ELECTRONICS FUNDAMENTALS for Aircraft Engineeers

ELECTRONICS FUNDAMENTALS for Aircraft Engineeers EASA Part-66 Cat. B1,B2 SHAHZAD KHALIL ELECTRONIC FUNDAMENTALS Shahzad Khalil EASA part-66-B1, B2 Electronic Fundamentals 4.1 Semiconductors 4.1.1 Diodes (a) Diode symbols; Diode characteristics and properties; Diodes in series and parallel; Main characteristics and use of silicon controlled rectifiers (thyristors), light emitting diode, photo conductive diode, varistor, rectifier diodes; Functional testing of diodes. (b) Materials, electron configuration, electrical properties; P and N type materials: effects of impurities on conduction, majority and minority characters; PN junction in a semiconductor, development of a potential across a PN junction in unbiased, forward biased and reverse biased conditions; Diode parameters: peak inverse voltage, maximum forward current, temperature, frequency, leakage current, power dissipation; Operation and function of diodes in the following circuits: clippers, clampers, full and half wave rectifiers, bridge rectifiers, voltage doublers and triplers; Detailed operation and characteristics of the following devices: silicon controlled rectifier (thyristor), light emitting diode, Schottky diode, photo conductive diode, varactor diode, varistor, rectifier diodes, Zener diode. 4.1.2 Transistors (a) Transistor symbols; Component description and orientation; Transistor characteristics and properties. (b) Construction and operation of PNP and NPN transistors; Base, collector and emitter configurations; Testing of transistors. Basic appreciation of other transistor types and their uses. Application of transistors: classes of amplifier (A, B, C); Simple circuits including: bias, decoupling, feedback and stabilisation; Multistage circuit principles: cascades, push-pull, oscillators, multivibrators, flip-flop circuits. Page B1 2 level B2 2 - 1 2 2 - 2 Electronic Fundamentals 4.1.3 Integrated Circuits (a) Description and operation of logic circuits and linear circuits/operational amplifiers. (b) Description and operation of logic circuits and linear circuits; Introduction to operation and function of an operational amplifier used as: integrator, differentiator, voltage follower, comparator; Operation and amplifier stages connecting methods: resistive capacitive, inductive (transformer), inductive resistive (IR), direct; Advantages and disadvantages of positive and negative feedback. 4.2 Printed Circuit Boards Description and use of printed circuit boards. 4.3 Servomechanisms (a) Understanding of the following terms: Open and closed loop systems, feedback, follow up, analogue transducers; Principles of operation and use of the following synchro system components/features: resolvers, differential, control and torque, transformers, inductance and capacitance transmitters. (b) Understanding of the following terms: Open and closed loop, follow up, servomechanism, analogue, transducer, null, damping, feedback, deadband; Construction operation and use of the following synchro system components: resolvers, differential, control and torque, E and I transformers, inductance transmitters, capacitance transmitters, synchronous transmitters; Servomechanism defects, reversal of synchro leads, hunting. Page Level B1 B2 1 - 2 1 2 - 1 - 2 4.1-SEMICONDUCTORS Semiconductor Materials: Semiconductor materials are insulators at absolute zero temperature that conduct electricity in a limited way at room temperature. They have negative temperature coefficient. There resistivity lies in between conductors and insulators. The defining property of a semiconductor material is that there electronic properties (conductivity) can be controlled either by increasing temperature or by throwing light or by doping or by increasing electrical potential across them. Selected groups of Periodic Table of Elements (Semiconductors) ii(+2) Zn Zinc Cd Cadmium iii(+3) B Boron Al Aluminum Ga Gallium In indium iv(+- 4) C Carbon Si Silicon Ge Germanium Sn Tin v (-3) N Nitrogen P Phosphorus As Arsenic Sb Antimony vi (-2) O oxygen S Sulphur Se Selenium Te Tellurium Note: fig. within the bracket shows the valency. Elemental semiconductors include Silicon and germanium; atoms of these materials are given below. SILICON GERMANIUM From figure it is clear that each atom has four electrons in its outer most shell; these electrons are known as valence electrons. Valence electrons are at a greater distance from the nucleus therefore these are less tightly bound and have an active role in electrical conduction. There exists also Compound Semiconductors; composed of elements from two or more different groups of the periodic table. For e.g. group-III (B, Al, Ga, In) and group-V (N, P, As, Sb, Bi) combine to form binary (two elements, e.g. GaAs), ternary (three elements, e.g. InGaAs) and quaternary (four elements, e.g. AlInGaP). Same is the case for group-ii and vi elements. The essential characteristic of Silicon crystal structure is that each atom has four electrons to share with adjacent atoms in forming bonds. The nature of a bond between two silicon atoms is such that each atom provides one electron to share with the other. The two electrons thus shared are in fact shared equally between the two atoms. This type of sharing is known as a covalent bond. Such a bond is very stable, and holds the two atoms together very tightly, so that it requires a lot of energy to break this bond. This is the reason that pure Si behaves as an insulator. At room temperature the atoms are vibrating sufficiently in the lattice for a few bonds to break, setting free some valence electrons, leaving a hole where an electron was. Free electrons are attracted towards the hole as the atom considered is now positively charged. Covalent bonds break when temperature increases If an electric potential is applied across pure semiconductor material, electrons are