electronics

PHY 311 ELECTRONICS

1. SEMICONDUCTOR DEVICES
SEMICONDUCTORS Semiconductors are solid or liquid materials that conduct electricity at room temperature more readily than an insulator, but less easily than a metal. Electrical conductivity, which is the ability to conduct electrical current under the application of a voltage, has one of the widest ranges of values of any physical property of matter. Such metals as copper, silver, and aluminums are excellent conductors, but such insulators as diamond and glass are very poor conductors. At low temperatures, pure semiconductors behave like insulators. Under higher temperatures or light or with the addition of impurities, however, the conductivity of semiconductors can be increased dramatically, reaching levels that may approach those of metals. Electrons and Holes The common semiconductors include chemical elements and compounds such as silicon, germanium, selenium, gallium arsenide, zinc selenide, and lead telluride. The increase in conductivity with temperature, light, or impurities arises from an increase in the number of conduction electrons, which are the carriers of the electrical current. In a pure, or intrinsic, semiconductor such as silicon, the valence electrons, or outer electrons, of an atom are paired and shared between atoms to make a covalent bond that holds the crystal. These valence electrons are not free to carry electrical current. To produce conduction electrons, temperature or light is used to excite the valence electrons out of their bonds, leaving them free to conduct current. Deficiencies, or holes, are left behind that contribute to the flow of electricity. (These holes are said to be carriers of positive electricity.) This is the physical origin of the increase in the electrical conductivity of semiconductors with temperature. The energy required to excite the electron and hole is called the energy gap. Types of Semiconductor There are two types of semiconductors; intrinsic semiconductor and extrinsic semiconductor i. Intrinsic Semiconductor An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities. In intrinsic semiconductors the number of excited electrons and the number of holes are equal: n = p. The electrical conductivity of intrinsic semiconductors can be due to crystallographic defects or electron excitation. In an intrinsic semiconductor the number of electrons in the conduction band is equal to the number of holes in the valence band. An example is Hg0.8, Cd0.2 and Te at room temperature. An indirect band gap intrinsic semiconductor is one in which the maximum energy of the valence band occurs at a different k (k-space wave vector) than the minimum energy of the conduction band. Examples include silicon and germanium. A direct band gap intrinsic semiconductor is one where the maximum energy of the valence band occurs at the same k as the minimum energy of the conduction band. Examples include gallium arsenide. A silicon crystal is different from an insulator because at any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice
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will be knocked loose from its position, leaving behind an electron deficiency called a "hole". If a voltage is applied, then both the electron and the hole can contribute to a small current flow. The conductivity of a semiconductor can be modeled in terms of the band theory of solids. The band model of a semiconductor suggests that at ordinary temperatures there is a finite possibility that electrons can reach the conduction band and contribute to electrical conduction. ii. Extrinsic Semiconductor An extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than the intrinsic (pure) semiconductor. Doping involves adding dopant atoms to an intrinsic semiconductor, which changes the electron and hole carrier concentrations of the semiconductor at thermal equilibrium. Dominant carrier concentrations in an extrinsic semiconductor classify it as either an n-type or p-type semiconductor. The electrical properties of extrinsic semiconductors make them essential components of many electronic devices. Semiconductor Doping Semiconductor doping is the process that changes an intrinsic semiconductor to an extrinsic semiconductor. During doping, impurity atoms are introduced to an intrinsic semiconductor. Impurity atoms are atoms of a different element than the atoms of the intrinsic semiconductor. Impurity atoms act as either donors or acceptors to the intrinsic semiconductor, changing the electron and hole concentrations of the semiconductor. Impurity atoms are classified as donor or acceptor atoms based on the effect they have on the intrinsic semiconductor. Donor impurity atoms have more valence electrons than the atoms they replace in the intrinsic semiconductor lattice. Donor impurities "donate" their extra valence electrons to a semiconductor's conduction band, providing excess electrons to the intrinsic semiconductor. Excess electrons increase the electron carrier concentration (n0) of the semiconductor, making it n-type. Acceptor impurity atoms have fewer valence electrons than the atoms they replace in the intrinsic semiconductor. They "accept" electrons from the semiconductor's valence band. This provides excess holes to the intrinsic semiconductor. Excess holes increase the hole carrier concentration (p 0) of the semiconductor, creating a p-type semiconductor. Semiconductors and dopant atoms are defined by the column of the periodic in which they fall. The column definition of the semiconductor determines how many valence electrons its atoms have and whether dopant atoms act as the semiconductor's donors or acceptors. Group IV semiconductors use group V atoms as donors and group III atoms as acceptors. Group III-V semiconductors, the compound semiconductors, use group VI atoms as donors and group II atoms as acceptors. Group III-V semiconductors can also use group IV atoms as either donors or acceptors. When a group IV atom replaces the group III element in the semiconductor lattice, the group IV atom acts as a donor. Conversely, when a group IV atom replaces the group V element, the group IV atom acts as an acceptor. Group IV atoms can act as both donors and acceptors; therefore, they are known as amphoteric impurities.
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The two types of extrinsic semiconductor i. N-type semiconductors

Extrinsic semiconductors with a larger electron concentration than hole concentration are known as n-type semiconductors. The phrase 'n-type' comes from the negative charge of the electron. In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. N-type semiconductors are created by doping an intrinsic semiconductor with donor impurities. In an n-type semiconductor, the Fermi energy level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band.

