Basic of Electronics Ppt Pdm 19nov

PDM ECE*Semiconductor PhysicsSession A

PDM ECE

PDM ECE*Introduction Semiconductors are materials whose electronic SeSeSeproperties are intermediate between te of Metals and Insulators.

They have conductivities in the range of 10 -4 to 10 +4S/m.

The interesting feature about semiconductors is that they are bipolar and current is transported by two charge carriers of opposite sign.

These intermediate properties are determined by 1.Crystal Structure bonding Characteristics. 2.Electronic Energy bands.

PDM ECE

Silicon and Germanium are elemental semiconductors and they have four valence electrons which are distributed among the outermost S and p orbital's.

These outer most S and p orbital's of Semiconductors involve in Sp3 hybridanisation.

These Sp3 orbital's form four covalent bonds of equal angular separation leading to a tetrahedral arrangement of atoms in space results tetrahedron shape, resulting crystal structure is known as Diamond cubic crystal structure

PDM ECE*

PDM ECE

Types of Semiconductors1.Intrinsic Seconductor

2.Extrinsic SemiconductorPDM ECE*

PDM ECE

Intrinsic Semiconductor

A Semiconductor which does not have any kind of impurities, behaves as an Insulator at 0k and behaves as a Conductor at higher temperature is known as Intrinsic Semiconductor or Pure Semiconductors.

Germanium and Silicon (4th group elements) are the best examples of intrinsic semiconductors and they possess diamond cubic crystalline structure.

PDM ECE*

PDM ECE

PDM ECE* Intrinsic Semiconductor

A Semiconductor which does not have any kind of impurities, behaves as an Insulator at 0k and behaves as a Conductor at higher temperature is known as Intrinsic Semiconductor or Pure Semiconductors.

Germanium and Silicon (4th group elements) are the best examples of intrinsic semiconductors and they possess diamond cubic crystalline structure.

PDM ECE

Carrier Concentration in Intrinsic SemiconductorWhen a suitable form of Energy is supplied to a Semiconductor then electrons take transition from Valence band to Conduction band.

Hence a free electron in Conduction band and simultaneously free hole in Valence band is formed. This phenomenon is known as Electron - Hole pair generation.

In Intrinsic Semiconductor the Number of Conduction electrons will be equal to the Number of Vacant sites or holes in the valence band.

PDM ECE*

PDM ECE

PDM ECE*

PDM ECE

Extrinsic SemiconductorsThe Extrinsic Semiconductors are those in which impurities of large quantity are present. Usually, the impurities can be either 3rd group elements or 5th group elements.

Based on the impurities present in the Extrinsic Semiconductors, they are classified into two categories. 1. N-type semiconductors 2. P-type semiconductors

PDM ECE*

PDM ECE

N - type SemiconductorsWhen any pentavalent element such as Phosphorous, Arsenic or Antimony is added to the intrinsic Semiconductor , four electrons are involved in covalent bonding with four neighboring pure Semiconductor atoms.

The fifth electron is weakly bound to the parent atom. And even for lesser thermal energy it is released Leaving the parent atom positively ionized.

PDM ECE*

PDM ECE

N - type SemiconductorsThe Intrinsic Semiconductors doped with pentavalent impurities are called N-type Semiconductors. The energy level of fifth electron is called donor level. The donor level is close to the bottom of the conduction band most of the donor level electrons are excited in to the conduction band at room temperature and become the Majority charge carriers.

Hence in N-type Semiconductors electrons are Majority carriers and holes are Minority carriers.

PDM ECE*

PDM ECE

*PDM ECEPDM ECE

PDM ECE

Carrier Concentration in N-type SemiconductorConsider Nd is the donor Concentration i.e., the number of donor atoms per unit volume of the material and Ed is the donor energy level.

At very low temperatures all donor levels are filled with electrons.

With increase of temperature more and more donor atoms get ionized and the density of electrons in the conduction band increases.

PDM ECE*

PDM ECE

Variation of Fermi level with temperatureTo start with ,with increase of temperature Ef increases slightly.

As the temperature is increased more and more donor atoms are ionized.

Further increase in temperature results in generation of Electron - hole pairs due to breading of covalent bonds and the material tends to behave in intrinsic manner.

The Fermi level gradually moves towards the intrinsic Fermi level Ei.

PDM ECE*

PDM ECE

P-type semiconductorsWhen a trivalent elements such as Al, Ga or Indium have three electrons in their outer most orbits , added to the intrinsic semiconductor all the three electrons of Indium are engaged in covalent bonding with the three neighboring Si atoms.Indium needs one more electron to complete its bond. this electron maybe supplied by Silicon , there by creating a vacant electron site or hole on the semiconductor atom.Indium accepts one extra electron, the energy level of this impurity atom is called acceptor level and this acceptor level lies just above the valence band.These type of trivalent impurities are called acceptor impurities and the semiconductors doped the acceptor impurities are called P-type semiconductors.

PDM ECE*

PDM ECE

PDM ECE*

PDM ECE

Direct band gap and indirect band gap semiconductors:We known that the energy spectrum of an electron moving in the presence of periodic potential field is divided into allowed and forbidden zones.

In crystals the inter atomic distances and the internal potential energy distribution vary with direction of the crystal. Hence the E-k relationship and hence energy band formation depends on the orientation of the electron wave vector to the crystallographic axes.

In few crystals like gallium arsenide, the maximum of the valence band occurs at the same value of k as the minimum of the conduction band as shown in below. this is called direct band gap semiconductor.

PDM ECE*

PDM ECE

Direct band gap and indirect band gap semiconductors:PDM ECE*

PDM ECE

In few semiconductors like silicon the maximum of the valence band does not always occur at the same k value as the minimum of the conduction band as shown in figure. This we call indirect band gap semiconductor.

In direct band gap semiconductors the direction of motion of an electron during a transition across the energy gap remains unchanged.

Hence the efficiency of transition of charge carriers across the band gap is more in direct band gap than in indirect band gap semiconductorsPDM ECE*

PDM ECE

Forward BiasForward bias is the condition that allows current through a pn junction. This external bias voltage is designated as VBIAS. The resistor R limits the current to a value that will not damage the pn structure.*PDM ECE

PDM ECE

The Effect of Forward Bias on the Depletion RegionAs more electrons flow into the depletion region, the number of positive ions is reduced. As more holes effectively flow into the depletion region on the other side of the pn junction, the number of negative ions is reduced. This reduction in positive and negative ions during forward bias causes the depletion region to narrow.*PDM ECE

PDM ECE

*PDM ECE

PDM ECE

Effect of the Barrier Potential During Forward BiasWhen forward bias is applied, the free electrons are provided with enough energy to overcome the barrier potential and effectively climb" the energy hill and cross the depletion region. The energy that the electrons require in order to pass through the depletion region is equal to the barrier potential energy. In other words, the electrons give up an amount of energy equivalent to the barrier potential when they cross the depletion region. This energy loss results in a voltage drop across the pn junction*PDM ECE

PDM ECE

Reverse BiasThe initial flow of charge carriers is transitional and lasts for only a very short time after the reverse bias voltage is applied. As the depletion region widens, the availability of majority carriers decreases.the electric field between the positive and negative charges increases in strength a very small reverse current that can usually be neglected *PDM ECE

PDM ECE

the width of the depletion zone will increase. This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing minimal electric current to cross the p-n junction.The strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p-n junction depletion zone breaks-down and current begins to flow, usually by either the Zeneror avalanche breakdown processes..

