Unit: 1Diode

Lecture notes for FE 7 and FE 9 students

NOTE: 1) Notes are prepared from reference books, various internet sources, my experience and knowledge.2) Carryout as many as you can, numerical from the sources other than from books(Internet, other notes etc.)3) Refer this notes for knowledge and exam point of view.

By.Dr.A.P.Dhande

Note:

What is diode?Diode,an electrical component that allows the flow of current in only one direction. In circuit diagrams, a diode is represented by a triangle with a line across one vertex.

Conduction: It is transfer of energy from one point to other in the form of heat,electrical current and radiation. There are 3 types of conduction of energy 1) Conduction:The process by which heat or electricity is directly transmitted through the material of a substance when there is a difference of temperature or of electrical potential between adjoining regions, without movement of the material.Eg.

The process by which sound waves travel through a medium.The transmission of impulses along nerves.

2) Convection is the circular motion that happens when warmer air or liquid - which has faster moving molecules, making it less dense - rises, while the cooler air or liquid drops down. Convection is a major factor in weather.

Radiation:The emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles which cause ionization.

Conducting, non-conducting and semi-conductors is a classification of materials available in nature. Basically conductor exhibits a property to conduct electrical current, heat or radiation. It is due to low resistance path among molecules and availability of plenty of charge carriers.Non conducting materials are also referred to as insulator. It is a material that do not allow a flow of charge (current) in them but may be heat or radiation conducted. We will be dealing only with the property of current conduction. Electrical current is basically motion of charges.Semiconductor: Its conductivity of this material lies between insulator and conductor. At absolute zero temperature material behaves as insulator and as its temperature increases, it becomes conductor.Ex silicon (Si atomic no.14) and germanium (Geatomic no. 32)

(Electronic configuration)

(Crystalline structure)

Diode is an electrical component that allows the flow of current in only one direction. ... When

this junction is forward biased (that is, a positive voltage is applied to the p-side), electrons can

easily move across the junction to fill the holes, and a current flows through the diode. Basically

it hastwo electrodes called the anode and the cathode. Most diodes are made

with semiconductor materials such as silicon, germanium, or selenium. Some diodes are

comprised of metal electrodes in a chamber evacuated or filled with a pure elemental gas at low

pressure. Diodes can be used as rectifiers, signal limiters, voltageregulators, switches, signal

modulators, signal mixers, signal demodulators, and oscillators.

Construction:

A diode is formed by joining two equivalently doped P-Type and N-Type semiconductor. When they are joined an interesting phenomenon takes place. The P-Type semiconductor has excess holes and is of positive charge. The N-Type semiconductor has excess electrons. At the point of contact of the P-Type and N-Type regions, the holes in the P-Type attract electrons in the N-Type material. Hence the electron diffuses and occupies the holes in the P-Type material. Causing a small region of the N-type near the junction to lose electrons and behave like intrinsic semiconductor material, in the P-type a small region gets filled up and behaves like a intrinsic semiconductor.Animation

Definition

Drift current: There are plenty of holes in P-type region and would like to move to N-region via diffusion but are prevented by the electric field (or the energy barrier) at equilibrium. The drift and diffusion currents cancel each other

Similarly, there are plenty of electrons in N-type region and would like to move to P-region via diffusion but are prevented by the electric field (or the energy barrier) at equilibrium. The drift and diffusion currents again cancel each other.

The fraction of electrons that are able to cross over to the P-side or the fraction of holes that are able to cross over to the N-side and contribute to current goes exponentially with the barrier

height (remember,  )K:Boldsmans’ constant, T= Temperature= charge and ψ=work function

See:space charge region on Google under Wikipedia for more details.

Reverse Bias:

The holes are now required by the applied bias to move from  and electrons from  as shown below:

Although the electric field favors the flow of holes to the P-region, there are very few holes in N-

region to begin with! The number of holes in N-region is   , a very small number.Further, the number of holes is fixed and unaffected by the bias.Similarly, the number of available electrons in P-region for current flow is very small and unaffected by the applied bias.The only thing that the applied reverse bias does is to increase the junction electric field or the barrier height as shown below

The increased electric field does not alter the current flow because the bottleneck is the small number of carriers available for current conduction.