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· Band structure of an n-type semiconductor Dark circles in the conduction band are electrons and light circles in the valence band are holes. The image shows that the electrons are the majority charge carrier. ii. P-type semiconductors

As opposed to n-type semiconductors, p-type semiconductors have a larger hole concentration than electron concentration. The phrase 'p-type' refers to the positive charge of the hole. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities. P-type semiconductors have Fermi energy levels below the intrinsic Fermi energy level. The Fermi energy level lies closer to the valence band than the conduction band in a p-type semiconductor.

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· Band structure of a p-type semiconductor Dark circles in the conduction band are electrons and light circles in the valence band are holes. The image shows that the holes are the majority charge carrier. Use of extrinsic semiconductors Extrinsic semiconductors are components of many common electrical devices. A semiconductor diode (devices that allow current in only one direction) consists of p-type and n-type semiconductors placed in junction with one another. Currently, most semiconductor diodes use doped silicon or germanium.
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Transistors (devices that enable current switching) also make use of extrinsic semiconductors. Bipolar junction transistors (BJT) are one type of transistor. The most common BJTs are NPN and PNP type. NPN transistors have two layers of n-type semiconductors sandwiching a p-type semiconductor. PNP transistors have two layers of p-type semiconductors sandwiching an n-type semiconductor. Field-effect transistors (FET) are another type of transistor implementing extrinsic semiconductors. As opposed to BJTs, they are unipolar and considered either N-channel or P-channel. FETs are broken into two families, junction gate FET (JFET) and insulated gate FET (IGFET). Other devices implementing the extrinsic semiconductor:
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Lasers Solar cells Photo detectors

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Light-emitting diodes Thyristors

Valence Band and Conduction Band

A valence band in a semiconductor means a band that originates from the atomic valence shell and that is completely filled at zero temperature. While a conduction band in a semiconductor means a band that originates from the atomic valence shell and that is completely empty at zero temperature. Conduction band can also be defined as the range of electron energies enough to free an electron from binding with its atom to move freely within the atomic lattice of the material as a 'delocalised electron'. Various materials may be classified by their band gap: this is defined as the difference between the valence and conduction bands.
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In insulators, the conduction band is higher than that of the valence band, so it takes high energies to delocalise their valence electrons. They are said to have a non-zero band gap. In conductors, such as metals, that have many free electrons under normal circumstances, the conduction band overlaps with the valence band--there is no band gap. In semiconductors, the band gap is small. This explains why it takes a little energy (in the form of heat or light) to make semiconductors' electrons delocalise and conduct electricity, hence the name, semiconductor.

Electrons within the conduction band are mobile charge carriers in solids, responsible for conduction of electric currents in metals and other good electrical conductors

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Diode In electronics, a diode is a two-terminal electronic component with an asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p-n junction connected to two electrical terminals. A vacuum tube diode is a vacuum tube with two electrodes, a plate (anode) and heated cathode. The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking current in the opposite direction (the reverse direction). Thus, the diode can be viewed as an electronic version of a check valve. This unidirectional behaviour is called rectification, and is used to convert alternating current to direct current, including extraction of modulation from radio signals in radio receivers, these diodes are forms of rectifiers.

· Structure of a vacuum tube diode However, diodes can have more complicated behaviour than this simple on off action. Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction (a state in which the diode is said to be forward-biased). The voltage drop across a forward-biased diode varies only a little with the current, and is a function of temperature; this effect can be used as a temperature sensor or voltage reference. Semiconductor diodes' nonlinear current voltage characteristic can be tailored by varying the semiconductor materials and doping, introducing impurities into the materials. These are exploited in special-purpose diodes that perform many different functions. For example, diodes are used to regulate voltage (Zener diodes), to protect circuits from high voltage surges (avalanche diodes), to electronically tune radio and TV receivers (varactor diodes), to generate radio frequency oscillations (tunnel diodes, Gunn diodes, IMPATT diodes), and to produce light (light emitting diodes). Tunnel diodes exhibit negative resistance, which makes them useful in some types of circuits. Diodes were the first semiconductor electronic devices. The discovery of crystals' rectifying abilities was made by German physicist Ferdinand Braun in 1874. The first semiconductor diodes, called cat's whisker diodes, developed around 1906, were made of mineral crystals such as galena. Today most diodes are made of silicon, but other semiconductors such as germanium are sometimes used. P N Junction A p n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. It is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy (growing a layer of crystal doped with one type of dopant on top of a
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layer of crystal doped with another type of dopant). If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that severely inhibits its utility by scattering the electrons and holes. p n junctions are elementary "building blocks" of most semiconductor electronic devices such as diodes, transistors, solar cells, LEDs, and integrated circuits; they are the active sites where the electronic action of the device takes place.