*PDM ECE

PDM ECE

Reverse current The small number of free minority electrons in the p region are "pushed" toward the pn junction by the negative bias voltage. When these electrons reach the wide depletion region, they "fall down the energy hill" and combine with the minority holes in the n region as valence electrons and flow toward the positive bias voltage, creating a small hole current. The minority electrons easily pass through the depletion region because they require no additional energy *PDM ECE

PDM ECE

Reverse BreakdownAt the breakdown voltage, the reverse current will drastically increase.The high reverse-bias voltage imparts energy to the free minority electrons so that as they speed through the p region, they collide with atoms with enough energy to knock valence electrons out of orbit . The newly created conduction electrons are also high in energy and repeat the process. If one electron knocks only two others out of their valence orbit during its travel through the h region, the numbers quickly multiply. As these high-energy electrons go through the depletion region, they have enough energy to go through the n region as conduction electrons. rather than combining with holes.The multiplication of conduction electrons just discussed is known as avalanche and results in a very high reverse current that can damage the pn structure because of excessive heat dissipation.*PDM ECE

PDM ECE

CURRENT-VOLTAGE CHARACTERISTIC OF A PN JUNCTION IV characteristics for forward biasPoint A corresponds to zero-bias condition. Point B corresponds to where the forward voltage is less than the barrier potential of 0.7 V. Point C corresponds to where the forward voltage approximately equals the barrier potential and the external bias voltage and forward current have continued to increase.

*PDM ECE

PDM ECE

Terminal Characteristics of Junction Diodes:

The diode: a two terminal device having the circuit symbol shown: Polarity of voltage: V positive Forward Direction of current: I positive Forward Three regions: 1- Forward bias region: V > 0 2- Reverse bias region: -VZK

PDM ECE*

PDM ECE

AmplifierAnamplifieris a device for increasing thepowerof asignal by use of an external energy source.In anelectronic amplifier the input "signal" is usually a voltage or a current. Other types exist; afluidic amplifier increases the power of signals represented as flow of gas or liquid, for example. Amplifiers may be classified in a variety of ways depending on their application, the frequency range they cover, or the active devices used. Ideally an amplifier increases the power of a signal without otherwise altering it; practical amplifiers have finite distortion and noise which they invariably add to the signal.A device that converts signals from one type to another (for example, alight signal inphotons to aDC signal inamperes) is atransducer atransformer or asensor However, none of these amplifypowerPDM ECE*

PDM ECE

Thegainof an amplifier is theratioof output to input power or amplitude, and is usually measured in decibel Bandwidth Thebandwidth of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance" Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the3 dBbandwidth. Efficiency Efficiency is a measure of how much of the power source is usefully applied to the amplifier's output. Noise This is a measure of how muchnoiseis introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components; also, much noise results from intentional economies of manufacture and design time. The metric for noise performance of a circuit isnoise figureor noise factor. Noise figure is a comparison between the output signal to noise ratio and the thermal noise of the input signal.

Characteristics of Amplifier

PDM ECE*

PDM ECE

Stability Stability is an issue in all amplifiers with feedback, whether that feedback is added intentionally or results unintentionally. It is especially an issue when applied over multiple amplifying stages. PDM ECE*

PDM ECE

Multistage AmplifiersPDM ECE*

PDM ECE

Multistage AmplifierPDM ECE*

PDM ECE

R-C Coupled Amplifier: When a.c. signal is applied to the base of the first transistor, it is amplified and developed across the out of the 1st stage. This amplified voltage is applied to the base of next stage through the coupling capacitor Cc where it is further amplified and reappears across the out put of the second stage. Thus the successive stages amplify the signal and the overall gain is raised to the desired level. Much higher gains can be obtained by connecting a number of amplifier stages in succession (one after the other). PDM ECE*

PDM ECE

. Resistance-capacitance (RC) coupling is most widely used to connect the output of first stage to the input (base) of the second stage and so on. It is the most popular type of coupling because it is cheap and provides a constant amplification over a wide range of frequencies. Fig. shows the circuit arrangement of a two stage RC coupled CE mode transistor amplifier where resistor R is used as a load and the capacitor C is used as coupling element between the two stages of the amplifier.

PDM ECE*

PDM ECE

Frequency response curveThe curve representing the variation of gain of an amplifier with frequency isknown as frequency response curve. It is shown in Fig The voltage gain of theamplifier increases with the frequency, f and attains a maximum value. The maximumvalue of the gain remains constant over a certain frequency range and afterwards the gainstarts decreasing with the increase of the frequency. It may be seen to be divided intothree regions. 1) Low frequency range ( 20 kHz ).PDM ECE*

PDM ECE

Negative Feedback Anegative feedback amplifier(or more commonly simply afeedback amplifier) is an amplifier which combines a fraction of the output with the input so that anegative feedbackopposes the original signal. The applied negative feedback improves performance (gain stability, linearity, frequency response,step response and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, negative feedback is used in this way in many amplifiers and control systems. A negative feedback amplifier is a system of three elements (see Figure 1): anamplifierwith gainAOL, an attenuatingfeedback networkwith a constant < 1 and a summing circuit acting as asubtractor(the circle in the figure). The amplifier is the only obligatory; the other elements may be omitted in some cases. For example, in a voltage (emitter,source,op-amp) follower the feedback network and the summing circuit are not necessary.PDM ECE*

PDM ECE

Below, the voltage gain of the amplifier with feedback, theclosed-loop gainAfb, is derived in terms of the gain of the amplifier without feedback, theopen-loop gainAOLand thefeedback factor, which governs how much of the output signal is applied to the input. See Figure 1, top right. The open-loop gainAOLin general may be a function of both frequency and voltage; the feedback parameter is determined by the feedback network that is connected around the amplifier. For anoperational amplifier two resistors forming a voltage divider may be used for the feedback network to set between 0 and 1. This network may be modified using reactive elements likecapacitors orinductors (a) give frequency-dependent closed-loop gain as in equalization/tone-control circuits or (b) construct oscillators. The gain of the amplifier with feedback is derived below in the case of a voltage amplifier with voltage feedback.Without feedback, the input voltageV'inis applied directly to the amplifier input. The according output voltage isGain reduction*PDM ECE

PDM ECE

Suppose now that an attenuating feedback oop applies a fraction .Voutof the output to one of the subtractor inputs so that it subtracts from the circuit input voltageVinapplied to the other subtractor input. The result of subtraction applied to the amplifier input is

Substituting forV'inin the first expression,

Rearranging\

Then the gain of the amplifier with feedback, called the closed-loop gain,Afbis given by,PDM ECE*

PDM ECE

IfAOL>> 1, thenAfb 1 / and the effective amplification (or closed-loop gain)Afbis set by the feedback constant , and hence set by the feedback network, usually a simple reproducible network, thus making linearizing and stabilizing the amplification characteristics straightforward. Note also that if there are conditions where AOL= 1, the amplifier has infinite amplification it has become an oscillator, and the system is unstable. The stability characteristics of the gain feedback product AOLare often displayed and investigated on aNyquist plot(a polar plot of the gain/phase shift as a parametric function of frequency). A simpler, but less general technique, usesBode plotsThe combinationL= AOLappears commonly in feedback analysis and is called theloop gain. The combination ( 1 + AOL) also appears commonly and is variously named as thedesensitivity factoror theimprovement factor.