Current in Reverse bias is very small and almost constant

Drift current: drift current is the electriccurrent, or movement of charge carriers, which is due to the applied electric field, often stated as the electromotive force over a given distance. The drift velocity is the average velocity of the charge carriers in the drift current.Itdepends on the electric field applied: if there's no electric field, there's no drift current.Avalanche and zener breakdown :Animation1

Concept of Vmax,Vrms and Vavgaverage voltages and currents:V max also referred to as Vpeak is the maximum value of voltage i.e. with full power.Vrms is root mean square is the square root of the arithmetic mean of the squares of a set of

values.it is for voltage i.e. Vmax/√2 =0.707.Vavg is average value of valtage which is zero. Vmax + vmin /2Applications of Diode:PN junctions have been used as rectifiers in power supplies, detectors in RF,circuits, Zener diodes which are voltage regulators, clippers, LED's, PIN diodes are RF switches.Rectifiers: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, since it "straightens" the direction of current. Physically, rectifiers take a number of forms, including vacuum tube diodes, mercury-arc valves, stacks of copper and selenium oxide plates, semiconductor diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches.The process is known as rectification, since it "straightens" the direction of current.Rectifier circuits may be single-phase or multi-phase (three phase 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 rectifiersHalf-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. Mathematically, it is a step function (for positive pass, negative block).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.

Positive Half wave rectifier

Negative Half wave rectifier

The no-load output DC voltage of an ideal half-wave rectifier for a sinusoidal input voltage is,

Ripplesandfilters:Ripple (specifically ripplevoltage) in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the alternating waveform after rectification. Ripple voltage originates as the output of a rectifier or from generation and commutation of DC power.An electronic filter like inductors and capacitors with high impedance at the ripple frequency are be used to reduce ripple voltage and increase or decrease DC output; such a filter is often called a smoothing filter.

Full-wave rectificationA full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Mathematically, this corresponds to the absolute value function. Full-wave rectification converts both polarities of the input waveform to pulsating DC (direct current), and yields a higher average 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. Single semiconductor diodes, double diodes with common cathode or common anode,and four-diode bridges, are manufactured as single components.

Working :AnimationFor both circuit Vin is input voltage. Output is Vout. T is step down transformer. n1 is the turn ratio of primary coil and n2 is the turn ratio of secondary coil of transformer. Four diodes (D1, D2, D3, D4) are placed to form bridge (Wheatstone bridge) between for both Vn2 and Vout. Voltage drops at diodes are Vd. R is load resistor. For this bridge structure this rectifier is known as bridge rectifier.Input voltage is applied in primary coil of transformer. This voltage is step down and transferred to secondary coil. Based on the polarity of input voltage and the position of diode, the diodes are forward biased (diode on) or reverse biased (diode off) and generate the output.For first circuit, when Vin positive, Vn2 positive, D1, D3 are forward biased, D2 , D4 are reverse biased i.e. D1, D3 on, D2, D4 off. Vout = Vn2. Vout is positive as Vn2 positive. when Vin negative, Vn2 negative, D1, D3 are reverse biased, D2 , D4 are forward biased i.e. D1, D3 off and D2, D4 on. Vout = -Vn2. As Vn2 is negative, so Vout is positive.For second circuit, when Vin positive, Vn2 positive, D1, D3 are forward biased, D2 , D4 are reverse biased i.e. D1, D3 on, D2, D4 off. Vout = -Vn2. Vout is negative as Vn2 positive. when Vin negative, Vn2 negative, D1, D3 are reverse biased, D2 , D4 are forward biased i.e. D1, D3 off and D2, D4 on. Vout = Vn2. As Vn2 is negative, so Vout is negative.

Peak Inverse Voltage (PIV): It’s the maximum voltage that the rectifying diode has to withstand when it is reverse biased. The peak inverse voltage (PIV) rating of a diode is of the primary importance in the design of rectification systems. When diode is reverse biased no current flows through load resistance and so causes no voltage drop across the load resistance and consequently the whole of the input voltage of secondary coil appears across the diode, is equal to the peak value of the secondary voltage.

Power supply filters and capacitors:The purpose of power supply filters is to smooth out the ripple contained in the pulses of DC obtained from the rectifier circuit while increasing the average output voltage or current. Filter circuits used inpower supplies are of two general types: Capacitorinput and Choke input. Types of filters are pi (л),Land TXc = 1/2л f c XL = 2л f L

Film capacitors,Ceramic capacitors,Tantalum capacitors,Aluminum electrolytic capacitors.

Half wave rectifier Efficiency

The ratio of input AC to output DC is known asefficiency (?). The efficiency of a half wave rectifier is 40.6% when RF is neglected. Ripple content is defined as the amount of AC content present in the output DC. ... The ripple factor value is 1.21 for a half wave rectifier.Ripple (specifically ripple voltage) in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. . Ripple voltage originates as the output of a rectifier or from generation and commutation of DC power.

i = v / (rf + RL)As we know,

v = Vm sin θTherefore,

i = Vm sin θ / (rf + RL)When sin θ = 1, then current = maximum. Therefore,Average and RMS power are related by relation that it is an amount of power that will produce same heating effect as it is produced by dc current.You can introduce the rms current which is Irms=Io/√2, then you have the same formula for the average power you would get in case of Irms=Io/√2 DC current. Pav=Irms2R. 