· A p n junction. The circuit symbol is shown: the triangle corresponds to the p side For example, a common type of transistor, the bipolar junction transistor, consists of two p n junctions in series, in the form n p n or p n p. The discovery of the p n junction is usually attributed to American physicist Russell Ohl of Bell Laboratories. A Schottky junction is a special case of a p n junction, where metal serves the role of the p-type semiconductor. Depletion Layers In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an electric field. The only elements left in the depletion region are ionized donor or acceptor impurities. The 'depletion region' is so named because it is formed from a conducting region by removal of all free charge carriers, leaving none to carry a current. Understanding the depletion region is key to explaining modern semiconductor electronics: diodes, bipolar junction transistors, field-effect transistors, and variable capacitance diodes all rely on depletion region phenomena Current voltage characteristic A semiconductor diode s behaviour in a circuit is given by its current voltage characteristic, or I V graph (see graph below). The shape of the curve is determined by the transport of charge carriers through the so-called depletion layer or depletion region that exists at the p n junction between differing semiconductors. When a p n junction is first created, conduction-band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (vacant places for electrons) with which the electrons "recombine". When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor (dopant) on the N side and negatively charged acceptor (dopant) on the P side. The region around the p n junction becomes depleted of charge carriers and thus behaves as an insulator. However, the width of the depletion region (called the depletion width) cannot grow without limit. For each electron hole pair that recombines, a positively charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds more ions are created, an increasing electric field develops through the depletion zone that acts to slow and then finally stop recombination. At this point, there is a "built-in" potential across the depletion zone.
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If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current flow (unless electron/hole pairs are actively being created in the junction by, for instance, light. see photodiode). This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in substantial electric current through the p n junction (i.e. substantial numbers of electrons and holes recombine at the junction). For silicon diodes, the built-in potential is approximately 0.7 V (0.3 V for Germanium and 0.2 V for Schottky). Thus, if an external current is passed through the diode, about 0.7 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be "turned on" as it has a forward bias. A diode s I V characteristic can be approximated by four regions of operation.

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I V characteristics of a p n junction diode

At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs that causes a large increase in current (i.e., a large number of electrons and holes are created at, and move away from the p n junction) that usually damages the device permanently. The avalanche diode is deliberately designed for use in the avalanche region. In the Zener diode, the concept of PIV is not applicable. A Zener diode contains a heavily doped p n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material, such that the reverse voltage is "clamped" to a known value (called the Zener voltage), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power in the clamped reverse-voltage region. Also, following the end of forward conduction in any diode, there is reverse current for a short time. The device does not attain its full blocking capability until the reverse current ceases. The second region, at reverse biases more positive than the PIV, has only a very small reverse saturation current. In the reverse bias region for a normal P N rectifier diode, the current through the device is very low (in the µA range). However, this is temperature dependent, and at sufficiently high temperatures, a substantial amount of reverse current can be observed (mA or more). The third region is forward but small bias, where only a small forward current is conducted. As the potential difference is increased above an arbitrarily defined "cut-in voltage" or "on-voltage" or "diode
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forward voltage drop (Vd)", the diode current becomes appreciable (the level of current considered "appreciable" and the value of cut-in voltage depends on the application), and the diode presents a very low resistance. The current voltage curve is exponential. In a normal silicon diode at rated currents, the arbitrary cut-in voltage is defined as 0.6 to 0.7 volts. The value is different for other diode types Schottky diodes can be rated as low as 0.2 V, Germanium diodes 0.25 to 0.3 V, and red or blue light-emitting diodes (LEDs) can have values of 1.4 V and 4.0 V respectively. At higher currents the forward voltage drop of the diode increases. A drop of 1 V to 1.5 V is typical at full rated current for power diodes. Types of Diode Various types of diodes are available which enable different kinds of application to be met. Diodes for high current and voltage, for small application, light emission and detection, as well as for giving variable capacitance and low forward voltage drops are available today. Some of the different types of diodes are: Light Emitting Diode (LED) LEDs or light emitting diodes are the most popularly known diodes today. They are p-n junction diodes that permit transfer of electrons between the electrodes and produce light. However, not all LEDs emit visible light. There are those that emit infrared light, which cannot be seen by human eyes. Such LEDs are used in remote controls of television, DVD players, etc. When the diode is switched on or forward biased, the electrons recombine with the holes and release energy in the form of light. This means electronic excitation spearheads photon emission, which in turn results in light emission or electroluminescence. Aluminum gallium indium phosphide or aluminum gallium arsenide are generally the conducting materials used in LEDs. The color emitted by the LED, will depend on the combination of semiconductor material used. Avalanche Diode Slightly doped p-n junctions at times encounter avalanche breakdown, which is the sudden multiplication of voltage (voltage transients) across a diode. This sudden increase often destroys diodes. However, avalanche diodes, available with breakdown voltages of 4000V are built in such a way, that they can break down the voltage and permit passage of reverse bias voltage. Thus, these diodes are used to protect circuits (especially high voltage circuits) from transient voltage. They are often used along with Zener diodes and often confused with the same. Zener Diode Zener diode is a type of diode that not only allows current to flow through it in one direction like any other diode, but in reverse bias, also allows current to flow in the reverse direction, if the voltage exceeds a certain limit. This voltage limit is known as Zener voltage and is fixed for Zener diodes with breakdown voltage ranging from 1.8V to 200V. Thus, Zener diodes protect circuits from damages. Unlike, avalanche diodes, such diodes have heavily-doped p-n junctions and the doping is done differently to achieve different Zener breakdown voltage. These diodes are mostly used to control voltage in electrical circuits. Schottky Diode Also referred to as hot-carrier diodes, Schottky diodes are diodes that feature lower forward voltage drop, as compared to the ordinary silicon p-n junction diodes. The voltage drop may be somewhere between 0.15 and 0.4 volts at low currents, as compared to the 0.6 volts for a silicon diode. The lower voltage drop helps the diode switch from conducting state to non-conducting state in shorter time. Thus, they come in handy for preventing transistor saturation and are also used in voltage-clamping applications. In order to
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achieve this performance, these diodes are constructed differently from normal diodes, with metal to semiconductor contact. Schottky diodes are used in RF applications and rectifier applications. Laser Diode This type of diode is different from the LED type, as it produces coherent light, which is nothing but radiation in which the waves are of the same frequency and in the same phase. These diodes are small in size, however, compared to their size, the output is commendable. Laser diodes can again be divided into two types: low power diodes and high power diodes. The coherent light produced by these diodes make them perfect for devices such as DVD and CD drives, laser pointers, high-definition TVs, barcode readers, etc. Laser diodes are more expensive than LEDs. However, they are cheaper than other forms of laser generators. Moreover, these laser diodes have limited life. Photodiode Photodiodes are used to detect light and convert light falling on it into electric current. They feature wide, transparent p-n junctions and work on the mechanism of photoelectric effect. These diodes operate in reverse bias, wherein even small amounts of current flow, resulting from the light, can be detected with ease. Photodiodes can also be used to generate electricity, used as solar cells and even in photometry. Some photodiodes feature an undoped layer sandwiched between the p and n layers, and such diodes are called PIN photodiodes. This kind of photodiode is more popularly used today, because of its higher efficiency. Application of Diodes Signal rectifier If the input is not a sine wave, we usually do not think of it as a rectification in the sense as it was for power supply. For instance, we might want to have a series of pulses corresponding to the rising edge of a square wave (see figure below, left hand side and right hand side of the capacitor C). While both, the rising and the falling, pulses are in the output after differentiation performed by CR circuit. The simplest way is to rectify the differentiated wave. i.