PDM ECE*

PDM ECE

Figure: Gain vs. frequency for a single-pole amplifier with and without feedback; corner frequencies are labeled.Feedback can be used to extend the bandwidth of an amplifier at the cost of lowering the amplifier gain.Figure shows such a comparison. The figure is understood as follows. Without feedback the so-calledopen-loopgain in this example has a single time constant frequency response given by wherefCis thecutofforcorner frequency of the amplifier: in this examplefC= 104Hz and the gain at zero frequency A0= 105V/V. The figure shows the gain is flat out to the corner frequency and then drops. When feedback is present the so-calledclosed-loopgain, as shown in the formula of the previous section, becomes.The last expression shows the feedback amplifier still has a single time constant behaviour, but the corner frequency is now increased by the improvement factor ( 1 + A0), and the gain at zero frequency has dropped by exactly the same factor. This behavior is called thegain-bandwidth trade-off in Figure ( 1 + A0) = 103, soAfb(0)= 105/ 103= 100 V/V, andfCincreases to 104 103= 107Hz.Bandwidth extension *PDM ECE

PDM ECE

Feedback topologiesVoltage- voltagePDM ECE*

PDM ECE

Current voltage feedback

PDM ECE*

PDM ECE

Voltage current feedback

PDM ECE*

PDM ECE

Current-current feedbackPDM ECE*

PDM ECE

Type of feedbackPositive---distorted output +input Amplified (distorted output)

Negative ----input - Fraction of distorted output Amplified (undistorted output)

PDM ECE*

PDM ECE

PDM ECE*

PDM ECE

PDM ECE*Basic ElectronicsSection-B Oscillators : Criteria for oscillations, analysis of LC, RC and Crystal oscillators, Study of Wien Bridge Oscillators.

Operational Amplifiers : Op-amps, its characteristics and its applications.

Power Suppliers : Introduction and Working of Switched Mode Power.

Voltage Regulator, Introduction to Inverters and UPS.

PDM ECE

PDM ECE*OscillatorsOscillators convert dc to ac.Oscillators utilize positive feedback.An amplifier will oscillate if it has positive feedback and has extra gain than loss in the feedback path.Sinusoidal oscillators have positive feedback at only on its own frequency.A lead-lag network produces a phase shift of 0 degrees at only one frequency

PDM ECE

VinVoutABFeedbackThis amplifier has positive feedback.It oscillates if A > B.*PDM ECE

PDM ECE

PDM ECE*Basic Linear OscillatorandIf Vs = 0, the only way that Vo can be nonzero is that loop gain A=1 which implies that(Barkhausen Criterion)

PDM ECE

PDM ECE*Basic Oscillator Feedback Circuit

Without Feedback

With Feedback

PDM ECE

Oscillators feedbackOscillators are circuits that produce an output waveform without an external signal source. The key to oscillator operation is positive feedback. A positive feedback network produces a feedback voltage (Vf) that is in phase with the input signal (Vin ) . The amplifier shown in the figure produces a 180 voltage phase shift, and the feedback network introduces another 180 voltage shift. This results in a combined 360 voltage phase shift, which is the same as a 0 shift. PDM ECE*

PDM ECE

Tuned collector oscillator circuit The tuned circuit is connected to the collector consist with capacitor C and transformer primary coil L, forms the load impedance and determines the frequency of oscillation.R1, R2 and RE form the dc biasing circuit the transistor. Capacitors C1 and CE are bypass capacitors for R2 and RE C1 provides ac ground for the transformer secondary. The output voltage developed across coil L1. The feedback voltage appears across the base-emitter junction, as the junction point of resistors R1 and R2 is at ac ground due to bypass capacitor C1phase shift of 180 is provided by the transistor amplifier, Another phase shift of 180 is provided by the transformer. Thus a total phase shift of 360 appears between the input and output voltages i.e. there is a positive feedback between the input and output voltagePDM ECE*

PDM ECE

Working of Tuned Collector Oscillator

When Vcc is switched on, a transient current is caused in the tuned L-C circuit. It is due to increase of collector current to its quiescent value. This transient current initiates normal oscillations in the tuned circuit. These natural oscillations induce some voltage into L1 by mutual induction which causes corresponding variations in base current. These variations in base current are amplified times and appear in the collector circuit. A part of this amplified energy is used to meet the losses that occur in the tuned circuit and the rest is radiated out in the form of electro-magnetic waves. The turn-ratio of L and L1 is determined by the total losses. Higher is the turn-ratio, lesser is the feedback voltage applied and vice-versa. The frequency of oscillation is the resonant frequency of the tuned circuit.

where:L is the Inductance in HenriesC is the Capacitance in Faradsr is the Output Frequency in Hertz

PDM ECE*

PDM ECE

Colpitts Oscillator Capacitors C1, C2, and inductor L form a positive feedback network feedback signals from both ends of capacitor C2, Feedback signal in phase with the input voltage to meet the oscillation LC resonant circuit Q value is high oscillation frequency is approximately equal to

Among

high oscillation frequency more than 100MHz. when the oscillation frequency is high C1, C2's value is very smallPDM ECE*

PDM ECE

Clapp oscillatorThe Clapp oscillator is simply a Colpitts oscillator with an extra capacitor C3 in series with the coil. The function of is to reduce the effects of junction capacitance on operating frequency. C1 is in parallel with the Miller input capacitance.C2 is in parallel with the Miller output capacitance C3 is always much lower in value than either C1 or C2, so it becomes the dominant capacitor in any frequency calculation.C1 and C2 is to provide the phase shift needed for regenerative feedback. C3 has not replaced C1 and C2. It is simply there to determine the operating frequency. Since C1 and C2 are eliminated from the frequency calculation, junction capacitance has little or no effect on operating frequency.

PDM ECE*

PDM ECE

Hartley oscillatorIt is widely used as local oscillator in radio receivers. phase-shift network consists of two inductors L r and L2 and a capacitor C. The output of the amplifier is applied across inductor Lx and the voltage across inductor L2 forms the feedback voltage.Coil L1 is inductively coupled to coil L2, the combination functions as an auto-transformer. However, because of direct connection, the junction of L1 and L2 cannot be directly grounded. Instead, another capacitor CL is used.Considering the fact that there exists mutual inductance between coils L1 and L2 because the coils are wound on the same core, their net effective inductance is increased by mutual inductance M.

