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Identify the types of diodes.Small Signal DiodeA small signal diode is a small non-linear semiconductor device with disproportional characteristics and whose applications are mainly involved at high frequency and very low currents devices such as television, radio and digital logic circuits. These diodes have small PN junction layer, as stated it its name.Small signal diode, as shown in Figure 1.1a is smaller in size compared to regular power diode. The PN junction of a small signal diode is usually encapsulated in glass in order to protect it and generally has a black or red band at one end of its body identifying it as the cathode end of the terminal. The symbol for a small signal diode is displayed in Figure 1.1b. (a) (b)Fig. 1.1 Small signal diode (a) Diode Polarity; (b) Symbol.Small signal diodes have lower current and power ratings in comparison to rectifier diodes but can function better in high frequency applications or in switching and clipping applications which deal with short-duration pulse waveforms.With respect to the functional frequencies of the signal diode the carrying capacity of the current and power are very low which are maximum nearly at 150mA and 500mW.Large Signal DiodeLarge signal diodes, as shown in Figure 1.2, are semiconductor devices that have large PN junction layer, causing the transformation of AC to DC voltages to be unbounded. Since it has large PN junction layer, the current forward capacity and reverse blocking voltage increases.Large signals disrupt functional point, which makes large signal diodes not suitable for high frequency applications.Large signal diodes are mainly applied in battery charging devices such as inverters.  These diodes’ forward resistance ranges in ohms while the reverse blocking resistance ranges in mega ohms.Large signal diodes have high current and voltage performance, allowing it to be used in many electrical devices that requires suppression of high peak voltages. Figure 1.2 Large Signal Diode.Zener DiodeA Zener diode, shown in Figure 1.3a is a silicon semiconductor device that allows the current to flow in either forward or reverse direction. The diode’s symbol is shown in Figure 1.3b. What makes Zener diode different from other diodes is that Zener diodes will also allow current to flow in the reverse direction when the voltage is above a certain value. This voltage is known as the breakdown voltage or Zener voltage. (a) (b)Fig. 1.1 (a) Zener diode; (b) Zener Diode Symbol.In a standard diode, the breakdown voltage is high and if a reverse current above the value is allowed to pass through it, the diode will permanently be damaged. However, the Zener diode has a well-defined reverse-breakdown voltage, at which it starts conducting current, and continues operating continuously in the reverse-bias mode without getting damaged.  Figure 1.4 Zener Diode I-V CharacteristicsFrom the I-V characteristics curve in Figure 1.4, we can see that the Zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max). This ability to control itself can be used to regulate or stabilize a voltage source against supply or load variations.Light Emitting Diode (LED) A Light Emitting Diode (LED) is an optical semiconductor device that converts electrical energy into light energy; it emits light when voltage is applied.  (a) (b)Figure 1.5 Light Emitting Diode (LED); (a) LED Polarity; (b) LED symbol.An LED has two pins called anode and cathode, as shown in Figure 1.5a. The anode is the longest pin, which is the pin connected to positive supply voltage. The cathode however is connected to the negative supply voltage. An LED must be connected correctly in order to work.  Figure 1.6 Graph of Forward Current vs. Output Light of an LEDThe amount of output light emitted by the LED is directly proportional to the amount of forward current flowing through the LED. More the forward current, the greater is the emitted output light. The graph of forward current vs output light is shown in Figure 1.6.Constant-Current DiodeConstant-current diode, also known as current-limiting diode (CLD) or current-regulating diode (CRD), is an electronic device that limits current to a maximum specified value for the device. They allow current through them to rise to a certain value, and then level off at a specific value.  Unlike Zener diodes, these diodes keep the current constant instead of the voltage constant. (a) (b)Figure 1.7 Constant-current diode (a) Symbol; (b) Internal Structure Constant-current diode consists of an n-channel JFET transistor with the gate shorted to the source, which functions like a two-terminal current limiter, as shown in Figure 1.7.Current in a constant-current diode will remain unchanged even though the voltage changes.Schottky DiodeA Schottky diode is a metal-semiconductor junction diode that has less forward voltage drop than the PN junction diode. It is also frequently used in high-speed switching applications.In Schottky diode, metals such as aluminum or platinum are used in place of P-type semiconductor. When a metal is joined with N-type semiconductor, a junction, known as metal-semiconductor junction or M-S junction is formed. The symbol and structure of a Schottky diode can be seen in Figure 1.8.  (a) (b)Figure 1.8 (a)Symbol of a Schottky diode; (b)Structure of a Schottky diode.Schottky diodes are widely used in radio frequency (RF) applications. A Schottky diode can switch on and off much faster than the regular diode. Also, Schottky diode produces less unwanted noise, which makes it very useful in high-speed switching applications.Shockley DiodeUnlike other types of semiconductor diodes, a Shockley diode has two PN junctions, so it is described as PNPN. Since it is composed of four layers of semiconductor sections, as shown in Figure 1.9, it is also known as four-layer diode. (a)  (b)Figure 1.9 Shockley Diode (a) Symbol; (b) Structure   The Shockley diode is used primarily for switching applications. It features switching properties similar to those of a lamp.Step Recovery DiodesThe step recovery diode (SRD) is a form of a semiconductor diode that can be used as a charge controlled switch and it has the ability to generate very sharp pulses.  SRD is also known as “Snap-off” diode, “charge storage” diode or “memory varactor”. Figure 1.10 Schematic Symbol of Step Recovery DiodeFigure 1.10 shows the schematic symbol of a step recovery diode. The step recovery diode or SRD is able to be used as a microwave radio frequency generator and pulse sharpener.Tunnel DiodeA tunnel diode, also known as Esaki diode, is a type of semiconductor device that is capable of very fast operation. It uses the quantum mechanical effect which is called tunneling. It is a heavily doped PN junction diode in which the electric current decreases as the voltage increases. The symbol for a tunnel diode is shown in Figure 1.11. Figure 1.11 Schematic Symbol of Tunnel DiodeVaractor DiodeA varactor diode is a tuning diode. It is a semiconductor device that has a variable capacitance which is a function of the voltage that is impressed on its terminals. The schematic symbol of a varactor diode is shown in Figure 1.12. Figure 1.12 Schematic Symbol of Varactor DiodeVaractor Diode is used in many applications such as in oscillatory circuits, microwave circuits and as resistant to nuclear radiation.Laser DiodeA laser diode is an optoelectronic device, which converts electrical energy into light energy to produce high-intensity coherent light. In a laser diode, the p-n junction of the semiconductor diode acts as the laser medium or active medium. Figure 1.13 Schematic Symbol of Laser DiodeFigure 1.13 shows the symbol for a laser diode. Laser diodes are used in fiber optic communications, barcoderreaders, laser pointers, CD/DVD/Blue-ray reading and recording as well as Laser printing.Transient Voltage Suppression DiodeTransient voltage suppression diodes are used in overcoming the damage of device’s output response. The reason behind this damage is due to the fact that in semiconductor devices due to the sudden change in the state voltage transients will occur.  Figure 1.14 Transient Voltage Suppression DiodeGold Doped DiodeGold doped diodes are semiconductors that use gold as dopant. One advantage of a gold doped diode is that it is faster than other diodes. In these diodes the leakage current in reverse bias condition is also less. Even at the higher voltage drop it allows the diode to operate in signal frequencies. This is due to the fact that gold helps for the faster recombination of minority carriers.Super Barrier DiodeA super barrier diode is a type of rectifier diode that has low forward voltage drop, just like schottky diode, with surge handling capability and low reverse leakage current, just like p-n junction diode. It was designed for high power, fast switching and low-loss applications. Super barrier rectifiers are the next generation rectifiers with low forward voltage than schottky diode.Peltier DiodeIn this type of diode, at the two material junction of a semiconductor it generates a heat which flows from one terminal to another terminal. This flow is done in only single direction that is as equal to the direction of current flow.A peltier diode is mainly used in cooling and heating applications.Crystal DiodeA crystal diode, also known as Cat’s whisker, is a type of point contact diode that operates depending on the pressure of contact between semiconductor crystal point.A crystal diode is used in applications such as crystal diode rectifier, crystal diode detector and crystal radio receiver.Avalanche DiodeAvalanche diode is a passive semiconductor device that works under the principle of avalanche breakdown, meaning, it works in the reverse bias condition. It results large currents from the ionization produced by PN junction during reverse bias condition. Figure 15 shows the representation of an avalanche diode. Figure 1.15 Schematic Symbol for Avalanche DiodeSilicon Controlled Rectifier (SCR)A silicon controlled rectifier (SCR) is almost the same as a normal diode, unless it consists of another terminal which is the gate. As its name indicates it is mainly used for the control purpose when small voltages are applied in the circuit. The symbol of the Silicon Controlled Rectifier is as shown below: Figure 1.16 Schematic Symbol for Silicon Controlled RectifierVacuum DiodeA vacuum diode is the simplest form of vacuum tube. It is a two electrode vacuum tube. The