The figure above is a series of pulses' rectifier. We should remember about forward drop voltage of the diode: This circuit gives no output for signal for input smaller then, forward drop voltage, let us say 0.5 V pp (peak to peak). If this is a problem, there are various tricks that help to combat this limitation. For instance: · Use Schottky diodes with smaller forward drop voltage (approximately 0.2V), · Use so called circuit solution, which means modifying the circuit structure and compensating the drop, · Use matched-pair compensation, use transistors, FETs. Diode gates Another application of diode is to pass the higher of two voltages without affecting the lower. A good example is battery backup, a method of keeping s device running (for instance a precision electronic clock) in case of power failure. The figure below shows a circuit that does the job.
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ii.

[OR gate: The output of OR gate is HIGH if either input (or both) is HIGH. In general, gates can have any number of inputs. The output is LOW only if all inputs are LOW]. 1. The battery does nothing until the power fails. 2. Then the battery takes over the control, without interruption. Diode clamps Sometimes it is necessary to limit the range of signal (for instance not to exceed certain voltage limit and not to destroy a device). The circuit in the figure [Diode voltage clamp] below will accomplish this. iii.

The diode prevents the output from exceeding @ 5.6V, with no effect on voltages smaller than this, including negative voltages. The only limitation is that the input must not be so negative that the reverse breakdown voltage is exceeded. Diode clamps are the standard equipment on all inputs in the CMOS family of digital logic (Complementary Metal Oxide Semiconductor). Without them, the delicate input circuits are easily destroyed by static electricity. iv. 0.6V. Limiter The circuit in the figure below [Diode limiter] limits the output swing to one diode drop, roughly

. It might seem very small, but if the next device is an amplifier with large voltage amplification, its input has to be always near zero voltage. Otherwise the output is in state of saturation. For instance we have an op amp with a gain of 1000. The amplifier operates with supply voltage ±15V. Sometimes it can be ±12V or ±18V or something in between. It will never give output voltage bigger than the supply voltage, i.e. ±15V. It means that the input signal ±15mV (±15V/1000) or bigger will saturate the output. This particular amplifier
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gives the output proportional to the input (proportionality factor is 1000) only for input signals from the interval (-15mV,+15mV). This diode limiter is often used as input protection for high-gain amplifiers.