Iav = ʃ (idθ) / 2π   ….. (i) Avearge current is integration over a cycle.

Integrate equation (i) from 0 to π,

Iav = (1 / 2π) * ʃ Im sin θ dθ= (Im / 2π) * ʃ sin θ dθ= (Im / 2π) [ –cos θ ]= (Im / 2π) [ -(-1-1)]

= 2 (Im / 2π)= (Im / π)

Therefore, DC power output is given as,

Pdc = Idc2 RL = (Im / π)2 RL

And AC power input is given as,

Pac =Irms2 (rf + RL)

Where,

** Irms = ʃ (i2 dθ) / 2π ….. (ii) rms current over a cycle.Integrate equation (ii) from 0 to π,

= √ (1 / 2π) * ʃ Im2 sin2 θ dθ take 2π outside integration as per bold eq.,

= √ (Im2 / 2π) * ʃ ( 1- cos 2θ)/ 2  dθ As sin2(θ) = 1/2*(1 - cos(2θ))

= √ (Im2 / 4π) * [ ʃdθ – ʃ cos 2θ dθ ]

= √ (Im2 / 4π) * [[θ] – [sin 2θ / 2]]

= √ (Im2 / 4π) * [π – 0]= Im / 2

Therefore, AC power input is given as,

Pac =Irms2 (rf + RL)

=(Im / 2)2 (rf + RL)As we know,

Rectifier Efficiency (η) = Pdc / Pac

Put the values of Pdc and Pac from above equations, therefore,η =[ (Im / π)2 * RL ] / ( Im / 2)2 * (rf + RL)

= 0.406 RL / (rf + RL)= 0.406 / (1+ rf RL)

If rf is neglected as compare to RL then the efficiency of the rectifier is maximum. Therefore,η max =0.406 = 40.6%

Average value

Consider first half cycle i.e. when   varies from 

We get,

 

 

Or, 

         …..(5)

Similarly, Eav = 0.637 Emax

Mathematical derivation of Full wave rectifier efficiency:Let the v = Vm sin θ be the AC voltage to be rectified, rf be the forward resistance of crystal diode and RL be the load resistance. The diagram given below shows the input voltage wave and rectified output wave.

The instantaneous value of current is given by the equation:

i = v / (rf + RL)As we know,

v = Vm sin θTherefore,

i = Vm sin θ / (rf + RL)When sin θ = 1, then current = maximum. Therefore,

m = Vm / (rf + RL)Where,

 i = Im sin θSince output is obtained across RL , therefore

D.C power output = Idc2 RL

                                 = *Iav2 RL

Where,

Iav = ʃ (i dθ) / π   ….. (i)Integrate equation (i) from 0 to π,

 Iav = (1 / π) * ʃ Im sin θ dθ     = (Im / π) * ʃ sin θ dθ

     = (Im / π) [ – cos θ ]     = (Im / π) [ -(-1-1)]

= 2 (Im / π)     = (2Im / π)

Therefore, DC power output is given as,

Pdc = Idc2 RL = (2Im / π)2 RL

And AC power input is given as,

Pac =Irms2 (rf + RL)

Where,

** Irms = √  ʃ (i2 dθ) / π ….. (ii)                     = √ (1 / π) * ʃ Im

2 sin2 θ dθ                                  = √ (Im

2 / π) * ʃ ( 1- cos 2θ)/ 2  dθ                                     = √ (Im

2 / 2π) * [ ʃ dθ – ʃ cos 2θ dθ ]                                   = √ (Im

2 / 2π) * [[θ] – [sin 2θ / 2]]                = √ (Im

2 / 2π) * [π – 0]= Im / √ 2

Therefore, AC power input is given as,

Pac =Irms2 (rf + RL)

      =( Im / √2)2 (rf + RL)As we know,

Rectifier Efficiency (η) = Pdc / Pac

Put the values of Pdc and Pac from above equations, therefore,η =[ (2Im / π)2 * RL ] / [( Im / √2)2 * (rf + RL)]

   = 0.812 RL / (rf + RL)  = 0.812 / (1+ rf /RL)

If rf is neglected as compared to RL then the efficiency of the rectifier is maximum. Therefore,η max =0.812 = 81.2%

1) Calculate Vdc,Vdc, Vrms,I rms, through 2 Kohm load connected to half wave rectifier.Transformer turn ration is 230 to 15 with voltage 230v,50 hz.

2) A sine wave of 25 v,50 Hz is applied to HWR with dynamitic resistance of diode of 1W.Find maximum DC and AC current through load.(Explain dynamic courve and then find load resistance.)