PDM ECE*

PDM ECE

PDM ECE* Total inductance LT = L1 + L2 + 2 M

Oscillation frequency is given by the expression

Hartley oscillator can also be correctly used for generating RF signals. The frequency can be easily varied by varying the inductances which can be done by making the core movable. Another method of varying frequency is of varying capacitance.

It is not suitable for low frequency work because at low frequency, the value of inductance required becomes large.

PDM ECE

Working of Tuned Collector Oscillator

When Vcc is switched on, a transient current is caused in the tuned L-C circuit. It is due to increase of collector current to its quiescent value. This transient current initiates normal oscillations in the tuned circuit. These natural oscillations induce some voltage into L1 by mutual induction which causes corresponding variations in base current. These variations in base current are amplified times and appear in the collector circuit. A part of this amplified energy is used to meet the losses that occur in the tuned circuit and the rest is radiated out in the form of electro-magnetic waves. The turn-ratio of L and L1 is determined by the total losses. Higher is the turn-ratio, lesser is the feedback voltage applied and vice-versa. The frequency of oscillation is the resonant frequency of the tuned circuit.

where:L is the Inductance in HenriesC is the Capacitance in Faradsr is the Output Frequency in Hertz

PDM ECE*

PDM ECE

Colpitts Oscillator Capacitors C1, C2, and inductor L form a positive feedback network feedback signals from both ends of capacitor C2, Feedback signal in phase with the input voltage to meet the oscillation LC resonant circuit Q value is high oscillation frequency is approximately equal to

Among

high oscillation frequency more than 100MHz. when the oscillation frequency is high C1, C2's value is very smallPDM ECE*

PDM ECE

Clapp oscillatorThe Clapp oscillator is simply a Colpitts oscillator with an extra capacitor C3 in series with the coil. The function of is to reduce the effects of junction capacitance on operating frequency. C1 is in parallel with the Miller input capacitance.C2 is in parallel with the Miller output capacitance C3 is always much lower in value than either C1 or C2, so it becomes the dominant capacitor in any frequency calculation.C1 and C2 is to provide the phase shift needed for regenerative feedback. C3 has not replaced C1 and C2. It is simply there to determine the operating frequency. Since C1 and C2 are eliminated from the frequency calculation, junction capacitance has little or no effect on operating frequency.

PDM ECE*

PDM ECE

Hartley oscillatorIt is widely used as local oscillator in radio receivers. phase-shift network consists of two inductors L r and L2 and a capacitor C. The output of the amplifier is applied across inductor Lx and the voltage across inductor L2 forms the feedback voltage.Coil L1 is inductively coupled to coil L2, the combination functions as an auto-transformer. However, because of direct connection, the junction of L1 and L2 cannot be directly grounded. Instead, another capacitor CL is used.Considering the fact that there exists mutual inductance between coils L1 and L2 because the coils are wound on the same core, their net effective inductance is increased by mutual inductance M.

PDM ECE*

PDM ECE

PDM ECE* Total inductance LT = L1 + L2 + 2 M

Oscillation frequency is given by the expression

Hartley oscillator can also be correctly used for generating RF signals. The frequency can be easily varied by varying the inductances which can be done by making the core movable. Another method of varying frequency is of varying capacitance.

It is not suitable for low frequency work because at low frequency, the value of inductance required becomes large.

PDM ECE

Crystal OscillatorPDM ECE*A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit.

PDM ECE

Crystal Oscillator

Crystal reactance versus frequency (neglecting the small resistance r ).A series resonance frequency at

A parallel resonance frequency at

PDM ECE*

PDM ECE

PDM ECE*+VCCRB2RB1RFCREC2C1CEvoutXtalReplaces thetank circuit

PDM ECE

Phase-shift oscillator voltage divider R1-R2 provides dc emitter base bias, RE and CE combination provides temperature stability and prevent ac signal degeneration and collector resistor RC controls the collector voltage. The oscillator output voltage is capacitively coupled to the load by Cc.The feedback signal is coupled through the feedback resistor R in series with the amplifier stage input resistance h^. The value of R should be such that when added with amplifier stage input resistance hie, it is equal to R i.e., R + hie = R.This variation in base current is amplified in collector circuit. The output of the amplifier is supplied to an R-C feedback network. The R-C network produces a phase shift of 180 between output and input voltages. CE amplifier produces a phase reversal of the input signal, total phase shift becomes 360 or 0 which is essential for regeneration or for sustained oscillationsPDM ECE*

PDM ECE

PDM ECE*If R1 = R2 = R3 = R and C1 = C2 = C3 = C, then

PDM ECE

Av

(Av < 0)

R1

R2

R3

C1

C2

C3

Feedback network

Wien bridge oscillatorIt is used in audio and sub-audio frequency ranges (20 20 kHz). This type of oscillator is simple in design, compact in size, and extremely stable in its frequency output. its output is relatively free from distortion and its frequency can be varied easily. the maximum frequency output of a typical Wien bridge oscillator is only about 1 MHz. It employs two transistors, each producing a phase shift of 180, and thus producing a total phase-shift of 360 or 0.

PDM ECE*

PDM ECE

Wien Bridge Oscillator

PDM ECE*

PDM ECE

Wien Bridge OscillatorPDM ECE*Advantages

Provides a stable low distortion sinusoidal output over a wide range of frequency.The frequency range can be selected simply by using decade resistance boxes.The frequency of oscillation can be easily varied by varying capacitances C1 and C2 simultaneously. The overall gain is high because of two transistors.

Disadvantages

The circuit needs two transistors and a large number of other components.The maximum frequency output is limited because of amplitude and the phase-shift characteristics of amplifier.

PDM ECE

PDM ECE*Operational Amplifier have a Very high differential gain (typically 200,000) Direct coupledNegative feedbackHigh input impedanceLow output impedanceProvide voltage changes (amplitude and polarity)Used in oscillator, filter and instrumentationAccumulate a very high gain by multiple stages

The Operational Amplifier

The Op Amp Symbol

PDM ECE

The Operational Amplifier

Op Amp Pin-out connections LM741 PDM ECE* 1. Gain--infinite 2. Input impedance--infinite 3. Output impedance--zero 4. Bandwidth--infinite 5. Voltage out--zero (when voltages into each other are equal) 6. Current entering the amp at either terminal--extremely small

PDM ECE

PDM ECE*Ideal Vs Practical Op-Amp

IdealPracticalOpen Loop gain A105Bandwidth BW10-100HzInput Impedance Zin>1MOutput Impedance Zout0 10-100 Output Voltage VoutDepends only on Vd = (V+V)Differential mode signalDepends slightly on average input Vc = (V++V)/2 Common-Mode signalCMRR10-100dB

PDM ECE

Inverting amplifier An inverting amplifier inverts and scales the input signal. The op-amp gain is very large, the amplifier gain is determined by two stable external resistors (the feedback resistor Rf and the input resistor Rin ) and not by op-amp parameters which are highly temperature dependent.