2. SINGLE PHASE RECTIFIER CIRCUITS AND FILTERS
RECTIFIER A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Physically, rectifiers take a number of forms, including vacuum tube diodes, mercury-arc valves, solid-state diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches. Historically, even synchronous electromechanical switches and motors have been used. Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulphide) to serve as a pointcontact rectifier or "crystal detector". Rectifiers have many uses, but are often found serving as components of DC power supplies and high-voltage direct current power transmission systems. Rectification may serve in roles other than to generate direct current for use as a source of power. The simple process of rectification produces a type of DC characterized by pulsating voltages and currents (although still unidirectional). Depending upon the type of end-use, this type of DC current may then be further modified into the type of relatively constant voltage DC characteristically produced by such sources as batteries and solar cells. A device which performs the opposite function (converting DC to AC) is known as an inverter. Rectifier devices Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used. With the introduction of semiconductor electronics, vacuum tube rectifiers became obsolete, except for some enthusiasts of vacuum tube audio equipment. For power rectification from very low to very high current, semiconductor diodes of various types (junction diodes, Schottky diodes, etc.) are widely used. Other devices which have control electrodes as well as acting as unidirectional current valves are used where more than simple rectification is required, e.g., where variable output voltage is needed. High-power rectifiers, such as are used in high-voltage direct current power transmission, employ silicon semiconductor devices of various types. These are thyristors or other controlled switching solid-state switches which effectively function as diodes to pass current in only one direction. Rectifier circuits Rectifier circuits may be single-phase or multi-phase (three being the most common number of phases). Most low power rectifiers for domestic equipment are single-phase, but three-phase rectification is very important for industrial applications and for the transmission of energy as DC (HVDC). Single-phase rectifier A single phase rectifier is a semiconductor device that converts single-phase AC into DC. In a single-phase rectifier, the sine waves produced by the AC power supply reach their peak at 90° simultaneously.
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Half-wave rectification In half wave rectification of a single-phase supply, 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, mean voltage is lower. Half-wave rectification requires a single diode in a single-phase supply, or three in a three-phase supply. Rectifiers yield a unidirectional but pulsating direct current; half-wave rectifiers produce far more ripple than full-wave rectifiers, and much more filtering is needed to eliminate harmonics of the AC frequency from the output.

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A Half-wave rectifier

The no-load output DC voltage of an ideal half wave rectifier is:

Where: , , , . 3.14159 A real rectifier will have a characteristic which drops part of the input voltage (a voltage drop, for silicon devices, of typically 0.7 volts plus an equivalent resistance, in general non-linear), and at high frequencies will distort waveforms in other ways; unlike an ideal rectifier, it will dissipate power. Full-wave rectification A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and yields a higher mean output voltage. Two diodes and a center tapped transformer, or four diodes in a bridge configuration and any AC source (including a transformer without center tap), are needed.[2] Single semiconductor diodes, double diodes with common cathode or common anode, and fourdiode bridges, are manufactured as single components.

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Graetz bridge rectifier: a full-wave rectifier using 4 diodes.

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (cathode-tocathode or anode-to-anode, depending upon output polarity required) can form a full-wave rectifier. Twice as many turns are required on the transformer secondary to obtain the same output voltage than for a bridge rectifier, but the power rating is unchanged.

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Full-wave rectifier using a center tap transformer and 2 diodes.

The average and root-mean-square no-load output voltages of an ideal single-phase full-wave rectifier are: 2

2 Voltage Doubler A voltage doubler is an electronic circuit which charges capacitors from the input voltage and switches these charges in such a way that, in the ideal case, exactly twice the voltage is produced at the output as at its input. The simplest of these circuits are a form of rectifier which take an AC voltage as input and output a doubled DC voltage. The switching elements are simple diodes and they are driven to switch state merely by the alternating voltage of the input. DC to DC voltage doublers cannot switch in this way and require a driving circuit to control the switching. They frequently also require a switching element that can be controlled directly, such as a transistor, rather than relying on the voltage across the switch as in the simple AC to DC case.

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i.

Villard circuit

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Villard circuit

The Villard circuit consists simply of a capacitor and a diode. While it has the great benefit of simplicity, its output has very poor ripple characteristics. Essentially, the circuit is a diode clamp circuit. The capacitor is charged on the negative half cycles to the peak AC voltage (Vpeak). The output is the superposition of the input AC waveform and the steady DC of the capacitor. The effect of the circuit is to shift the DC value of the waveform. The negative peaks of the AC waveform are "clamped" to 0 V (actually VF, the small forward bias voltage of the diode) by the diode, therefore the positive peaks of the output waveform are 2Vpeak. The peak-to-peak ripple is an enormous 2Vpeak and cannot be smoothed unless the circuit is effectively turned into one of the more sophisticated forms. ii. Greinacher circuit

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Greinacher circuit

The Greinacher voltage doubler is a significant improvement over the Villard circuit for a small cost in additional components. The ripple is much reduced, nominally zero under open-circuit load conditions, but when current is being drawn depends on the resistance of the load and the value of the capacitors used. The circuit works by following a Villard cell stage with what is in essence a peak detector or envelope detector stage. The peak detector cell has the effect of removing most of the ripple while preserving the peak voltage in the output.