Refer https://www.sanfoundry.com/analog-circuits-questions-answers-halfwave-rectifier-1/ for questions.

Diode Applications: Clippers and Clampers

Clippers: The 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. For example, the half-wave rectifier is a clipper circuit, since all voltages below zero are eliminated.

A clipping circuit or a clipper is a device used to ‘clip’ the input voltage to prevent it from attaining a value larger than a predefined one.

The basic components required for a clipping circuit are – an ideal diode and a resistor. In order to fix the clipping level to the desired amount, a dc battery must also be included.  When the diode is forward biased, it acts as a closed switch, and when it is reverse biased, it acts as an open switch. Different levels of clipping can be obtained by varying the amount of voltage of the battery and also interchanging the positions of the diode and resistor.Depending on the features of the diode, the positive or negative region of the input signal is “clipped” off and accordingly the diode clippers may be positive or negative clippers.There are two general categories of clippers: series and parallel (or shunt). The series configuration is defined as one where a diode is in series with the load, while the shunt clipper has the diode in a branch parallel to the load.

Positive Diode Clipper

As shown in the figure, the diode is kept in series with the load. During the positive half cycle of the input waveform, the diode ‘D’ is reverse biased, which maintains the output voltage at 0 Volts. This causes the positive half cycle to be clipped off. During the negative half cycle of the input, the diode is forward biased and so the negative half cycle appears across the output.In Figure (b), the diode is kept in parallel with the load. This is the diagram of a positive shunt clipper circuit. During the positive half cycle, the diode ‘D’ is forward biased and the diode acts as a closed switch. This causes the diode to conduct heavily. This causes the voltage drop across the diode or across the load resistance RL to be zero. Thus output voltage during the positive half cycles is zero, as shown in the output waveform. During the negative half cycles of the input signal voltage, the diode D is reverse biased and behaves as an open switch. Consequently, the entire input voltage appears across the diode or across the load resistance RL if R is much smaller than RL. Actually the circuit behaves as a voltage divider with an output voltage of [RL / R+ RL] Vmax = -Vmax when RL >> RNegative Diode ClipperThe negative clipping circuit is almost the same as the positive clipping circuit, with only one difference. If the diode in figures (a) and (b) is reconnected with reversed polarity, the circuits will become for a negative series clipper and negative shunt clipper respec-tively. The negative series and negative shunt clippers are shown in figures (a) and (b) as given below.

In all the above discussions, the diode is considered to be the ideal one. In a practical diode, the breakdown voltage will exist (0.7 V for silicon and 0.3 V for

Germanium). When this is taken into account, the output waveforms for positive and negative clippers will be of the shape shown in the figure below.

Biased Positive Clipper and Biased Negative Clipper

A biased clipper comes in handy when a small portion of positive or negative half cycles of the signal voltage is to be removed. When small portion of positive is removed, ciccuit is positive bias else negative.

Combination ClipperWhen a portion of both positive and negative of each half cycle of the input voltage is to be clipped (or removed), combination clipper is employed. The circuit for such a clipper is given in the figure below.The action of the circuit is summarized below. For positive input voltage signal when input voltage exceeds battery voltage ‘+ V1‘ diode D1 conducts heavily while diode ‘D2‘ is reversed biased and so voltage ‘+ V1‘ appears across the output. This output voltage ‘+ V1‘ stays as long as. the input signal voltage exceeds ‘+ V1‘. On the other hand for the negative input voltage signal, the diode ‘D1‘ remains reverse biased and diode ‘D2‘ conducts heavily only when input voltage exceeds battery

voltage ‘V2‘ in magnitude. Thus during the negative half cycle the output stays at ‘- V2‘ so long as the input signal voltage is greater than ‘-V2‘.

Drawbacks of Series and Shunt Diode Clippers In series clippers, when the diode is in ‘OFF’ position, there will be no

transmission of the input signal to output. But in the case of high-frequency signals transmission occurs through diode capacitance which is undesirable. This is the drawback of using the diode as a series element in such clippers.

In shunt clippers, when the diode is in the ‘off condition, transmission of input signal should take place to output. But in the case of high-frequency input signals, diode capacitance affects the circuit operation adversely and the signal gets attenuated (that is, it passes through diode capacitance to ground).Applications of clipping circuits

Used in FM transmitters to reduce noise To limit the voltage input to a device To modify an existing waveform to the desired output