attracted towards positive terminal and holes towards negative terminal of the battery. This current flow is very small and is called as ‘intrinsic conduction’ and the pure semiconductor material itself is known as ‘intrinsic material’. The concept of hole is understood by considering it as a vacancy or deficiency of electron. As the electron moves in one direction, this vacancy moves in opposite direction. If the temperature is increased, electron pairs break and more electron-holes are generated which increases conductivity and hence decreasing resistance. More heat is generated and increasing more conduction and leads to thermal runaway. This eventually destroys crystal structure. Doping: The conductivity of semiconductors is altered by adding some impurities in a small quantity typically 1 in billionth. The material is then called as extrinsic semiconductor. An N-type semiconductor (N for Negative) is obtained by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free charge carriers. When the doping material is added, it gives away (donates) weakly-bound outer electrons to the semiconductor atoms. This type of doping agent is also known as donor material since it gives away some of its electrons. The purpose of N-type doping is to produce an abundance of mobile or "carrier" electrons in the material. To help understand how n-type doping is accomplished, consider the case of silicon (Si). Si atoms have four valence electrons, each of which is covalently bonded with each of the four adjacent Si atoms. If an atom with five valence electrons, such as those from group V(e.g. phosphorus (P), arsenic (As), or antimony (Sb)), is incorporated into the crystal lattice in place of a Si atom, then that atom will have four covalent bonds and one un-bonded electron. This extra electron is only weakly bound to the atom and can easily be excited into the conduction band. At normal temperatures, virtually all such electrons are excited into the conduction band. Since excitation of these electrons does not result in the formation of a hole, the number of electrons in such a material far exceeds the number of holes. In this case the electrons are the majority carriers and the holes are the minority carriers. Because the five-electron atoms have an extra electron to "donate", they are called donor atoms. Note that each movable electron within the semiconductor is never far from an immobile positive dopant ion, and the N-doped material normally has a net electric charge of zero. Free electrons can migrate through the inter-atomic space and can therefore act as current carriers when a very low voltage is applied. Intentionally hidden A p-n junction is a junction formed by combining P-type and N-type semiconductors together in very close contact. Both pieces are neutral up to the instant of contact. The term junction refers to the region where the two regions of the semiconductor meet. It can be thought of as the border region between the p-type and n-type blocks as shown in the following diagram: + ++ + + + ++ ++ + + +++ P ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ N ‘+’ represents a hole and ‘–‘ an electron As the holes are the vacancies for the electrons so as the two regions contact each other, electrons migrate towards the junction to fill in the holes. As electron leaves the N type material it becomes positively charged and the P-type material which acquires an electron becomes negatively charged. In an equilibrium PN junction, electrons near the PN interface tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (donors) on the n region. Similarly holes near the PN interface begin to diffuse in the n-type region leaving fixed ions (acceptors) with negative charge. The regions nearby the PN interfaces lose their neutrality and become charged, forming the space charge region or depletion layer. T Intentionally hidden Forward Bias Forward-bias occurs when the P-type semiconductor material is connected to the positive terminal of a battery and the N-type semiconductor material is connected to the negative terminal, as shown below. With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed towards the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type material repels the holes, while the negative charge applied to the Ntype material repels the electrons. As electrons and holes are pushed towards the junction, the distance between them decreases. This lowers the barrier in potential. With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field can't counteract charge carrier motion across the p-n junction, consequently reducing electrical resistance. The electrons which cross the p-n junction into the P-type material (or holes which cross into the N-type material) will diffuse in the near-neutral region. Therefore, the amount of minority diffusion in the near-neutral zones determines the amount of current that may flow through the diode. To maintain the flow of current through the PN junction requires a voltage greater than barrier potential. Intentionally hidden Semicond ductor Diodee: Most modeern diodes are based on sem miconductor p-n p junctions.. In a p-n diode, connventional cuurrent can flow w from the p-ttype side (thee anode) to the n-type side (the cathode), but cannot flo ow in the oppposite directioon. mbol Diode Sym Diode Ch haracteristicss: A semicondductor diode'ss current–volttage characterristic, or I–V curve, is related to the transport of carriers thhrough the so--called deplettion layer or ddepletion regiion that exists at th he p-n junctioon between diiffering semicconductors. If an exterrnal voltage is placed across the diode w with the samee polarity as thhe built-in po otential, the depletion zone continu ues to act as ann insulator, prreventing anyy significant eelectric currennt flow. This is the reveerse bias phennomenon. Hoowever, if the polarity of thhe external vooltage