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Voltage quadrupler

two Greinacher cells of opposite polarities

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This circuit was first invented by Heinrich Greinacher in 1913 to provide the 200 300 V he needed for his newly invented ionometer (The term ionometer was originally applied to a device for measuring the intensity of ionising radiation), the 110 V AC supplied by the Zurich power stations of the time being insufficient. He later (1920) extended this idea into a cascade of multipliers. This cascade of Greinacher cells is often inaccurately referred to as a Villard cascade. It is also called a Cockcroft Walton multiplier after the particle accelerator machine built by John Cockcroft and Ernest Walton, who independently rediscovered the circuit in 1932. The concept in this topology can be extended to a voltage quadrupler circuit by using two Greinacher cells of opposite polarities driven from the same AC source. The output is taken across the two individual outputs. As with a bridge circuit, it is impossible to simultaneously ground the input and output of this circuit. iii. Bridge circuit

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Bridge (Delon) voltage doubler

The Delon circuit uses a bridge topology for voltage doubling. This form of circuit was, at one time, commonly found in cathode ray tube television sets where it was used to provide an e.h.t. voltage supply. Generating voltages in excess of 5 kV with a transformer has safety issues in terms of domestic equipment and in any case is uneconomic. However, black and white television sets required an e.h.t. of 10 kV and colour sets even more. Voltage doublers were used to either double the voltage on an e.h.t winding on the mains transformer or were applied to the waveform on the line flyback coils. The circuit consists of two half-wave peak detectors, functioning in exactly the same way as the peak detector cell in the Greinacher circuit. Each of the two peak detector cells operates on opposite half-cycles of the incoming waveform. Since their outputs are in series, the output is twice the peak input voltage. Ripple Factors The most common meaning of ripple in electrical science is the small unwanted residual periodic variation of the direct current (dc) output of a power supply which has been derived from an alternating current (ac) source. This ripple is due to incomplete suppression of the alternating waveform within the power supply. As well as this time-varying phenomenon, there is a frequency domain ripple that arises in some classes of filter and other signal processing networks. In this case the periodic variation is a variation in the insertion loss of the network against increasing frequency. The variation may not be strictly linearly periodic. In this
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meaning also, ripple is usually to be considered an unwanted effect, its existence being a compromise between the amount of ripple and other design parameters. Time-domain ripple

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Full-wave rectifier circuit with a reservoir capacitor on the output for the purpose of smoothing ripple

Ripple factor ( ) may be defined as the ratio of the root mean square (rms) value of the ripple voltage to the absolute value of the dc component of the output voltage, usually expressed as a percentage. However, ripple voltage is also commonly expressed as the peak-to-peak value. This is largely because peak-to-peak is both easier to measure on an oscilloscope and is simpler to calculate theoretically. Filter circuits intended for the reduction of ripple are usually called smoothing circuits. The simplest scenario in ac to dc conversion is a rectifier without any smoothing circuitry at all. The ripple voltage is very large in this situation; the peak-to-peak ripple voltage is equal to the peak ac voltage. A more common arrangement is to allow the rectifier to work into a large smoothing capacitor which acts as a reservoir. After a peak in output voltage the capacitor (C) supplies the current to the load (R) and continues to do so until the capacitor voltage has fallen to the value of the now rising next half-cycle of rectified voltage. At that point the rectifiers turn on again and deliver current to the reservoir until peak voltage is again reached. If the time constant, CR, is large in comparison to the period of the ac waveform, then a reasonably accurate approximation can be made by assuming that the capacitor voltage falls linearly. A further useful assumption can be made if the ripple is small compared to the dc voltage. In this case the phase angle through which the rectifiers conduct will be small and it can be assumed that the capacitor is discharging all the way from one peak to the next with little loss of accuracy.

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Ripple voltage from a full-wave rectifier, before and after the application of a smoothing capacitor

With the above assumptions the peak-to-peak ripple voltage can be calculated as: For a full-wave rectifier: =

For a half-wave rectification: where

=

is the peak-to-peak ripple voltage is the current in the circuit is the frequency of the ac power is the capacitance For the rms value of the ripple voltage, the calculation is more involved as the shape of the ripple waveform has a bearing on the result. Assuming a saw tooth waveform is a similar assumption to the ones above and yields the result: = where is the ripple factor is the resistance of the load Another approach to reducing ripple is to use a series choke. A choke has a filtering action and consequently produces a smoother waveform with less high-order harmonics. Against this, the dc output is close to the average input voltage as opposed to the higher voltage with the reservoir capacitor which is close to the peak input voltage. With suitable approximations, the ripple factor is given by: = where is the angular frequency 2 is the inductance of the choke
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Effects of ripple Ripple is undesirable in many electronic applications for a variety of reasons:
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The ripple frequency and its harmonics are within the audio band and will therefore be audible on equipment such as radio receivers, equipment for playing recordings and professional studio equipment. The ripple frequency is within television video bandwidth. Analogue TV receivers will exhibit a pattern of moving wavy lines if too much ripple is present. The presence of ripple can reduce the resolution of electronic test and measurement instruments. On an oscilloscope it will manifest itself as a visible pattern on screen. Within digital circuits, it reduces the threshold, as does any form of supply rail noise, at which logic circuits give incorrect outputs and data is corrupted. High-amplitude ripple currents shorten the life of electrolytic capacitors.