But 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. Then diode clipping circuits can be used in voltage limiting applications.A Zener diode is a particular type of diode that, unlike a normal one, allows current to flow not only from its anode to its cathode, but also in the reverse direction, when the Zener voltage is reached.Zener diodes have a highly doped p-n junction. Normal diodes will also break down with a reverse voltage but the voltage and sharpness of the knee are not as well defined as for a Zener diode. Also normal diodes are not designed to operate in the breakdown region, but Zener diodes can reliably operate in this region.The device was named after Clarence Melvin Zener, who discovered the Zener effect. Zener reverse breakdown is due to electron quantum tunnelling caused by a high-strength electric field. However, many diodes described as "Zener" diodes rely instead on avalanche breakdown. Both breakdown types are used in Zener diodes with the Zener effect predominating under 5.6 V and avalanche breakdown above.Zener diodes are widely used in electronic equipment of all kinds and are one of the basic building blocks of electronic circuits. They are used to generate low power stabilized supply rails from a higher voltage and to provide reference voltages for circuits, especially stabilized power supplies. They are also used to protect circuits from overvoltage, especially electrostatic discharge (ESD).

The Zener diode's operation depends on the heavy doping of its p-n junction. The depletion region formed in the diode is very thin (<1 µm) and the electric field is consequently very high (about 500 kV/m) even for a small reverse bias voltage of about 5 V, allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material.At the atomic scale, this tunnelling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of doping on both sides. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.07% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available Zener diodes can vary widely from 1.2 volts to 200 volts.For diodes that are lightly doped the breakdown is dominated by the avalanche effect rather than the Zener effect. Consequently, the breakdown voltage is higher (over 5.6 V) for these devices. Waveform clipperTwo Zener diodes facing each other in series will act to clip both halves of an input signal. Waveform clippers can be used not only to reshape a signal, but also to prevent voltage spikes from affecting circuits that are connected to the power supply.

Voltage shifter[A Zener diode can be applied to a circuit with a resistor to act as a voltage shifter. This circuit lowers the output voltage by a quantity that is equal to the Zener diode's breakdown voltage.

Level Shifting With a Zener DiodeZener diodes are diodes that allow current to flow from anode to cathode just like a regular diode, but also allows current to flow from cathode to anode up to a particular voltage, called the breakdown voltage. Zener diodes come in various breakdown voltages, so you can choose the one that works for your application. For example, the 1N5226 is a common 3.3V Zener diode, and the the 1N4733 is a common 5.1V Zener diode. When you’re using a Zener diode as a voltage shifter, you connect the cathode to the source, and the anode to the output, usually with a pull down resistor as shown below.Voltage regulatorA Zener diode can be applied in a voltage regulator circuit to regulate the voltage applied to a load, such as in a linear regulator.

Voltage multipliers are similar in many ways to rectifiers in that they convert AC-to-DC voltages for use in many electrical and electronic circuit applications such as in microwave ovens, strong electric field coils for cathode-ray tubes, electrostatic and high voltage test equipment, etc, where it is necessary to have a very high DC voltage generated from a relatively low AC supply.

Generally, the DC output voltage (Vdc) of a rectifier circuit is limited by the peak value of its sinusoidal input voltage. But by using combinations of rectifier diodes and capacitors together we can effectively multiply this input peak voltage to give a DC output equal to some odd or even multiple of the peak voltage value of the AC input voltage. Consider the basic voltage multiplier circuit below.Full wave voltage multiplier

The above circuit shows a basic symmetrical voltage multiplier circuit made up from two half-wave rectifier circuits. By adding a second diode and capacitor to the output of a standard half-wave rectifier, we can increase its output voltage by a set amount. This type of voltage multiplier configuration is known as a Full Wave Series Multiplier because one of the diodes is conducting in each half cycle, the same as for a full wave rectifier circuit.When the sinusoidal input voltage is positive, capacitor C1 charges up through diode D1 and when the sinusoidal voltage is negative, capacitor C2charges up through diode, D2. The output voltage 2VIN is taken across the two series connected capacitors.The voltage produced by a voltage multiplier circuit is in theory unlimited, but due to their relatively poor voltage regulation and low current capability there are generally designed to increase the voltage by a factor less than ten. However, if designed correctly around a suitable transformer, voltage multiplier circuits are capable of producing output voltages in the range of a few hundred to tens’s of thousand’s of volts, depending upon their original input voltage value but all with low currents in the milli amperes range.

The Voltage Doubler

As its name suggests, a Voltage Doubler is a voltage multiplier circuit which has a voltage multiplication factor of two. The circuit consists of only two diodes, two capacitors and an oscillating AC input voltage (a PWM waveform could also be used). This simple diode-capacitor pump circuit gives a DC output voltage equal to the peak-to-peak value of the sinusoidal input. In other words, double the peak voltage value because the diodes and the capacitors work together to effectively double the voltage.