opposees the built-in potential, recombinatioon can once again proceed,, resulting in substantial ellectric currentt through the p-n junction. For silicoon diodes, the built-in potenntial is approximately 0.6 V. Thus, if an n external current is passed through the diode, about 0.6 V will w be develooped across thhe diode suchh that the Pdoped reggion is positiv ve with respecct to the N-dooped region an nd the diode is i said to be "turned on" as it has a forward biass. At very laarge reverse bias, b beyond the peak inverrse voltage orr PIV, a proceess called reveerse breakdow wn occurs which causes a laarge increase in current thaat usually dam mages the dev vice permanenntly. The avalaanche diode is i deliberatelyy designed forr use in the avvalanche region. In the Zener diode, the conceept of PIV is not n applicablee. A Zener dioode contains a heavily dopped p-n junction allowing a electtrons to tunneel from the vaalence band off the p-type material m to thee conduction band of thhe n-type material, such thaat the reversee voltage is "cclamped" to a known valuee (called the Zener volttage), and avaalanche does not occur. Booth devices, however, h do hhave a limit too the maximum m current and power in the clamped reveerse voltage region. Also, ffollowing thee end of forward conduction in any diode, th here is reversee current for a short time. T The device do oes not attain its full blo ocking capabiility until the reverse curreent ceases. Intentionally hidden Diode parameters: Peak Inverse voltage: PIV is the maximum voltage that a diode can withstand in the reverse direction without breaking down or avalanching. If this voltage is exceeded the diode may be destroyed. Diodes must have a peak inverse voltage rating that is higher than the maximum voltage that will be applied to them in a given application. Maximum Forward Voltage (VF): usually specified at the diode's rated forward current. Ideally, this figure would be zero: the diode providing no opposition whatsoever to forward current. In reality, the forward voltage is described by the “diode equation.” Maximum (average) forward current (IF(AV)): the maximum average amount of current the diode is able to conduct in forward bias mode. This is fundamentally a thermal limitation: how much heat can the PN junction handle, given that dissipation power is equal to current (I) multiplied by voltage (V or E) and forward voltage is dependent upon both current and junction temperature. Ideally, this figure would be infinite. Maximum (peak or surge) forward current (IFSM or if(surge)): The maximum peak amount of current the diode is able to conduct in forward bias mode. Again, this rating is limited by the diode junction's thermal capacity, and is usually much higher than the average current rating due to thermal inertia (the fact that it takes a finite amount of time for the diode to reach maximum temperature for a given current). Ideally, this figure would be infinite. Maximum total dissipation (PD): The amount of power (in watts) allowable for the diode to dissipate, given the dissipation (P=IE) of diode current multiplied by diode voltage drop, and also the dissipation (P=I2R) of diode current squared multiplied by bulk resistance. Fundamentally limited by the diode's thermal capacity (ability to tolerate high temperatures). Maximum DC reverse voltage (VR or VDC): The maximum amount of voltage the diode can withstand in reverse-bias mode on a continual basis. Ideally, this figure would be infinite. Operating junction temperature (TJ ): The maximum allowable temperature for the diode's PN junction, usually given in degrees Celsius (oC). Maximum reverse current (IR): The amount of current through the diode in reverse-bias operation, with the maximum rated inverse voltage applied (VDC). Sometimes referred to as leakage current. Ideally, this figure would be zero, as a perfect diode would block all current when reverse-biased. In reality, it is very small compared to the maximum forward current. Typical junction capacitance (CJ): The typical amount of capacitance intrinsic to the junction, due to the depletion region acting as a dielectric separating the anode and cathode connections. This is usually a very small figure, measured in the range of picofarads (pF). Reverse recovery time (trr): The amount of time it takes for a diode to “turn off” when the voltage across it alternates from forward-bias to reverse-bias polarity. Ideally, this figure would be zero: the diode halting conduction immediately upon polarity reversal. For a typical rectifier diode, reverse recovery time is in the range of tens of µ-Sec.; for a “fast switching” diode, it may be a nanoseconds. v Intentionally hidden Half Wave rectifier A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one phase supply, or with three diodes in a three-phase supply. When anode is positive with respect to cathode the diode conducts, this causes a current to flow across the circuit and a voltage will be developed across R. When the input polarity reverses the diode becomes reverse biased and will switch off. The voltage developed across R is therefore half sine wave and is known as half wave Rectifier. The output is DC but its magnitude varies. The average value is half of that of supply i.e. 0.318 of peak voltage. The output ripple frequency is equal to supply frequency. Half wave Rectifier Characteristics Peak input voltage Peak output voltage DC Output Output Frequency 1st Approx. 