RC CIRCUIT A resistor capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors driven by a voltage or current source. A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit. RC circuits can be used to filter a signal by blocking certain frequencies and passing others. The two most common RC filters are the high-pass filters and low-pass filters; band-pass filters and band-stop filters usually require RLC filters, though crude ones can be made with RC filters. There are three basic, linear passive lumped analog circuit components: the resistor (R), the capacitor (C), and the inductor (L). These may be combined in the RC circuit, the RL circuit, the LC circuit, and the RLC circuit, with the abbreviations indicating which components are used. These circuits, among them, exhibit a large number of important types of behaviour that are fundamental to much of analog electronics. In particular, they are able to act as passive filters. This article considers the RC circuit, in both series and parallel forms, as shown in the diagrams below. Natural response The simplest RC circuit is a capacitor and a resistor in series. When a circuit consists of only a charged capacitor and a resistor, the capacitor will discharge its stored energy through the resistor. The voltage across the capacitor, which is time dependent, can be found by using Kirchhoff's current law, where the current through the capacitor must equal the current through the resistor. This results in the linear differential equation + Solving this equation for where = 0 =

yields the formula for exponential decay:

is the capacitor voltage at time = 0

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The time required for the voltage to fall to

is called the RC time constant and is given by

Complex impedance The complex impedance, ZC (in ohms) of a capacitor with capacitance C (in farads) is

The complex frequency s is, in general, a complex number,

where represents the imaginary unit: 1 is the exponential decay constant (in radians per second), and is the sinusoidal angular frequency (also in radians per second). Sinusoidal steady state Sinusoidal steady state is a special case in which the input voltage consists of a pure sinusoid (with no exponential decay). As a result, 0 and the evaluation of s becomes

Series circuit

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Series RC circuit

By viewing the circuit as a voltage divider, the voltage across the capacitor is:

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and the voltage across the resistor is:

. Parallel circuit

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Parallel RC circuit

The parallel RC circuit is generally of less interest than the series circuit. This is largely because the output voltage is equal to the input voltage as a result; this circuit does not act as a filter on the input signal unless fed by a current source. LC CIRCUIT

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LC circuit diagram

An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C. When connected together, they can act as an electrical resonator, an electrical analogue of a tuning fork, storing energy oscillating at the circuit's resonant frequency. LC circuits are used either for generating signals at a particular frequency, or picking out a signal at a particular frequency from a more complex signal. They are key components in many electronic devices, particularly radio equipment, used in circuits such as oscillators, filters, tuners and frequency mixers. An LC circuit is an idealized model since it assumes there is no dissipation of energy due to resistance. Any practical implementation of an LC circuit will always include loss resulting from small but
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non-zero resistance within the components and connecting wires. The purpose of an LC circuit is usually to oscillate with minimal damping, so the resistance is made as low as possible. While no practical circuit is without losses, it is nonetheless instructive to study this ideal form of the circuit to gain understanding and physical intuition. Operation An LC circuit can store electrical energy oscillating at its resonant frequency. A capacitor stores energy in the electric field between its plates, depending on the voltage across it, and an inductor stores energy in its magnetic field, depending on the current through it. If a charged capacitor is connected across an inductor, charge will start to flow through the inductor, building up a magnetic field around it and reducing the voltage on the capacitor. Eventually all the charge on the capacitor will be gone and the voltage across it will reach zero. However, the current will continue, because inductors resist changes in current. The energy to keep it flowing is extracted from the magnetic field, which will begin to decline. The current will begin to charge the capacitor with a voltage of opposite polarity to its original charge. When the magnetic field is completely dissipated the current will stop and the charge will again be stored in the capacitor, with the opposite polarity as before. Then the cycle will begin again, with the current flowing in the opposite direction through the inductor. The charge flows back and forth between the plates of the capacitor, through the inductor. The energy oscillates back and forth between the capacitor and the inductor until (if not replenished by power from an external circuit) internal resistance makes the oscillations die out. Its action, known mathematically as a harmonic oscillator, is similar to a pendulum swinging back and forth, or water sloshing back and forth in a tank. For this reason the circuit is also called a tank circuit. The oscillation frequency is determined by the capacitance and inductance values. In typical tuned circuits in electronic equipment the oscillations are very fast, thousands to millions of times per second. Resonance effect The resonance effect occurs when inductive and capacitive reactances are equal in magnitude. The frequency at which this equality holds for the particular circuit is called the resonant frequency. The resonant frequency of the LC circuit is

where L is the inductance in henries, and C is the capacitance in farads. The angular frequency of radians per second. The equivalent frequency in units of hertz is

has units

LC circuits are often used as filters; the L/C ratio is one of the factors that determines their "Q" and so selectivity. For a series resonant circuit with a given resistance, the higher the inductance and the lower the
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capacitance, the narrower the filter bandwidth. For a parallel resonant circuit the opposite applies. Positive feedback around the tuned circuit ("regeneration") can also increase selectivity (see Q multiplier and Regenerative circuit). Stagger tuning can provide an acceptably wide audio bandwidth, yet good selectivity. Applications The resonance effect of the LC circuit has many important applications in signal processing and communications systems. 1. The most common application of tank circuits is tuning radio transmitters and receivers. For example, when we tune a radio to a particular station, the LC circuits are set at resonance for that particular carrier frequency. 2. A series resonant circuit provides voltage magnification. 3. A parallel resonant circuit provides current magnification. 4. A parallel resonant circuit can be used as load impedance in output circuits of RF amplifiers. Due to high impedance, the gain of amplifier is maximum at resonant frequency. 5. Both parallel and series resonant circuits are used in induction heating. LC circuits behave as electronic resonators, which are a key component in many applications:
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Amplifiers Oscillators Filters Tuners Mixers

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Foster-Seeley discriminator Contactless cards Graphics tablets Electronic Article Surveillance

Series LC circuit

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Series LC Circuit

In the series configuration of the LC circuit, the inductor L and capacitor C are connected in series, as shown here. The total voltage v across the open terminals is simply the sum of the voltage across the
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inductor and the voltage across the capacitor. The current i flowing into the positive terminal of the circuit is equal to the current flowing through both the capacitor and the inductor.