The circuit shows a half wave voltage doubler. During the negative half cycle of the sinusoidal input waveform, diode D1 is forward biased and conducts charging up the pump capacitor, C1 to the peak value of the input voltage, (Vp). Because there is no return path for capacitor C1 to discharge into, it remains fully charged acting as a storage device in series with the voltage supply. At the same time, diode D2conducts via D1 charging up capacitor, C2.During the positive half cycle, diode D1 is reverse biased blocking the discharging of C1 while diode D2 is forward biased charging up capacitor C2. But because there is a voltage across capacitor C1 already equal to the peak input voltage, capacitor C2 charges to twice the peak voltage value of the input signal.In other words, V(positive peak) + V(negative peak), so on the negative half-cycle, D1 charges C1 to Vp and on the positive half-cycle D2 adds the AC peak voltage to Vp onC1 and transfers it all to C2. The voltage across capacitor, C2 discharges through the load ready for the next half cycle.Then the voltage across capacitor, C2 can be calculated as: Vout = 2Vp, (minus of course the voltage drops across the diodes used) where Vp is the peak value of the input voltage. Note that this double output voltage is not instantaneous but increases slowly on each input cycle, eventually settling to 2Vp.As capacitor C2 only charges up during one half cycle of the input waveform, the resulting output voltage discharged into the load has a ripple frequency equal to the supply frequency, hence the name half wave voltage doubler. The disadvantage of this is that it can be difficult to smooth out this large ripple frequency in much the same way as for a half wave rectifier circuit. Also, capacitor C2 must have a DC voltage rating at least twice the value of the peak input voltage.The advantage of “Voltage Multiplier Circuits” is that it allows higher voltages to be created from a low voltage power source without a need for an expensive high voltage transformer as the voltage doubler circuit makes it possible to use a transformer with a lower step up ratio than would be need if an ordinary full wave supply were used. However, while voltage multipliers can boost the voltage, they can only supply low currents to a high-resistance (+100kΩ) load because the generated output voltage quickly drops-off as load current increases.By reversing the direction of the diodes and capacitors in the circuit we can also reverse the direction of the output voltage creating a negative voltage output. Also, if we connected the output of one multiplying circuit onto the input of another (cascading), we can continue to increase the DC output voltage in integer steps to produce voltage triplers, or voltage quadruplers circuits, etc, as shown.

DC Voltage Tripler Circuit

 By adding an additional single diode-capacitor stage to the half-wave voltage doubler circuit above, we can create another voltage multiplier circuit that increases its input voltage by a factor of three and producing what is called a Voltage Tripler Circuit.A “voltage tripler circuit” consists of one and a half voltage doubler stages. This voltage multiplier circuit gives a DC output equal to three times the peak voltage value (3Vp) of the sinusoidal input signal. As with the previous voltage doubler, the diodes within the voltage tripler circuit charge and block the discharge of the capacitors depending upon the direction of the input half-cycle. Then 1Vp is dropped across C3 and 2Vp across C2 and as the two capacitors are in series, this results in the load seeing a voltage equivalent to 3Vp.Note that the real output voltage will be three times the peak input voltage minus the voltage drops across the diodes used, 3Vp – V(diode).If a voltage tripler circuit can be made by cascading together one and a half voltage multipliers, then a Voltage Quadrupler Circuit can be constructed by cascading together two full voltage doubler circuits as shown.

Schottky diode: The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action.A metal–semiconductor junction is formed between a metal and a semiconductor, creating a Schottky barrier (instead of a semiconductor–semiconductor junction as in conventional diodes).Typical metals used are molybdenum, platinum, chromium or tungsten, and certain silicides (e.g., palladium silicide and platinum silicide), whereas the semiconductor would typically be n-type silicon.[1] The metal side acts as the anode, and n-type semiconductor acts as the cathode of the diode;

meaning conventional current can flow from the metal side to the semiconductor side, but not in the opposite direction. This Schottky barrier results in both very fast switching and low forward voltage drop.

Typical Schottky diodeIn a p–n diode, the reverse recovery time can be in the order of several microseconds to less than 100 ns for fast diodes. Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e., there is no charge carrier depletion region at the junction). The switching time is ~100 ps for the small-signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. With p–n-junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise. With Schottky diodes, switching is essentially "instantaneous" with only a slight capacitive loading, which is much less of a concern.Zener Diode:

Diode clamping conceptClamping (hold tightly, fasten)

Clamper circuit: This circuit binds upper or lower form of waveform to a fixed D.C. voltage level. These circuits are also referred to as voltage resistor. Circuit is capable of voltage both for positive or negative D.C. level.

Positive unbiased clamper

Negative unbiased clamper

For a practical diode (Si) positive voltage will not drop below 0.7 volts i.e. cut in voltage.

A Positive Clamper circuit is one that consists of a diode, a resistor and a capacitor and that shifts the output signal to the positive portion of the input signal.