two electrodes are cathode and anode (plate). The function of cathode is to emit electrons by thermionic emission. The emission of electron is accelerated by putting a cathode in a vacuum. Figure 1.17 Schematic Symbol for Vacuum DiodePIN DiodePIN diode is the improved version of the normal PN junction diode. Unlike the normal diode, doping is not necessary for PIN diode. Figure 1.17 shows the symbol for PIN diode. Figure 1.18 Schematic Symbol for PIN DiodePIN diodes are used in RF switches, attenuators as well as photo detectors.Gunn DiodeGunn diode is fabricated with n-type semiconductor material only. For a gunn diode, when voltage increases in the circuit the current also increases. After certain level of voltage the current will exponentially decrease thus this exhibits the negative differential resistance. The figure below shows the symbol for a gunn diode. Figure 1.19 Schematic Symbol for Gunn Diode?Discuss, illustrate and derive the related equations of various rectifier circuits. Half-wave rectifierA half-wave rectifier (HWR) is the first type in the rectifiers which rectifies the full wave input AC signal in to half wave pulsating DC signal. That’s why this is called half wave rectifier. In a half-wave rectifier, DC is available at its output terminals during one half cycle of the AC input.Construction Figure 2.1 Construction of a Half-wave RectifierFigure 2.1 above shows the construction of a half-wave rectifier. In half-wave rectifier, the diode is placed in series with the input AC sinusoidal signal. The transformer is placed at the input to step down the input AC signal to the corresponding output DC voltage.  Figure 2.2 Ideal Input and Output Waveforms of a Half-wave RectifierThe operation of half-wave rectifier is same as diode where it rectifies half of the input wave form and allows other half. When the input voltage is greater than diode built-in potential then it starts conducting and the input signal is produced at the output. i.e. until the diode is conducting state the output will be same as input. During the negative pulse, once again when the input voltage is less than built-in potential, the diode is reverse biased and stops conducting which blocks the input signal. So the resultant output during this period will be zero. The ideal input and output waveforms are shown in Figure 2.2.Specifications Average Output Voltage (VDC)Consider the output waveform of Figure 2.2 with Vm as output peak voltage and period is 0 to 2?.V_dc= (Average Value)/Period= 1/2? ?_0^???V_m sin?d??= V_m/2? |-cos?| ?([email protected])= V_m/2? (1+1)V_dc= V_m/? RMS Value of Output (VRMS)This value measures the AC content in the output pulsating DC Voltage. V_rms= ?(?Mean Value?^2 )         = ?(1/2? ?_0^???V_m sin?d??)= V_m/?2? ?(?_0^??((1-cos2?)/2)d?)= V_m/?2? ((2?-0)/2)V_rms=V_m/?2 Form FactorIt measures the percentage of RMS AC voltage to the Average DC Voltage.Form Factor= (RMS Value)/(Average Value)=(V_m/?2)/(V_m/?)Form Factor=1.11 Peak Inverse Voltage (PIV)The maximum voltage reading across the diode is called peak inverse voltage. The peak inverse voltage of the half wave rectifier is Vm.PIV= V_m Center Tapped Full Wave RectifierCenter tap is the contact made at the middle of the winding of the transformer. Figure 2.3 Diagram of Center Tapped Full Wave RectifierIn the center tapped full wave rectifier two diodes were used, as shown in Figure 2.3. These are connected to the center tapped secondary winding of the transformer. Above circuit diagram shows the center tapped full wave rectifier. It has two diodes. The positive terminal of two diodes is connected to the two ends of the transformer. Center tap divides the total secondary voltage into equal parts.Specifications DC Output VoltageAverage or dc value of voltage across the load is given asV_dc= ?2V?_m/? RMS Value of Voltage?V_rms= I?_rms R_LBut IRMS is ? I?_rms=? I?_m/?2V_rms=I_m/?2 R_L Form FactorForm Factor= (RMS Value)/(Average Value)=(V_m/?2)/(V_m/?)Form Factor=1.11 Peak Inverse Voltage (PIV)The maximum voltage reading across the diode is called peak inverse voltage. The peak inverse voltage of the half wave rectifier is Vm.PIV= ?2V?_max Full Wave Bridge RectifierA bridge rectifier is a type of full wave rectifier which uses four or more diodes in a bridge circuit configuration to efficiently convert the Alternating Current (AC) into Direct Current (DC). Figure 2.4 Diagram of Full Wave Bridge RectifierWhen input AC signal is applied across the bridge rectifier, during the positive half cycle diodes D1 and D3 are forward biased and allows electric current while the diodes D2 and D4 are reverse biased and blocks electric current. On the other hand, during the negative half cycle diodes D2 and D4 are forward biased and allow electric current while diodes D1 and D3 are reverse biased and blocks electric current.Specifications Average Output Voltage (VDC)Consider the output waveform of Figure 2.2 with Vm as output peak voltage and period is 0 to 2?.?V_dc= I?_dc R_LV_dc= 2/? ? I?_max R_L RMS Value of Output (VRMS)?V_rms= I?_rms R_LBut IRMS is ? I?_rms=? I?