Resonance Inductive reactance magnitude ( ) increases as frequency increases while capacitive reactance magnitude ( ) decreases with the increase in frequency. At a particular frequency these two reactances are equal in magnitude but opposite in sign. The frequency at which this happens is the resonant frequency ( ) for the given circuit. Hence, at resonance:

Solving for , we have

which is defined as the resonant angular frequency of the circuit. Converting angular frequency (in radians per second) into frequency (in hertz), we have

In a series configuration, XC and XL cancel each other out. In real, rather than idealised components the current is opposed, mostly by the resistance of the coil windings. Thus, the current supplied to a series resonant circuit is a maximum at resonance.
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In the limit as current is maximum. Circuit impedance is minimum. In this state a circuit is called an acceptor circuit. For , . Hence circuit is capacitive. For , . Hence circuit is inductive.

Impedance In the series configuration, resonance occurs when the complex electrical impedance of the circuit approaches zero. First consider the impedance of the series LC circuit. The total impedance is given by the sum of the inductive and capacitive impedances:
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By writing the inductive impedance as ZL = j L and capacitive impedance as ZC = (j C) we have

1

and substituting

. Writing this expression under a common denominator gives

. Finally, defining the natural angular frequency as

the impedance becomes

. The numerator implies that in the limit as the total impedance Z will be zero and otherwise nonzero. Therefore the series LC circuit, when connected in series with a load, will act as a band-pass filter having zero impedance at the resonant frequency of the LC circuit. Parallel LC circuit

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Parallel LC Circuit

In the parallel configuration, the inductor L and capacitor C are connected in parallel, as shown here. The voltage v across the open terminals is equal to both the voltage across the inductor and the voltage across the
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capacitor. The total current i flowing into the positive terminal of the circuit is equal to the sum of the current flowing through the inductor and the current flowing through the capacitor.

Resonance Let R be the internal resistance of the coil. When XL equals XC, the reactive branch currents are equal and opposite. Hence they cancel out each other to give minimum current in the main line. Since total current is minimum, in this state the total impedance is maximum. Resonant frequency given by: . Note that any reactive branch current is not minimum at resonance, but each is given separately by dividing source voltage (V) by reactance (Z). Hence I=V/Z, as per Ohm's law.
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At f0, line current is minimum. Total impedance is maximum. In this state a circuit is called a rejector circuit. Below f0, circuit is inductive. Above f0, circuit is capacitive.

Impedance The same analysis may be applied to the parallel LC circuit. The total impedance is then given by:

and after substitution of

and

and simplification, gives

which further simplifies to

where

Note that
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but for all other values of the impedance is finite. The parallel LC circuit connected in series with a load will act as band-stop filter having infinite impedance at the resonant frequency of the LC circuit. The parallel LC circuit connected in parallel with a load will act as band-pass filter. REGULATED POWER SUPPLY A regulated power supply is an embedded circuit, or stand-alone unit, the function of which is to supply a stable voltage (or less often current), to a circuit or device that must be operated within certain power supply limits. The output from the regulated power supply may be alternating or unidirectional, but is nearly always DC (Direct Current). The type of stabilization used may be restricted to ensuring that the output remains within certain limits under various load conditions, or it may also include compensation for variations in its own supply source. The latter is much more common today. Applications
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D.C. variable bench supply (a bench power supply usually refers to a power supply capable of supplying a variety of output voltages useful for bench testing electronic circuits, possibly with continuous variation of the output voltage, or just some preset voltages; a laboratory (lab) power supply normally implies an accurate bench power supply, while a balanced or tracking power supply refers to twin supplies for use when a circuit requires both positive and negative supply rails). Mobile Phone power adaptors Regulated power supplies in appliances

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3. Transistors: PNP and NPN transistors, transistors characteristics in common base (CB), common emitter (CE) and common collector (CC) configuration, load line, operating point, transistor parameter. 4. Amplifiers: Classification, RC couple amplifiers, and their frequency response, transistor amplifiers in three configurations, input impedance, distortion in amplifiers, gain band with product, feedback in amplifiers and its importance in stability and frequency response, element of class A, B and AB amplifier, transformer couple amplifier, power amplifier and noise in amplifiers. 5. Oscillators: Types of wave forms, different types of transistor oscillators, tuned oscillator, Colpitts oscillation, Hartley oscillation, phase shift oscillation, Wine bridge oscillation and crystal oscillation.

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