Remember the diode connection. It will conduct only when cathode becomes more negative than anode. During this point no output is available. When negative voltage attains peak value, the capacitor gets charged to its maximum negative value -Vm and diode gets forward bias and conducts.

During next positive half cycle, the capacitor gets charged + Vm ,diode gets reverse bias and open circuit. The output of circuit at this moment is V0 = Vi + Vm. Thus signal gets positively clamped.

The time constant τ requires to charge capacitor should be greater than half of time period. τ > ½ t. Where t is time period of input waveform.

Types of ClampersThere are few types of clamper circuits, such as

Positive Clamper Positive clamper with positive Vr Positive clamper with negative -Vr Negative Clamper

Negative clamper with positive +Vr Negative clamper with negative -Vr

Positive Clamper with Positive Vr

A Positive clamper circuit if biased with some positive reference voltage, that voltage will be added to the output to raise the clamped level. Using this, the circuit of the positive clamper with positive reference voltage is constructed as below.

During the positive half cycle, the reference voltage is applied through the diode at the output and as the input voltage increases, the cathode voltage of the diode increase with respect to the anode voltage and hence it stops conducting. During the negative half cycle, the diode gets forward biased and starts conducting. The voltage across the capacitor and the reference voltage together maintain the output voltage level.

And remaining works on same principal.

Light Emitting Diode (LED)

The “Light Emitting Diode” or LED as it is more commonly called, is basically just a specialized type of diode as they have very similar electrical characteristics to a PN junction diode. That is LED will pass current in its forward direction but block the flow of current in the reverse direction.Light emitting diodes are made from a very thin layer of fairly heavily doped semiconductor material and depending on the semiconductor material used and the amount of doping, when forward biased an LED will emit a coloured light at a particular spectral wavelength.

When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single colour) of light. Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a coloured light output.when operated in a forward biased direction Light Emitting Diodes are semiconductor devices that convert electrical energy into light energy. The construction of a Light Emitting Diode is very different from that of a normal signal diode. The PN junction of an LED is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell or body which protects the LED from both vibration and shock.An LED junction does not actually emit that much light so the epoxy resin body is constructed in such a way that the photons of light emitted by the junction are reflected away from the surrounding substrate base to which the diode is attached and are focused upwards through the domed top of the LED, which itself acts like a lens concentrating the amount of light. This is why the emitted light appears to be brightest at the top of the LED.However, not all LEDs are made with a hemispherical shaped dome for their epoxy shell. Some indication LEDs have a rectangular or cylindrical shaped construction that has a flat surface on top or their body is shaped into a bar or arrow. Generally, all LED’s are manufactured with two legs protruding from the bottom of the body.Also, nearly all modern light emitting diodes have their cathode, ( – ) terminal identified by either a notch or flat spot on the body or by the cathode lead being shorter than the other as the anode ( + ) lead is longer than the cathode (k). Unlike normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, the light emitting diode produces a “cold” generation of light which leads to high efficiencies than the normal “light bulb” because most of the generated energy radiates away within the visible spectrum. Because LEDs are solid-state devices, they can be extremely small and durable and provide much longer lamp life than normal light sources.

Light Emitting Diode Colours

Typical LED Characteristics

Semiconductor

Material

Wavelength

Colour

VF @ 20mA

GaAs850-

940nmInfra-Red

1.2v

GaAsP630-

660nmRed 1.8v

GaAsP605-

620nmAmb

er2.0v

GaAsP:N585-

595nmYello

w2.2v

AlGaP550-

570nmGree

n3.5v

SiC 430- Blue 3.6v

505nm

GaInN 450nmWhit

e4.0v

LED Series Resistance.

The series resistor value RS is calculated by simply using Ohm´s Law, by knowing the required forward current IF of the LED, the supply voltage VS across the combination and the expected forward voltage drop of the LED, VF at the required current level, the current limiting resistor is calculated as:LED Series Resistor Circuit

An amber coloured LED with a forward volt drop of 2 volts is to be connected to a 5.0v stabilised DC power supply. Using the circuit above calculate the value of the series resistor required to limit the forward current to less than 10mA. Also calculate the current flowing through the diode if a 100Ω series resistor is used instead of the calculated first.1). series resistor required at 10mA.

2). with a 100Ω series resistor.