Hence, the amplifier output is related to the input as

So voltage gain of the amplifier is

where the negative sign is a convention indicating that the output is negated.

PDM ECE*

PDM ECE

The Non-inverting Op-AmpPDM ECE*The input voltage signal, (Vin) is applied directly to the non-inverting ( + ) input terminal which means that the output gain of the amplifier becomes "Positive" . The result of this is that the output signal is "in-phase" with the input signal.

PDM ECE

PDM ECE*The overall closed-loop gain of a non-inverting amplifier will always be greater but never less than one (unity), it is positive in nature and is determined by the ratio of the values of Rf and R2. If the value of the feedback resistor Rf is zero, the gain of the amplifier will be exactly equal to one (unity). If resistor R2 is zero the gain will approach infinity, but in practice it will be limited to the operational amplifiers open-loop differential gain, (Ao).

PDM ECE

Differential amplifier

PDM ECE* The circuit computes the difference of two voltages multiplied by some constant the output voltage is: The differential input impedance Z in is approximately R1 + R2 Under the condition that the R f /R1 = R g/R2, the output expression becomes:

where is the differential gain of the circuit. If Rf /R1 = Rg/R2, as before, and Rf = R1, the differential gain A = 1, and the circuit is a differential follower with:

PDM ECE

Voltage follower Op-amp as a buffer amplifier to eliminate loading effects (connecting a device with a high source impedance to a device with a low input impedance). V out = V in Z in =

Reasonably, the differential input impedance of the op-amp itself, 1 M to 1 T)PDM ECE*

PDM ECE

Summing amplifier A summing amplifier sums several (weighted) voltages:

When , and Rf independent

When

Output is invertedInput impedance of the nth input is (V_ is a virtual ground)PDM ECE*

PDM ECE

Integrating amplifier

The insertion of a capacitor in the feedback path of an op-amp results in an output signal that is a time integral of the input signal. A circuit arrangement for a simple inverting integrator is given in

PDM ECE*(where Vin and V out are functions of time, V initial is the output voltage of the integrator at time t = 0.)

PDM ECE

Common Mode Rejection Ratio (CMRR)

PDM ECE*It is the ability of an op amp to reject the signal which is present at its both inputs simultaneously i.e. the common mode signalCMRR = AOL / ACM, where ACM is common mode voltage gain defined by Vout / VCMIdeally CMRR is infiniteFor IC 741 it is 90 dB

PDM ECE

Op-Amp differentiatorPDM ECE*

PDM ECE

ECE 101-F : Basic ElectronicsSection- CPDM ECE*Section- CDigital Electronics : Number system and conversions.Boolean Algebra. Truth tables of logic gates & NAND, NOR as universal gates.Difference between combinational circuits and sequential circuits. Introduction to flip-flops (S-R & J-K).Electronics Instruments : Millimeter Digital & AnalogCathode Ray Oscilloscope (CRO).Function/Signal Generator.

PDM ECE

PDM ECE*How to add binary numbersHow to add binary numbersConsider adding two 1-bit binary numbers x and y0+ 0 = 00+ 1 = 11+ 0 = 11+ 1 = 10

Carry is x AND ySum is x X OR yThe circuit to compute this is called a half-adder

xyCarrySum0000010110011110

PDM ECE

PDM ECE*Boolean functions are implemented in digital computer circuits called gates.A gate is an electronic device that produces a result based on two or more input values.In reality, gates consist of one to six transistors, but digital designers think of them as a single unit.Integrated circuits contain collections of gates suited to a particular purpose.Vs is ground = 0 VoltsVd high voltage for all the things were doing, this is +5V, but there are many possibilities.Vg gate voltage depending on this value, the electrons can or can not flow from high to low voltage.Logic Gates

PDM ECE

OR Operation

PDM ECE*Boolean expression for the OR operation: x =A + BThe above expression is read as x equals A OR B

PDM ECE

AND Operation

PDM ECE*Boolean expression for the AND operation: x =A BThe above expression is read as x equals A AND B

PDM ECE

NOT Operation

PDM ECE*The NOT operation is an unary operation, taking only one input variable.Boolean expression for the NOT operation: x = AThe above expression is read as x equals the inverse of AAlso known as inversion or complementation.Can also be expressed as: A

PDM ECE

NOR Gate

PDM ECE*Boolean expression for the NOR operation: x = A + B

PDM ECE

The Universal Property of NAND and NOR Gates NAND and NOR gates are universal because they can used to produce any of the other logic functions.

PDM ECE*

PDM ECE

The Universal Property of NAND Gates

PDM ECE*NAND Gate as an Inverter

PDM ECE

Two NAND Gates as an AND Gate*PDM ECE

PDM ECE

Three NAND Gates as an OR Gate*PDM ECE

PDM ECE

Four NAND Gates as NOR Gate*PDM ECE

PDM ECE

The Universal Property of NOR Gates

PDM ECE*NOR Gate as an Inverter

PDM ECE

Two NOR Gates as an OR Gate*PDM ECE

PDM ECE

Three NOR Gates as an AND Gate*PDM ECE

PDM ECE

Four NOR Gates as an NAND Gate*PDM ECE

PDM ECE

Circuits

PDM ECE**PDM ECE*Two general categories In a combinational circuit, the input values explicitly determine the outputIn a sequential circuit, the output is a function of the input values as well as the existing state of the circuitAs with gates, we can describe the operations of entire circuits using three notationsBoolean expressionslogic diagramstruth tables

PDM ECE

Sequential Logic Definition of sequential logic. Sequential logic can have one or more, inputs and one or more outputs. However, the outputs are a function of both the present value of the inputs and also the previous output values. Thus, sequential logic requires memory to store these previous outputs values.

PDM ECE*

PDM ECE

FLIP-FLOPS

PDM ECE**PDM ECE*Memory device capable of storing one bitMemory means circuit remains in one state after condition that caused the state is removed.Two outputs designated Q and Q-Not that are always opposite or complimentary.When referring to the state of a flip flop, referring to the state of the Q output.

PDM ECE

CLOCKED R-S FLIP-FLOP

PDM ECE*Symbols:Truth Table: Mode of operation Inputs Outputs Clk S R Q Q Hold + pulse 0 0 no change Reset + pulse 0 1 0 1 Set + pulse 1 0 1 0 Prohibited 1 1 0 0

NOTE: Active-High inputs

PDM ECE

D Flip-flop

PDM ECE*D flip-flop: single input D (data)D=HIGH a SET stateD=LOW a RESET stateQ follows D at the clock edge.Convert S-R flip-flop into a D flip-flop: add an inverter.A positive edge-triggered D flip-flop formed with an S-R flip-flop. = clock transition LOW to HIGH

PDM ECE

Definition for the PR (preset) and CLR (clear) Asynchronous input for a D flip-flop.

