Active region: In this region the collector is reverse biased and the emitter is forward biased. The collector current, IC response is most sensitive for changes in IB. In active region, the base and collector currents satisfy the condition (DC Current gain). β is a constant for a particular transistor, which varies from to for different transistors. In active region, as V BE > 0.7, in this mode (active), is basically a current amplifier. Cut-off region: Here the operating conditions of the transistor are zero input base current ( I B ), zero output collector current ( I C ) and maximum collector voltage ( V CE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched Fully-OFF. In cut-off, I B =0, as V BE >

16 Saturation region: Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. (saturation current). Therefore the transistor is switched Fully-ON. If I C /I B becomes less than β, the transistor is in saturation. 12. Explain input and output characteristics of common collector configuration. A test circuit for determining the static characteristic of an NPN transistor is shown in Fig. In this circuit the collector is common to both the input and the output circuits. To measure the base and the emitter currents, milli ammeters are connected in series with the base and the emitter circuits. Voltmeters are connected across the input and the output circuits to measure V CE and V CB. It has voltage gain slightly less than unity. INPUT CHARACTERISTICS :--- It is a curve which shows the relationship between the base current I B, and the collector base voltage VCB at constant VCE This method of determining the characteristic is as shown in fig. The base current is taken on the y-axis, and the input voltage is taken on the x-axis. Fig. shows the family of the input haracteristic at different collector- emitter voltages. Input voltage V CB is largly determined by V CE. V EB =V EC V BC as V CB increases with V CE constant V EB decreases hence IB decreases. Input resistance = V CB / I B constant V CE (very High) 16

17 Output Characteristics It is a curve which shows the relationship between the emitter current and collector-emitter voltage, the method of determining the output characteristic is as follows. First, by adjusting the input a suitable current I B is maintained. Next V CB increased in a number of steps from zero and corresponding values of IE are noted. 6. The above whole procedure is repeated for different values of I B. The emitter current is taken on the Y-axis and the collector-emitter voltage is taken on the X-axis. Fig shows the family of output characteristics at different base current values. The following points are noted from the family of characteristic curves. 7. Output resistance = V CE / IE constant IB 13. Explain working of transistor as an amplifier. 17

18 Fig shows the amplifier circuit. Consists of following differnet components: Biasing circuit- consists of R1, R2, R E to maintain the base emmiter junction as forward bias and emmiter collector circuit as reverse bias. 2. C1, C E and Cc are connected to block the dc and pass the ac. 3. Input is given in between emmiter and base. Since the input signal to the common emitter goes positive when the output goes negative, the two signals (input and output) are 180 degrees out of phase. The common-emitter circuit is the only configuration that provides a phase reversal. (Vcc = IcRc + Vc E As Ic increases IcRc increases and Vc E decreases because Vcc remain constant.) The common-emitter is the most popular of the three transistor configurations because it has the best combination of current and voltage gain. The term GAIN is used to describe the amplification capabilities of the amplifier. It is basically a ratio of output versus input The current gain in the common-emitter circuit is called BETA (β). Beta is the relationship of collector current (output current) to base current (input current). To calculate beta, use the following formula: for example ---- This simply means that a change in base current produces a change in collector current which is 44 times as large. The resistance gain of the common emitter can be found in a method similar to the one used for finding beta: Once the resistance gain is known, the voltage gain is easy to calculate since it is equal to the current gain (β) multiplied by the resistance gain (E = βr). And, the power gain is equal to the voltage gain multiplied by the current gain β (P = βe). a. Explain working of BJT as a switch. If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the I-V characteristics curves we have seen previously. Operating Regions Cut-off Region :- Here the operating conditions of the transistor are zero input base current ( I B ), zero output collector current ( I C ) and maximum collector voltage ( 18

19 V CE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched Fully-OFF. The input and Base are grounded ( 0v ) Base-Emitter voltage V BE < 0.7v Base-Emitter junction is reverse biased Base-Collector junction is reverse biased Transistor is fully-off ( Cut-off region ) No Collector current flows ( I C = 0 ) V OUT = V CE = V CC = 1 Transistor operates as an open switch 2. Saturation Region Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched Fully-ON. Saturation Characteristics The input and Base are connected to V CC Base-Emitter voltage V BE > 0.7v Base-Emitter junction is forward biased Base-Collector junction is forward biased Transistor is fully-on ( saturation region ) Max Collector current flows ( I C = Vcc/R L ) 19

20 V CE = 0 ( ideal saturation ) V OUT = V CE = 0 Transistor operates as a closed switch Then the transistor operates as a single-pole single-throw (SPST) solid state switch. With a zero signal applied to the Base of the transistor it turns OFF acting like an open switch and zero collector current flows. With a positive signal applied to the Base of the transistor it turns ON acting like a closed switch and maximum circuit current flows through the device. 20