LED Driver CircuitsLed can be controlled by switching it “ON” and “OFF”. The output stages of both TTL and CMOS logic gates can both source and sink useful amounts of current therefore can be used to drive an LED. Normal integrated circuits (ICs) have an output drive current of up to 50mA in the sink mode configuration, but have an internally limited output current of about 30mA in the source mode configuration.Either way the LED current must be limited to a safe value using a series resistor as we have already seen. Below are some examples of driving light emitting diodes using inverting ICs but the idea is the same for any type of integrated circuit output whether combinational or sequential.IC Driver Circuit

If more than one LED requires driving at the same time, such as in large LED arrays, or the load current is to high for the integrated circuit or we may just want to use discrete components instead of ICs, then an alternative way of driving the LEDs using either bipolar NPN or PNP transistors as switches is given below. Again as before, a series resistor, RS is required to limit the LED current.

The brightness of a light emitting diode cannot be controlled by simply varying the current flowing through it. Allowing more current to flow through the LED will make it glow brighter but will also cause it to dissipate more heat. LEDs are designed to produce a set amount of light operating at a specific forward current ranging from about 10 to 20mA.In situations where power savings are important, less current may be possible. However, reducing the current to below say 5mA may dim its light output too much or even turn the LED “OFF” completely. A much better way to control the brightness of LEDs is to use a control process known as “Pulse Width Modulation” or PWM, in which the LED is repeatedly turned “ON” and “OFF” at varying frequencies depending upon the required light intensity of the LED.

Transistor Driver Circuit

A Bi-colour LED

LED DisplaysAs well as individual colour or multi-colour LEDs, several light emitting diodes can be combined together within a single package to produce displays such as bargraphs, strips, arrays and seven segment displays.A 7-segment LED display provides a very convenient way when decoded properly of displaying information or digital data in the form of numbers, letters or even alpha-numerical characters and as their name suggests, they consist of seven individual LEDs (the segments), within one single display package.In order to produce the required numbers or characters from 0 to 9 and A to Frespectively, on the display the correct combination of LED segments need to be illuminated. A standard seven segment LED display generally has eight input connections, one for each LED segment and one that acts as a common terminal or connection for all the internal segments. The Common Cathode Display (CCD) – In the common cathode display, all

the cathode connections of the LEDs are joined together and the individual segments are illuminated by application of a HIGH, logic “1” signal.

The Common Anode Display (CAD) – In the common anode display, all the anode connections of the LEDs are joined together and the individual segments are illuminated by connecting the terminals to a LOW, logic “0” signal.

A Typical Seven Segment LED Display

Photo Diode

A photodiode is one type of light detector, used to convert the light into current or voltage based on the mode of operation of the device.

A photodiode is a device that helps in conversion of light into electric current. Current is produced in the photodiode when photons are absorbed and a small amount of current is also produced when there is no light present. With increase of the surface area, photodiodes have slower response times.Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, responsivity measures the electrical output per optical

input. The responsivity of a photodetector is usually expressed in units of either amperes or volts per watt of incident radiant power.The responsivity (or radiant sensitivity) of a photodiode or some other kind ofphotodetector is the ratio of generated photocurrent and incident (or sometimes absorbed) optical power (neglecting noise influences), determined in the linear region of response.

R= ηq/ɧƒ ≈ η ʎµm/1.2395(µµm/W/A)where η is the quantum efficiency (the conversion efficiency of photons to electrons) of the detector for a given wavelength, q is the electron charge, ƒ is the frequency of the optical signal, and ɧ is Planck's constant. This expression is also given in terms of ʎ, the wavelength of the optical signal, and has units of amperes per watt (A/W).Photodiode is very sensitive to light so when light or photons falls on the photodiode it easily converts light into electric current. Solar cell is also known as large area photodiode because it converts solar energy or light energy into electric energy. However, solar cell works only at bright light.The construction and working of photodiode is almost similar to the normal p-n junction diode. PIN (p-type, intrinsic and n-type) structure is mostly used for constructing the photodiode instead of p-n (p-type and n-type) junction structure because PIN structure provide fast response time. PIN photodiodes are mostly used in high-speed applications.In a normal p-n junction diode, voltage is used as the energy source to generate electric current whereas in photodiodes, both voltage and light are used as energy source to generate electric current.The external reverse voltage applied to the p-n junction diode will supply energy to the minority carriers but not increase the population of minority carriers.However, a small number of minority carriers are generated due to external reverse bias voltage. The minority carriers generated at n-side or p-side will recombine in the same material before they cross the junction. As a result, no electric current flows due to these charge carriers. For example, the minority carriers generated in the p-type material experience a repulsive force from the external voltage and try to move towards n-side. However, before crossing the junction, the free electrons recombine with the holes within the same material. As a result, no electric current flows. To overcome this problem, we need to apply external energy directly to the depletion region to generate more charge carriers. A special type of diode called photodiode is designed to generate more number of charge carriers in depletion region. In photodiodes, we use light or photons as the external energy to generate charge carriers in depletion region.

Advantages of PIN photodiode: Wide bandwidth, High quantum efficiency, High response speed

Rs=420Ω