PDM ECE**Asynchronous inputs (Preset & Clear) are used to override the clock/data inputs and force the outputs to a predefined state.The Preset (PR) input forces the output to:

The Clear (CLR) input forces the output to:

PRPRESETCLRCLEARCLKCLOCKDDATA110011111001XX10Asynchronous Preset10XX01Asynchronous Clear00XX11ILLEGAL CONDITION

PDM ECE

J/K Flip-Flop: Excitation Table

PDM ECE**Schematic symbol and excitation table for the J/K flip-flop.

JKCLK00No Change010Clear101Set11Toggle

: Rising Edge of Clock

PDM ECE

Edge-Triggered Flip-flops

PDM ECE*Flip-flops: synchronous bistable devicesOutput changes state at a specified point on a triggering input called the clock.Change state either at the positive edge (rising edge) or at. the negative edge (falling edge) of the clock signal

PDM ECE

PDM ECE*S-R, D and J-K edge-triggered flip-flops. Note the > symbol at the clock input.

Positive edge-triggered flip-flopsNegative edge-triggered flip-flops

PDM ECE

POS & NEG Edge Triggered J/KPDM ECE**Positive Edge TriggerNegative Edge Trigger

JKCLK0001010111 : Rising Edge of Clock

JKCLK0001010111 : Rising Edge of Clock

PDM ECE

Flip-Flop Vs. LatchPDM ECE*The primary difference between a D flip-flop and D latch is the EN/CLOCK input. The flip-flops CLOCK input is edge sensitive, meaning the flip-flops output changes on the edge (rising or falling) of the CLOCK input. The latchs EN input is level sensitive, meaning the latchs output changes on the level (high or low) of the EN input.

PDM ECE

PDM ECE*

PDM ECE

Electronic InstrumentsPDM ECE*

PDM ECE

INTRODUCTION The cathode-ray oscilloscope (CRO) is a multipurpose display instrument used for the observation, measurement , and analysis of waveforms by plotting amplitude along y-axis and time along x-axis. CRO is generally an x-y plotter; on a single screen it can display different signals applied to different channels. It can measure amplitude, frequencies and phase shift of various signals. Many physical quantities like temperature, pressureand strain can be converted into electrical signals by the use of transducers, and the signals can be displayed on the CRO. A moving luminous spot over the screen displays the signal. CROs are used to study waveforms, and other time-varying phenomena from very low to very high frequencies. The central unit of the oscilloscope is the cathode-ray tube (CRT), and the remaining part of the CRO consists of the circuitry required to operate the cathode-ray tube.PDM ECE*

PDM ECE

COMPONENTS OF THE CATHODE-RAY OSCILLOSCOPEThe CRO consists of the following:(i) CRT(ii) Vertical amplifier(iii) Delay line(iv) Horizontal amplifier(v) Time-base generator(vi) Triggering circuit(vii) Power supply

PDM ECE*

PDM ECE

Electron GunPDM ECE*

PDM ECE

CATHODE-RAY TUBE:PDM ECE*

PDM ECE

Electron Gun:In the electron gun of the CRT, electrons are emitted, converted into a sharp beam and focused upon the fluorescent screen. The electron beam consists of an indirectly heated cathode, a control grid, an accelerating electrode and a focusing anode. The electrodes are connected to the base pins. The cathode emitting the electrons is surrounded by a control grid with a fine hole at its centre. The accelerated electron beam passes through the fine hole. The negative voltage at the control grid controls the flow of electrons in the electron beam, and consequently, the brightness of the spot on the CRO screen is controlled.

PDM ECE*

PDM ECE

Deflection Systems:Electrostatic deflection of an electron beam is used in a general purpose oscilloscope. The deflecting system consists of a pair of horizontal and vertical deflecting plates. Let us consider two parallel vertical deflecting plates P1 and P2. The beam is focused at point O on the screen in the absence of a deflecting plate voltage.

If a positive voltage is applied to plate P1 with respect to plate P2, the negatively charged electrons are attracted towards the positive plate P1, and these electrons will come to focus at point Y1 on the fluorescent screen.

*PDM ECE

PDM ECE

Fluorescent Screen: Phosphor is used as screen material on the inner surface of a CRT. Phosphor absorbs the energy of the incident electrons. The spot of light is produced on the screen where the electron beam hits. The bombarding electrons striking the screen, release secondary emission electrons. These electrons are collected or trapped by an aqueous solution of graphite called Aquadag which is connected to the second anode. Collection of the secondary electrons is necessary to keep the screen in a state of electrical equilibrium. The type of phosphor used, determines the color of the light spot. The brightest available phosphor isotope, P31, produces yellowgreen light with relative luminance of 99.99%.PDM ECE*

PDM ECE

Display waveform on the screenPDM ECE* Figure shows a sine wave applied to vertical deflecting plates and a repetitive ramp or saw-tooth applied to the horizontal plates. The ramp waveform at the horizontal plates causes the electron beam to be deflected horizontally across the screen .If the waveforms are perfectly synchronized then the exact sine wave applied to the vertical display appears on the CRO display screen.

PDM ECE

Triangular waveform:PDM ECE*Similarly the display of the triangular waveform is as shown in Fig. .

PDM ECE

TIME-BASE GENERATORS: The CRO is used to display a waveform that varies as a function of time. If the wave form is to be accurately reproduced, the beam should have a constant horizontal velocity. As the beam velocity is a function of the deflecting voltage, the deflecting voltage must increase linearly with time. A voltage with such characteristics is called a ramp voltage. If the voltage decreases rapidly to zerowith the waveform repeatedly produced, as shown in Fig. we observe a pattern which is generally called a saw-tooth waveform. The time taken to return to its initial value is known as flyback or return time.PDM ECE*

PDM ECE

Vertical Amplifiers: Vertical amplifiers determines the sensitivity and bandwidth of an oscilloscope. Sensitivity, which is expressed in terms of V/cm of vertical deflection at the mid-band frequency. The gain of the vertical amplifier determines the smallest signal that the oscilloscope can satisfactorily measure by reproducing it on the CRT screen. The sensitivity of an oscilloscope is directly proportional to the gain of the vertical amplifier. So, as the gain increases the sensitivity also increases.The vertical sensitivity measures how much the electron beam will be deflected for a specified input signal. The CRT screen is covered with a plastic grid pattern called a graticule. The spacing between the grids lines is typically 10 mm. Vertical sensitivity is generally expressed in volts per division.The vertical sensitivity of an oscilloscope measures the smallest deflection factor that can be selected with the rotary switch.PDM ECE*

PDM ECE

TYPES OF THE CATHODE-RAY OSCILLOSCOPES:

1. Analog CRO: In an analog CRO, the amplitude, phase and frequency are measured from the displayed waveform, through direct manual reading.2. Digital CRO: A digital CRO offers digital read-out of signal information, i.e., the time, voltage or frequency along with signal display. It consists of an electronic counter along with the main body of the CRO.3. Storage CRO: A storage CRO retains the display up to a substantial amount of time after the first trace has appeared on the screen. The storage CRO is also useful for the display of waveforms of low-frequency signals. 4. Dual-Beam CRO: In the dual-beam CRO two electron beams fall on a single CRT. The dual-gun CRT generates two different beams. These two beams produce two spots of light on the CRT screen which make the simultaneous observation of two different signal waveforms possible. The comparison of input and its corresponding output becomes easier using the dual-beam CRO.PDM ECE*

PDM ECE

SWEEP FREQUENCY GENERATOR: A sweep frequency generator is a signal generator which can automatically vary its frequency smoothly and continuously over an entire frequency range. Figure 14-15 shows the basic block diagram of a sweep frequency generator. The sweep frequency generator has the ramp generator and the voltage-tuned oscillator as its basic components.PDM ECE*

PDM ECE

SWEEP FREQUENCY GENERATOR A sweep frequency generator is a signal generator which can automatically vary its frequency smoothly and continuously over an entire frequency range. Figure 14-15 shows the basic block diagram of a sweep frequency generator. The sweep frequency generator has the ramp generator and the voltage-tuned oscillator as its basic components.PDM ECE*

PDM ECE

Applications of the Sweep Frequency Generator:PDM ECE*

PDM ECE

FUNCTION GENERATOR:The basic components of a function generator are:(i) Integrator(ii) Schmitt trigger circuit(iii) Sine wave converter(iv) AttenuatorPDM ECE*

PDM ECE

Block Diagram of Function GeneratorPDM ECE*

PDM ECE

SINE WAVE GENERATOR:PDM ECE* A sine wave is produced by converting a triangular wave, applying proper circuits. The triangular wave is produced by employing an integrator and a Schmitt trigger circuit. This triangular wave is then converted to a sine wave using the diode loading circuit ,as shown in Fig. 14-19. Resistors R1 and R2 behave as the voltage divider. When VR2 exceeds V1, the diode D1 becomes forward-biased. There is more attenuation of the output voltage levels above V1 than levels below V1. With the presence of the diode D1 and resistor R3 in the circuit, the output voltage rises less steeply. The output voltage falls below V1 and the diode stops conducting, as it is in reverse-bias. The circuit behaves as a simple voltage-divider circuit. This is also true for the negative half-cycle of the input Vi . If R3 is carefully chosen to be the same as R4 , the negative and the positive cycles of the output voltage will be the same. The output is an approximate sine wave.

PDM ECE

SINE WAVE GENERATOR:PDM ECE* The circuit is adjusted by comparing a 1 kHz sine wave and the output of the triangular/sine wave converter on a dual-track CRO. R1, R2, R3 and the peak amplitude of Ei are adjusted in sequence for the best sinusoidal shape.

PDM ECE

CIRCUIT DIAGRAM OF SINE WAVE GENERATOR:PDM ECE*

PDM ECE

Two Types of Multimeters

PDM ECE*DMM(digitalVOM(analog

PDM ECE

Types of Meters

Analog meter:Uses a moving pointer and a printed scale to indicate values of voltage, current, or resistance.Volt-Ohm-Milliammeter (VOM):Allows all three kinds of measurements on a single scale or readout.Digital multimeter:Uses a numerical readout to indicate the measured value of voltage, current or resistance.

PDM ECE*

PDM ECE

Moving-Coil Meter Direct Current MetersDirect current in a moving-coil meter deflects the pointer in proportion to the amount of current.A current meter must be connected in series with the part of the circuit where the current is to be measured.

A dc current meter must be connected with the correct polarity.

PDM ECE*

PDM ECE

Analog instruments use a moving coil meter movementPDM ECE*

PDM ECE

VoltmetersA voltmeter is connected across two points to measure their difference in potential.A voltmeter uses a high-resistance multiplier in series with the meter movement.A dc voltmeter must be connected with the correct polarity.

A multiplier resistor is a large resistance in series with a moving-coil meter movement which allows the meter to measure voltages in a circuit.

PDM ECE*

PDM ECE

A multiplier resistor is a large resistance in series with a moving-coil meter movement which allows the meter to measure voltages in a circuit.

PDM ECE*

PDM ECE

Ohms-per-Volt RatingAnalog voltmeters are rated in terms of the ohms of resistance required for 1 V of deflection.This value is called the ohms-per-volt rating, or the sensitivity of the voltmeter.The ohms-per-volt rating is the same for all ranges. It is determined by the full-scale current IM of the meter movement.The voltmeter resistance RV can be calculated by multiplying the ohms-per-volt rating and the full-scale voltage of each range.

PDM ECE*

PDM ECE

OhmmetersAn ohmmeter consists of an internal battery in series with the meter movement, and a current limiting resistance.Power in the circuit being tested is shut off. Current from the internal battery flows through the resistance being measured, producing a deflection that is:Proportional to the current flow, and Displayed on a back-off scale, with ohm values increasing to the left as the current backs off from full-scale deflection.

PDM ECE*

PDM ECE

PDM ECE*How meter movement M can be used as an ohmmeter with a 1.5-V battery. (a) Equivalent closed circuit with R1 and the battery when ohmmeter leads are short-circuited for zero ohms of external R. (b) Internal ohmmeter circuit with test leads open, ready to measure an external resistance

PDM ECE

MultimetersMultimeters are also called multitesters.Multimeters are used to measure voltage, current, or resistance.Main types of multimeters are:Volt-ohm-milliammeter (VOM)Digital multimeter (DMM)

PDM ECE*

PDM ECE

MultimetersPDM ECE*

PDM ECE

Digital Multimeters (DMMs)The digital multimeter has become a very popular test instrument.The digital value of the measurement is displayed automatically with decimal point, polarity, and the unit for V, A, or .

PDM ECE*

PDM ECE

Digital multimeters Digital multimeters are generally easier to use.

They eliminate the human error that often occurs in reading different scales on an analog meter with a pointer.