2nd Approx. Vp Vp Vp (Output)/π fout=fin Vp Vp-0.7 Vp (Output)/π fout=fin Intentionally hidden Bridgge Rectifier A bridge rectifier maakes use of four fo diodes inn a bridge arrrangement tto achieve fuull-wave rectificattion. In each caase, the upperr right output remains posittive and loweer right outputt negative. Since this is true whethher the input is AC or DC,, this circuit nnot only produuces a DC outtput from an AC A input, it can also provide p what is i sometimes called "reverrse polarity prrotection". Thhat is, it perm mits normal functionin ng of DC-pow wered equipm ment when battteries have beeen installed backwards, b or when the leads (wirres) from a DC C power sourrce have beenn reversed, and protects thee equipment from f potential damage d causeed by reverse polarity. Fulll wave Bridg ge Rectifier Characteristics Peak input voltage v Peak output voltage DC Output Output Frequ uency 1st Approx. Vp Vp Vp /π (=0.637 Vp) fout=2fin The peak inverse voltaage across eacch diode shouuld be equal too the supply peak p voltage. Intentionally hidden Voltage Multipliers Voltage multipliers m aree used primariily to developp high voltagees where low current is reqquired. The most com mmon applicattion of the higgh voltage outtputs of voltaage multipliers is the anodee of cathoderay tubes (CRT), whichh are used forr radar scope presentationss, oscilloscopee presentationns, or TV picture tub bes and the High H Energy Ignition I Unit of o Engines. The T dc output of the voltage multiplier ranges fro om 1000 voltss to 30,000 voolts. The actual voltage deppends upon itts equipment application. Just like trransformers, when voltagee is stepped upp, the output current decreeases. This is also true of voltage multipliers. m Altthough the measured m outpuut voltage of a voltage multiplier may be b several times greaater than the input i voltage,, once a load is connected the value of the t output volltage decreases. Also any sm mall fluctuatio on of load imppedance causees a large flucctuation in thee output voltage off the multiplieer. For this reeason, voltagee multipliers are a used only in special appplications where thee load is consttant and has high h impedancce or where innput voltage sstability is noot critical. Voltage multipliers m maay be classifieed as voltage doublers, tripplers, or quadrruplers. The classification c depends on o the ratio off the output vo oltage to the iinput voltage. Voltage multipliers increease voltages through thhe use of seriees-aiding volttage sources. This can be compared c to the t connectionn of cells (batteries)) in series. During onne half cycle of o the supply,, upper capacitor will charge up u to V volts, on the other halff cycle lower capacitor c willl charge. As the two capacitors are a in series; then t the output is approximately a y 2V volts. Here is annother type off half wave vooltage doubleer circuit. By conneecting the ouutput of one multiplying circuit onto the input off the next (caascading) the dc vooltage outputt can be fourr times the acc input. Intentionally hidden Intentionally hidden Intentionally hidden Intentionally hidden Parallel Diode Clipping Circuit In this type of clippers, the diode is connected between output terminals. The on/off state of diode directly affects the output voltage. Following figures illustrate the clipping process. Intentionally hidden Zeneer diode A Zener diode d is a typpe of diode thhat permits currrent in the fo orward directiion like a norrmal diode, but also inn the reverse direction if thhe voltage is llarger than thee breakdown voltage know wn as "Zener knee voltaage" or "Zeneer voltage". A convenntional solid-state diode will not allow siignificant currrent if it is reeverse-biased below its reverse brreakdown volltage. When th he reverse biaas breakdown n voltage is exxceeded, a conventional diode is su ubject to highh current due to avalanche breakdown. Unless U this cuurrent is limitted by external circuitry, c the diode d will be permanently damaged. In case of large forward bias (current in the directiion of the arroow), the diode exhibits a vvoltage drop due d to its juncction built-in voltage v and internal reesistance. Thee amount of thhe voltage droop depends on the semiconnductor materrial and the doping cooncentrations. A Zener diode d exhibitts almost the same s propertiies, except thee device is sppecially designned so as to have a greeatly reduced breakdown voltage, v the soo-called Zeneer voltage. A Zener diode contains a heavily do oped p-n juncction allowing g electrons to tunnel from the t valence band of the p-ttype material to the connduction band d of the n-typee material. In the atomic sccale, this tunnneling correspponds to the transport of o valence baand electrons into the emptty conductionn band states; as a result off the reduced barrier between these bands b and higgh electric fiellds that are innduced due to the relativelyy high levels of dopings on both sidees. A reverse--biased Zenerr diode will exxhibit a contrrolled breakdoown and allow the current to keeep the voltagee across the Z Zener diode att the Zener vooltage. For exxample, a diode withh a Zener breakdown voltaage of 3.2 V will w exhibit a voltage drop of 3.2 V if reeverse bias voltage appplied across it is more thaan its Zener vooltage. Howeever, the curreent is not unliimited, so the Zener diode is typicallyy used to gennerate a refereence voltage for f an amplifier stage, or ass a voltage stabilizer for low-curreent applicationns. m th hat produces a similar effecct is the avalaanche effect ass in the avalanche diode. Another mechanism The two tyypes of diodee are in fact coonstructed thee same way and a both effeccts are presentt in diodes off this type. In silicon dioodes up to aboout 5.6 volts, the Zener efffect is the preddominant effeect and shows a marked m negatiive temperatu ure coefficientt. Above 5.6 volts, v the avalanche effect becomes predominant and exhib bits a positive temperature coefficient. In a 5.6 V diode, the tw wo effects occcur together aand their tempperature coeffficients neatlyy cancel each other out, thus the 5.6 V diode is thee component of choice in temperature-c t critical applications. Intentionally hidden Light Emitting Diode-LED A light-emitting-diode (LED) is a semiconductor diode that emits light when an electric current is applied in the forward direction of the device. The effect is a form of electroluminescence where incoherent and narrow-spectrum light is emitted from the p-n junction in a solid state material. LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. An LED is usually a small area (less than 1 mm2) light source, often with optics added directly on top of the chip to shape its radiation pattern and assist in reflection. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. Besides lighting, interesting applications include using UV-LEDs for sterilization of water and disinfection of devices, and as a grow light to enhance photosynthesis in plants. The LED consists of a chip of semiconducting material doped, with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side to n-side, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors. Intentionally hidden Intentionally hidden Intentionally hidden It is often said that the Schottky diode is a "majority carrier" semiconductor device. This means that if the semiconductor body is doped n-type, only the n-type carriers (mobile electrons) play a significant role in normal operation of the device. The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons (keep in mind the mobility of electron is greater than holes). Therefore no slow, random recombination of n- and p- type carriers is involved, so that this diode can cease conduction faster than an ordinary p-n rectifier diode. This property in turn allows a smaller device area, which also makes for a faster transition. This is another reason why Schottky diodes are useful in switch-mode power converters; the high speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 5 GHz. The Schottky diode is used in logic gates. Schottky metal-semiconductor junctions are featured in the successors to the 7400 TTL family of logic devices, the 74S, 74LS and 74ALS series. Shock kley diode A Shockleey diode is, inn effect, one of o the first inttegrated circu uits. It is just a four layer diode, a pnpn device, ass shown in Fig gure. Consider an alternativee rendering off the device's construction: Shown likke this, it appeears to be a seet of interconnnected bipolaar transistors, one PNP and d the other NPN. Draawn using stan ndard schemaatic symbols, and respectin ng the layer ddoping concen ntrations not shown in the last imagee, the Shockleey diode lookks like this: Let's conn nect one of thhese devices to o a source of variable voltaage and see w what happens:: Intentionally hidden DIAC Shockley diodes are un nidirectional devices; d that iis, they only conduct c curreent in one direection. If bidirectionnal (AC) operation is desirred, two Shocckley diodes may m be joinedd in parallel facing f different directions d to form f a new kiind of thyristoor, the DIAC:: The termss anode and cathode c no lon nger apply, soo the connectiions are simpply named term minal 1 (T1) and terminal t 2 (T22). Each term minal can serve as either an node or cathodde, according to the polarity of the applied voltage. A DIAC operated o with h a DC voltage across it behhaves exactly y the same as a Shockley diode. d With AC, howeever, the behaavior is differeent from whaat one might expect. e Becauuse alternatingg current repeatedlyy reverses dirrection, DIAC Cs will not staay latched lonnger than one--half cycle. Iff a DIAC becomes latched, l it willl continue to conduct currrent only as loong as there iss voltage avaiilable to pushh enough cuurrent in that direction. Whhen the AC poolarity reverses, as it must twice per cyccle, the DIAC C will drop out due to inssufficient currrent, necessitating anotherr breakover beefore it condu ucts again. Thhe result is a current waveeform that loooks like this: DIACs arre almost neveer used alone,, but in conjuunction with other o thyristorr devices. Intentionally hidden Intentionally hidden Intentionally hidden Intentionally hidden Typical SCR’s Intentionally hidden Photodiodes A photodiode is a diode optimized to produce an electron current flow in response to irradiation by ultraviolet, visible, or infrared light. Silicon is the most often used to fabricate photodiodes; though, germanium and gallium arsenide can be used. The junction through which light enters the semiconductor must be thin enough to pass most of the light on to the active region. As it operates in reverse bias mode there will be leakage current (minority carriers) which increase in proportion to the amount of light falling on the junction. The light energy breaks the bond in the crystal lattice of the semiconductor and produces electrons and holes to increase the leakage current. Intentionally hidden Intentionally hidden 4. Capacitance Compared to zener diodes, varistors have a higher capacitance. Depending on the application, transient suppressor capacitance can be a desirable or undesirable feature. In DC circuits, the capacitance of varistors provides both decoupling and transient voltage clamping functions. 5. Less Expensive Varistors are both cost and size effective compared with diode. Surge capability (typical) Response time Shunt capacitance Metal-oxide Up to 70,000 varistor (MOV) Amps @ 100 Amps, 8x20 ~1 µs pulse shape: 1000 nanosecond surges Typically 100 1000 pF +++ 10 microamps Avalanche diode 50 Amps @ 50 Amps, 8x20 µs pulse shape: infinite 50 pF 10 microamps Gas tube @ 500 Amps, 8x20 > 20,000 Amps µs pulse width: 200 surges Type Lifetime - number of surges Submicrosecond <5 < 1 pF microseconds Leakage current (approximate) picoamps Intentionally hidden Testing Silicon Diodes (Not LED Or Zener) To test a silicon diode such as a 1N914 or a 1N4001 all you need is an ohm-meter. If you are using an analog VOM type meter, set the meter to one of the lower ohms scales, say 02K, and measure the resistance of the diode both ways. If you get zero both ways, the diode is shorted. If you get