_m/?2V_rms=I_m/?2 R_L Form FactorIt measures the percentage of RMS AC voltage to the Average DC Voltage.Form Factor= (RMS Value)/(Average Value)=(V_m/?2)/(V_m/?)Form Factor=1.11 Peak Inverse Voltage (PIV)The maximum voltage reading across the diode is called peak inverse voltage. The peak inverse voltage of the half wave rectifier is Vm.PIV= V_maxDiscuss the block diagram of a power supply. You may illustrate it and the corresponding waveforms. Figure 3.1 Block Diagram of a Basic Power SupplyFigure 3.1 shows the basic block diagram of a power supply. As illustrated, the first section is the transformer. The transformer steps up or steps down the input line voltage and isolates the power supply from the power line. Next, the rectifier section converts the alternating current input signal to a pulsating direct current. However, we all know that pulsating dc is not desirable. For this reason a filter section is used to convert pulsating dc to a purer, more desirable form of dc voltage. The final section, the regulator, does just what the name implies. It maintains the output of the power supply at a constant level in spite of large changes in load current or input line voltages. Figure 3.2 Waveform Stages of Power SupplyFigure 3.2 shows the changes in waveform of the power supply in each stage. Initially, the input signal is an alternating current (AC) signal which comes from the mains. After passing through the transformer, the signal is stepped down into a smaller signal. The rectifier then converts the stepped down AC signal into a raw direct current (DC) signal. It can be either half wave or full wave rectified signal.The raw DC from the rectifier is then passed into a filter to smoother it out, removing most of the ripples of the signal.Finally, a regulator takes the smoothed voltage and provides a constant output regardless of the output current.State other applications of a diode. Aside from a rectifier, diode has many more applications. Diodes in Clipping CircuitsA Diode Clipper, also known as a Diode Limiter, is a wave shaping circuit that takes an input waveform and clips or cuts off its top half, bottom half or both halves together. This clipping of the input signal produces an output waveform that resembles a flattened version of the input. Figure 4.1 Diode in Clipping CircuitFigure 4.1 shows a basic clipping circuit using a diode. Diode Clipping Circuits can be used a variety of applications to modify an input waveform using signal and Schottky diodes or to provide over-voltage protection using zener diodes to ensure that the output voltage never exceeds a certain level protecting the circuit from high voltage spikes. Simply, diode clipping circuits can be used in voltage limiting applications. Diodes in Clamping CircuitsA clamper circuit is used to shift or alter either positive or negative peak of an input signal to a desired level. Figure 4.2 Diode in Clamping CircuitLooking at Figure 4.2, the diode application is shown in the clamping circuit. During the positive half-cycle of the input, diode is reverse-biased so the output voltage is equal to the sum of input voltage and capacitor voltage (considering the capacitor is initially charged). During the negative half-cycle of the input, diode is forward-biased and behaves as a closed switch so the capacitor charges to a peak value of the input signal. Diodes in Logic GatesDiodes can also perform digital logic operations. Low and high impedance states of logic switch are analogous to the forward and reverse-biased conditions of the diode respectively. Thus, the diode can perform logic operations such as AND, OR, etc.  Diodes in Voltage Multiplier CircuitsVoltage multiplier consist of two or more diode rectifier circuits which are cascaded to produce a DC output voltage equal to the multiplier of the applied input voltage. These multiplier circuits are of different types like voltage doubler, tripler, quadrupler, etc. By the usage of diodes combination with capacitors, we get the odd or even multiple of the input peak voltage at the output. Diodes in Reverse Current ProtectionThe reverse polarity or current protection is necessary to avoid the damage that occurs due to connecting the battery in a wrong way or reversing the polarities of the DC supply. This accidental connection of supply causes to flow a large amount current, thorough the circuit components results to explode them. Therefore, a protective or blocking diode is connected in series with the positive side of the input to avoid the reverse connection problem. Figure 4.3 Diode in Reverse Current ProtectionAs can be seen in Figure 4.3, a diode is connected in series with the applied voltage.  Diodes in Voltage Spike Suppression Figure 4.4 Voltage Spike SuppressionFigure 4.4 shows how a diode is connected in the order to avoid voltage spike suppression in circuits. In case of an inductor or inductive loads, sudden removal of supply source produces a higher voltage due to its stored magnetic field energy. These unexpected spikes in the voltage can cause the considerable damage to the circuit components. Hence, a diode is connected across the inductor or inductive loads to limit the large voltage spikes. These diodes are also called by different names in different circuits such as snubber diode, fly-back diode, suppression diode, and freewheeling diode and so on.


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