PDM ECE*

PDM ECE

SWEEP FREQUENCY GENERATOR A sweep frequency generator is a signal generator which can automatically vary its frequency smoothly and continuously over an entire frequency range. Figure 14-15 shows the basic block diagram of a sweep frequency generator. The sweep frequency generator has the ramp generator and the voltage-tuned oscillator as its basic components.PDM ECE*

PDM ECE

SEVEN SEGMENT DISPLAY

Often used to display BCD numbers (1 through 9) and a few alphabetsA group of eight LEDs physically mounted in the shape of the number eight plus a decimal point as shown (a) Each LED is called a segment and labeled as a through g.PDM ECE*

PDM ECE

TYPES:Two types of seven-segment LEDs Common anode Common cathode

decimal pointPDM ECE*

PDM ECE

COMMON ANODEIn a common anode seven-segment LEDAll anodes are connected together to a power supply and cathodes are connected to data linesLogic 0 turns on a segment.Example: To display digit 1, all segments except b and c should be off.Byte 11111001 = F9H will display digit 1.PDM ECE*

PDM ECE

COMMON CATHODEIn a common cathode seven-segment LEDAll cathodes are connected together to ground and the anodes are connected to data linesLogic 1 turns on a segment.Example To display digit 1, all segments except b and c should be off.Byte 00000110 = 06H will display digit 1.PDM ECE*

PDM ECE

LED 7-SEGMENT DISPLAY DRIVERPDM ECE*

PDM ECE

Seven-Segment ChipsALPHA/NUMERIC C/A DISPLAY PDM ECE*

PDM ECE

FOURTEEN SEGMENT DISPLAYAfourteen-segment display(sometimes referred to as astarburst displayor a "Union Jack" display) is a type of display based on 14segments that can be turned on or off to produce letters and numerals. It is an expansion of the more commonseven-segment display, having an additional four diagonal and two vertical segments with the middle horizontal segment broken in half. A seven-segment display suffices for numerals and certain letters, but unambiguously rendering theISO basic Latin alphabetrequires more detail.A slight variation is thesixteen-segment displaywhich allows additional legibility in displaying letters or other symbols.PDM ECE*

PDM ECE

PDM ECE*

PDM ECE

Fourteen-segmentgas-plasma displayswere used inpinball machinesfrom 1986 through 1991 with an additionalcommaandperiodpart making for a total of 16 segments.Fourteen and sixteen-segment displays were used to producealphanumericcharacters oncalculators and otherembedded systems. Applications today include displays fitted to telephoneCaller ID units, gymnasium equipment,VCRs,car stereos,microwave ovens,slot machines, and DVD players.Such displays were very common on pinball machines for displaying the score and other information, before the widespread use of dot-matrix display panels.

PDM ECE*

PDM ECE

LIGHT EMITTING DIODES *PDM ECE

PDM ECE

*A light emitting diode (LED) is essentially a PN junction opto-semiconductor that emits a monochromatic (single color) light when operated in a forward biased direction. LEDs convert electrical energy into light energy. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not. PDM ECE

PDM ECE

About LEDs *The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb as shown at the right. The chip has two regions separated by a junction. The p region is dominated by positive electric charges, and the n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and the n regions. Only when sufficient voltage is applied to the semi-conductor chip, can the current flow, and the electrons cross the junction into the p region. PDM ECE

PDM ECE

*PDM ECE

PDM ECE

How Does A LED Work? *When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions.

In the p region there are many more positive than negative charges.

When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region.PDM ECE

PDM ECE

How Does A LED Work? *Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy.

For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material (usually a combination of the chemical elements gallium, arsenicand phosphorus).. PDM ECE

PDM ECE

How Much Energy Does an LED Emit?*The energy (E) of the light emitted by an LED is related to the electric charge (q) of an electron and the voltage (V) required to light the LED by the expression: E = qV Joules.

This expression simply says that the voltage is proportional to the electric energy, and is a general statement which applies to any circuit, as well as to LED's. The constant q is the electric charge of a single electron, -1.6 x 10-19 Coulomb. PDM ECE

PDM ECE

Finding the Energy from the Voltage*Suppose you measured the voltage across the leads of an LED, and you wished to find the corresponding energy required to light the LED. Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts. So the Energy required to light the LED is

E = qV or E = -1.6 x 10-19 (1.71) Joule,

since a Coulomb-Volt is a Joule. Multiplication of these numbers then gives

E = 2.74 x 10-19 Joule.

PDM ECE

PDM ECE

Applications* Sensor Applications Mobile Applications Sign Applications Automative Uses LED Signals Illuminations IndicatorsPDM ECE

PDM ECE

Sensor ApplicationsMedical Instrumentation Bar Code Readers Color & Money Sensors Encoders Optical Switches Fiber Optic Communication *PDM ECE

PDM ECE

Mobile ApplicationsMobile Phone PDA's Digital Cameras Lap Tops General Backlighting *PDM ECE

PDM ECE

Sign ApplicationsFull Color Video Monochrome Message Boards Traffic/VMS Transportation - Passenger Information *PDM ECE

PDM ECE

Automative ApplicationsInterior Lighting - Instrument Panels & Switches, Courtesy Lighting Exterior Lighting - CHMSL, Rear Stop/Turn/Tail Truck/Bus Lighting - Retrofits, New Turn/Tail/Marker Lights

*PDM ECE

PDM ECE

Signal AppicationsTraffic Rail Aviation Tower Lights Runway Lights Emergency/Police Vehicle Lighting

LEDs offer enormous benefits over traditional incandescent lampsincluding:

Energy savings (up to 85% less power than incandescent) Reduction in maintenance costs Increased visibility in daylight and adverse weather conditions

*PDM ECE

PDM ECE

LIQUID CRYSTAL CELLS

Liquid crystal material is a liquid that exhibits some of the properties of solid .the molecules in ordinary liquids normally have random orientations. In liquid crystals the molecules are oriented in a definite crystal pattern.

*PDM ECE

PDM ECE

Used for numeric and alpha numeric character in dot matrix and segmental displays. popular liquid crystal structure is Nematic Crystal (NLC).The liquid is normally transparent but is subjected to a strong electric field disruption of the well ordered crystal structure takes place causing the liquid to polarize and turn opaque. the removal of the applied electric field allows the crystal structure to regain its original form and the material becomes transparent.*PDM ECE

PDM ECE

*PDM ECE

PDM ECE

TYPESDynamic Scattering typeField effect type

*PDM ECE

PDM ECE

Dynamic Scattering TypesThe display consists of two glass plates each coated with tin oxide (SnO2) on the inside with transparent electrodes separated by a liquid crystal layer 5 to 50 micrometer thick. The oxide coating on the front sheet is etched to produce a single or multi segment pattern of characters with each segment properly insulated from each other . a weak electric field applied to a liquid crystal tends to align molecules in the direction of the field.*PDM ECE

PDM ECE

Field Effect TypesTwo thin polarising optical filters are placed at the inside of each glass sheet. The LCD material is of twisted nematic type which twists the light passing through the cell when the latter is not energized. This allows light to pass through the optical filters and the cell appears bright. When the cell is energized no twisiting of light takes place and the cell appears dull.

*PDM ECE

PDM ECE

Liquid crystal cells are two typesTransmittive type cell, both glass sheets are transparent so that light from a rear source is scattered in the forward direction when the cell is activated.The Reflective type cell has a reflecting surface on one side of the glass sheet .The incident light on the front surface of the cell is dynamically scattered by an activated cell.

PDM ECE*

PDM ECE

AdvantageThe voltage required are smallThey have a low power consumption They are economical.

PDM ECE*

PDM ECE

DisadvantageLCD are very slow devices. The turn On and Off times are quite large .yhe turn on time is typically of the order few ms while the turn off is 10ms.When used on ac their life span is quite small.Occupy large area.

PDM ECE*

PDM ECE

Thank YouPDM ECE*

PDM ECE

*******

Documents

Basic of Electronics Ppt Pdm 19nov