INFINITY both ways, the diode is open. If you get INFINITY one way but some reading the other way (the value is not important) then the diode is good. If you use a digital multi-meter (DMM), then there should be a special setting on the Ohms range for testing diodes. Often the setting is marked with a diode symbol: Measure the diode resistance both ways. One way the meter should indicate an open circuit. The other way you should get a reading (often a reading around 600). That indicates the diode is good. If you measure an open circuit both ways, the diode is open. If you measure low resistance both ways, the diode is shorted. Testing Diodes in Circuit The procedures described above assume the diode under test is not part of any circuit. If you are trying to test a diode that is on a circuit board or otherwise connected to other components, then you should disconnect one end of the diode. On a circuit board you can unsolder one end of the diode and lift it off the board. Make sure that you first disconnect all power going to the circuit before you disconnect the diode. After disconnecting one end, proceed as described above. Intentionally hidden Transistor A transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Construction and Theory The bipolar or junction transistor consists of two p-n junctions back to back in the same crystal. If two PN junctions are fused together so that the two P regions form a very thin (0.1-1mm thick) lightly doped layer between the two more heavily doped N regions an NPN transistor is formed. Figure shows the layout of transistor and symbol. Collector Base Emitter The three electrodes are called as Emitter (represented by arrow in symbol), Base and the Collector. The emitter is more heavily doped than Collector which is more heavily doped than Base. The physical size of collector is much higher than emitter and that of Base is very much small as compared to Emitter. Similarly if two heavily doped P regions are separated by a very thin and lightly doped N regions then a PNP transistor is formed. Collector Base Emitter Operation of a Transistor (PNP) Again the base-emitter junction id forward biased and the collector-base junction is reverse biased. Under the influence of the electric field due to battery VCC, holes cross the junction into the base. Only 1-2% of holes recombine with the free electrons in the base due to it being very thin and lightly doped. The majority of the holes (98-99%) are accelerated towards the very strong negative influence of battery VCC. Holes are the majority carriers in the PNP transistor. Due to recombination of electrons and holes in the base, the base region loses free electrons and will therefore exhibit a positive charge. The electrons will be attracted by the battery VCC into the base to make up for those lost by recombine ing with holes. 9 R4 1.0kΩ 7 Q2 R3 8 1.0kΩ 6 Vcc 12 V BJT_PNP_4T_VIRTUAL Conventional Current flow in PNP Transistor Vbb 6V 10 The arrow on the emitter of the transistor symbol points in the direction of conventional current. IE= IB + IC Intentionally hidden Transistor Characteristics and Parameters: Consider the fig. shown where transistor is biased by two batteries, although in actual circuits a single supply VCC is normally taken directly from the power supply output and VBB which is smaller can be produced with a voltage divider bias circuit. DC Beta (βdc) and DC Alpha (α): The ratio of the dc collector current (IC) to the base current (IB) is the dc beta (βdc), which is the dc current gain of a transistor. βdc = IC/IB Typical values of βdc range from less than 20 to 200 or higher. βdc is designated as hFE on transistor data sheets. The ratio of the dc collector current (IC) to the emitter current (IE) is the dc alpha (α dc). αdc = IC/IE Typical values of αDC range from 0.95-0.99 or greater, but αdc is always less than 1. Example: Determine βdc and IE for a transistor where IB =50µA and IC = 3.65mA. Solution: βdc = IC/IB = 3.65mA/50µA = 73 IE= IB + IC = 3.65mA + 50µA = 3.70mA. A certain transistor has a βdc of 200. When the base current is 50 µA, determine the collector current. Intentionally hidden Current and Voltage Analysis: Consider the basic transistor bias circuit configuration in figure. Three transistor dc currents and three dc voltages can be identified. IB: dc base current IE: dc emitter current IC: dc collector current VBE = dc voltage at base w.r.t. emitter VCB = dc voltage at collector w.r.t. base VCE = dc voltage at collector w.r.t. emitter VBB forward biases the emitter base junction and VCC reverse biases the base collector junction. When the BE junction is forward biased, it is like a forward biased diode and has a nominal voltage drop of 0.7 volt. Although in actual transistor VBE can be as high as 0.9 volt and is dependent upon current. Since the emitter is at ground (0v), by Kirchhoff’s Voltage Law, the voltage across RB is VRB =VBB - VBE Also by ohm’s Law VRB = IBRB Substituting for IBRB = VBB -VBE Solving for IB, IB = (VBB -VBE) / RB The voltage at the collector w.r.t. the grounded emitter is VCE = VCC - VRC Since the drop across RC is, VRC = ICRC The voltage at the collector can be written as, VCE = VCC - ICRC Where IC = βdc IB The voltage across the reverse biased collector base junction is VCB = VCE- VBE Intentionally hidden Testing of Transistor Meter readings will be exactly opposite, of course, for an NPN transistor, with both PN junctions facing the other way. Intentionally hidden Class AB Class AB Amplifier Operation Amplifiers designed for class AB operation are biased so that collector current is zero (cutoff) for a portion of one alternation of the input signal. This is accomplished by making the forward-bias voltage less than the peak value of the input signal. By doing this, the base-emitter junction will be reverse biased during one alternation for the amount of time that the input signal voltage opposes and exceeds the value of forward-bias voltage. Therefore, collector current will flow for more than 180 degrees but less than 360 degrees of the input signal, as shown in figure view B. As compared to the class A amplifier, the dc operating point for the class AB amplifier is closer to cutoff. The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion. Class C: In class C operation, collector current flows for less than one half cycle of the input signal, as shown in figure. The class C operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow. The class C operated amplifier is used as a radio-frequency amplifier in transmitters. Common Emitter Amplifier (Class A, voltage divider bias) Let us first consider the biasing of the circuit. Here voltage divider bias is used. R1 and R2 divide the supply voltage into the same ratio as that of the resistors. So if the resistor values are 16kΩ and 4kΩ then with a supply voltage of 10volts, the voltage across R1 and R2 will be 8v and 2v respectively. The voltage across base-emitter must be 0.6volts to overcome the barrier potential. This could be achieved by removing RE and making R2 of such a value that 0.6volts is dropped across base-emitter junction but then R2 would be quite low and amplification will be restricted. VBE = VR2 - VRE So for this case VRE = 1.4volts leaving 0.6volts for VBE. So in the static conditions quiescent current flows through the Q1, R1, R2 and RE providing the bias necessary to make Q1 conduct. When transistor is conducting there will be a voltage drop across RL. Let it be 5volts so that the remaining voltage is 5volts. This is the condition that when dc is applied to the amplifier, all bias voltages available and a standing voltage is available at the collector of Q1. Now a small ac signal is applied in the base of Q1which is superimposed on dc. Capacitor C1 blocks any dc component and also the amplified ac output must only be passed to the next stage if again dc component is blocked using C3. These capacitors are known as coupling capacitors. It is also essential that VRE remains constant and therefore VBE remains constant so that ac input signal adds to and subtracts from the steady VBE bias. To ensure this capacitor C2 is connected across RE. This capacitor has a capacitive reactance lower than RE at the operating frequency. This means that if the ac bypasses RE then it will have a steady dc value. This capacitor is known as decoupling capacitor. Intentionally hidden Transistor Switching Circuit: This is a Common Emitter arrangement. Here the transistor either turns fully "OFF" (Cut-off) or fully "ON" (Saturated). An ideal transistor switch would have an infinite resistance when turned "OFF" resulting in zero current flow and zero resistance when turned "ON", resulting in maximum current flow. In practice when turned "OFF", small leakage currents flow through the transistor and when fully "ON" the device has a low resistance value causing a small saturation voltage (Vce) across it. In both the Cut-off and Saturation regions the power dissipated by the transistor is at its minimum. To make the Base current flow, the Base input terminal must be made more positive than the Emitter by increasing it above the 0.7 volts needed for a silicon device. By varying the Base-Emitter voltage Vbe, the Base current is altered and which in turn controls the amount of Collector current flowing through the transistor. When maximum Collector current flows the transistor is said to be saturated. The value of the Base resistor determines how much input voltage is required and corresponding Base current to switch the transistor fully "ON". Example1: For example, using the transistor values from the previous tutorials of: β = 200, Ic = 4mA and Ib = 20uA, find the value of the Base resistor (Rb) required to switch the load "ON" when the input terminal voltage exceeds 2.5v. RB = (Vin - VBE) /IB = (2.5-0.7) / 20x10-6 = 90kΩ Example 2: Again using the same values, find the minimum Base current required to turn the transistor fully "ON" (Saturated) for a load that requires 200mA of current. IB = IC/ß = 200mA /200 = 1mA Transistor switches are used for a wide variety of applications such as interfacing large current or high voltage devices like motors, relays or lamps to low voltage digital logic IC's or gates like AND Gates or OR Gates. Types of Bias 1. Fixed bias 2. Voltage divider bias 3. Emitter bias Fixed bias (Base bias) This form of biasing is also called base bias. In the fig. on the right, the single power source is used for both collector and base of transistor, although separate batteries can also be used. In the given circuit, VCC = IBRB + Vbe Therefore, IB = (VCC - Vbe)/RB For a given transistor, Vbe does not vary significantly during use. As VCC is of fixed value, on selection of RB, the base current IB is fixed. Therefore this type is called fixed bias type of circuit. Also for given circuit, Therefore, VCC = ICRC + Vce Vce = VCC - ICRC From this equation we can obtain Vce. Since IC = βIB, we can obtain IC as well. In this manner, operating point given as (VCE,IC) can be set for given transistor. Merits: • • It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (RB). Very few number of components are required. Demerits: • • • The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable. Changes in Vbe will change IB and thus cause RE to change. This in turn will alter the gain of the stage. When the transistor is replaced with another one, considerable change in the value of β can be expected. Due to this change the operating point will shift. Usage: Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits, ie. those circuits which use the transistor as a current source. Instead it is often used in circuits where transistor is used as a switch. However, one application of 'fixed' bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a dc signal derived from the ac output of a later stage. Intentionally hidden Resistive-Capacitive (RC) Coupling Inductive-capacitive (LC) coupling: Transformer coupling: amplifier: Direct coupled amplifier: Integrated Circuits: Integrated Circuits are arrangements of several electronic components in a common housing. The major advantage is the very high density of the components; the total arrangement therefore will be very compact. As well they are quite resistant to mechanical stress. The small housing and therefore the small surface is a disadvantage because some additional cooling might be required. A heat sink or fan must be attached then. Another disadvantage is that IC’s cannot be repaired; a defective IC must always be replaced. Usually the following components are integrated in IC’s: 1. Semiconductors (Transistors, Diodes) 2. Resistors 3. Capacitors Inductances usually cannot be integrated due to their large space requirements. IC’s can be found in each and every modern appliance, in analogues as well as in digital ones. Functional blocks can be found in a single IC, requiring only a very small amount of space, i.e. Processors (Computer), Amplifier. Differential Amplifier: It consists of two transistors with two inputs and a single output. The circuit is symmetrical i.e. the two transistors have identical characteristics. The emitter resistor RE is common to both transistors. Collector load resistors R2=R3. The two input (signals) circuits are also identical. And also R1=R4 Intentionally hidden Differential-mode input (Non-common mode operation): The two transistors are connected in differential mode, receive input sine wave from opposite ends of a centre-tapped transformer. The input signals to the bases of Q1 and Q2 are equal in magnitude but opposite in phase, the condition for differential mode operation. Assume an instant of time when input to the base of Q1 is positive going and that on Q2 is negative going. Now consider the action of Q1 as there is no Q2 connected. With a positive going signal on the base of Q1, an amplified negative-going waveform appears at the collector of Q1. Moreover a positivegoing sine wave appears across RE, the un-bypassed emitter resistor, because of the emitter follower action of Q1. Now consider the action of Q2 as there is no Q1 connected. With a negative going signal on the base of Q2, an amplified positive-going waveform appears at the collector of Q2. Moreover a negative-going sine wave appears across RE, the un-bypassed emitter resistor, because of the emitter follower action of Q2. The signal voltages appear across RE, because of the opposite actions of Q1 and Q2 are equal in amplitude but 180o out of phase. Therefore when we consider the action of both the Q1and Q2 acting together, the signal voltages across the emitter resistor cancel each other and no signal is developed across RE. In this case RE does not introduce degeneration. Now if VOUT is taken across the collector of Q1 and Q2, a positive going wave with amplitude twice the amplitude of the signal voltage from either the collector to ground is received. However it is possible to take two outputs from the differential amplifier equal in amplitude but opposite in phase. Intentionally hidden Operational Amplifier The term operational amplifier or "op-amp" refers to a class of high gain DC coupled amplifiers with two inputs and a single output. Some of the general characteristics of the ideal op-amp are: • • • • • Infinite voltage gain (on the order of a million ) Infinite bandwidth Used with split supply, usually +/- 15V infinite input impedance Zero output impedance Typically the output of the op-amp is controlled either by negative feedback, which largely determines the magnitude of its output voltage gain, or by positive feedback, which facilitates regenerative gain and oscillation. Modern designs are electronically more rugged than earlier implementations and some can sustain direct short-circuits on their outputs without damage. Various op-amp ICs in 8-pin dual in-line packages "DIPs") Symbol and terminals: The circuit symbol for an op-amp is shown to the right, where: • • • • • V + : non-inverting input V − : inverting input Vout: output VS + : positive power supply VS − : negative power supply The power supply pins (VS + and VS −) can be labeled in different ways. Despite different labeling, the function remains the same. Intentionally hidden Comparator The extremely large open-loop gain of an op-amp makes it an extremely sensitive device for comparing its input with zero. For practical purposes, if the output is driven to the positive supply voltage and if it is driven to the negative supply voltage. The switching time for - to + is limited by the slew rate of the op-amp. at the slightest difference between its inputs. But there The basic comparator will swing its output to are many variations where the output is designed to switch between two other voltage values. Also, the input may be tailored to make a comparison to an input voltage other than zero Voltage follower Used as a buffer amplifier, to eliminate loading effects or to interface impedances (connecting a device with a high source impedance to a device with a low input impedance). Due to the strong feedback, this circuit tends to get unstable when driving a high capacity load. This can be avoided by connecting the load through a resistor. • (realistically, the differential input impedance of the op-amp itself, 1 MΩ to 1 TΩ) Summing amplifier Sums several (weighted) voltages • When , and Rf independent • When • • Output is inverted Input impedance Zn = Rn, for each input (V − is a virtual ground) Intentionally hidden Intentionally hidden Intentionally hidden Intentionally hidden Intentionally hidden Intentionally hidden Intentionally hidden Hundreds of EASA Part-66 questions Intentionally hidden THE END