EC2251- Electronic Circuits II

8/6/2019 EC2251- Electronic Circuits II

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Electronic circuitsElectronic circuits--IIII

EC2251EC2251


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UnitUnit--11Feedback amplifiersFeedback amplifiers


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FeedbackFeedback

Consists of returning part of the output of a system to the inputConsists of returning part of the output of a system to the input

Negative Feedback:Negative Feedback: a portion of the output signal is returned to the inputa portion of the output signal is returned to the inputin opposition to the original input signalin opposition to the original input signal

Positive Feedback:Positive Feedback: the feedback signal aids the original input signalthe feedback signal aids the original input signal

Negative Feedback Effects:Negative Feedback Effects:

Reduces gainReduces gain

Stabilizes gainStabilizes gain

Reduces non linear distortionReduces non linear distortion

Reduces certain types of noiseReduces certain types of noise

Controls input and output impedancesControls input and output impedances

Extends bandwidthExtends bandwidth

The disadvantage of reducing the gain can be overcome by adding fewThe disadvantage of reducing the gain can be overcome by adding fewmore stages of amplificationmore stages of amplification


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Feedback amplifier. Note that the signals are denoted as xi, xf, xo, and so on.

The signals can be either currents or voltages

FA

AA f

!

1Negative feedback(AfA)

Af--- closedloop gain

A---Open loop

gain

A----loop gain

F

Positive feedback provides an easy way to obtain large gain.

It leads to poor gain stability , a slight shift in power supply

Or temp can change the magnitude of loop gain to unity &

cause the Amplifier to break into oscillation .


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Types of Feedback

There are 4 basic types of feedback that have different effects:

Voltage series

Current series

Voltage shunt Current shunt

The units ofF are the inverse of the units of the amplifier gain

For series-voltage feedback A=Av andF is unit less

For series-current feedback A=Gm and

F is in;For voltage shunt feedback A=Rm and

F is in Siemens

For current shunt feedback A=Ai and

F is unit less


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The four basic feedback topologies: (a) voltage-sampling series-mixing (series-shunt) topology; (b)

current-sampling shunt-mixing (shunt-series) topology; (c) current-sampling series-mixing (series-

series) topology; (d) voltage-sampling shunt-mixing (shunt-shunt) topology.


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Effects of various types of feedback on gainEffects of various types of feedback on gain

FA

A

x

xA

s

f

!!1

0

FAv

Av

v

vA

s

vf

!!1

0

FGm

Gm

x

xG

s

mf

!!1

0

FRm

Rm

x

xR

s

mf

!!1

0

FAi

Ai

x

xA

sif

!!1

0

Gain Stabilization

If we design the amplifier so that AF >> 1, then theclosed loop gain Afis approximately 1/F

Under this condition Afdepends only on the stablepassive components (resistor or capacitors) used inthe feedback network, instead of depending on theopen loop gain A which in turn depends on activedevice parameters (gm) which tend to be highlyvariable with operating point and temperature


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Summary (Effects on feedback)


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Analysis of feedback amplifiersAnalysis of feedback amplifiers

StepsSteps

1.1.Identify the type of feedbackIdentify the type of feedback

2.Redraw the amplifier circuit without the effect of feedback .2.Redraw the amplifier circuit without the effect of feedback .

3.Use a thevenins source at the input for series mixing and use a3.Use a thevenins source at the input for series mixing and use a

Nortons source at the input for shunt mixing.Nortons source at the input for shunt mixing.4.After drawing the amplifier circuit without feedback determine the ac4.After drawing the amplifier circuit without feedback determine the ac

parameters of the circuit using the h parameter model.parameters of the circuit using the h parameter model.

5.Determine the feedback ratio5.Determine the feedback ratioFF= x= xff/ x/ xoo from the original circuitfrom the original circuit..

6. Find he desensivity factor(D).6. Find he desensivity factor(D).

7.Knowing A,D,Ri,and Ro , Find Af, Rif , Rof.7.Knowing A,D,Ri,and Ro , Find Af, Rif , Rof.


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Nyquist criterionNyquist criterion

Criterion OfNyquist:

The amplifier is unstableifthis curve encloses thepoint 1+j0 and theamplifier is stable ifthe

curve does not enclosethis point

Re (AF)

Im(AF)

-1+j0

AF

f2 f1

fn

1+ AF

Gain andphase margins

These are a measureofthe stabilityofa

circuit


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UnitUnit--22OscillatorOscillator


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Oscillators are circuits that produce a continuous signal ofsome type withoutthe need o aninput.

These signals serve a purpose for a varietyofpurposes.Communications systems, digital systems (includingcomputers), and test equipment make use ofoscillators.

Anoscillator is a circuitthat produces a repetitive signalfrom a dc voltage.

The feedback type oscillator which relyon a positive

feedback ofthe outputto maintainthe oscillations. The relaxation oscillator makes use ofanRC timing

circuittogenerate a non-sinusoidal signal such as squarewave.


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The requirements for oscillation are described byThe requirements for oscillation are described bythe Baukhausenthe Baukhausencriterion:criterion:

1.1.The magnitude ofthe loop gainThe magnitude ofthe loop gainAAFF must be 1must be 1

2.2.The phase shiftofthe loop gainThe phase shiftofthe loop gainAA FF must be 0must be 0rr or 360or 360rr ororinteger multiple of2piinteger multiple of2pi


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Amplitude stabilisation

in both the oscillators above, the loop gain is set by componentvalues

in practice the gain of the active components is very variable

if the gain of the circuit is too high it will saturate

if the gain of the circuit is too low the oscillation will die

real circuits need some means of stabilising the magnitude of theoscillation to cope with variability in the gain of the circuit

Mechanism ofstartofoscillation:

The starting voltage is provided by noise, which is produced due torandom motion of electrons in resistors used in the circuit.

The noise voltage contains almost all the sinusoidal frequencies.This low amplitude noise voltage gets amplified and appears at theoutput terminals.

The amplified noise drives the feedback network which is the phaseshift network. Because of this the feedback voltage is maximum at aparticular frequency, which in turn represents the frequency ofoscillation.


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RCRCor phaseor phase--shiftoscillatorshiftoscillator

one wayofproducing a phase shiftof180one wayofproducing a phase shiftof180rr is touse anis touse anRCRCladder networkladder network

this gives a phase shiftof180this gives a phase shiftof180rr

atthis frequencythe gainofthe network isatthis frequencythe gainofthe network is

62

1

CRf

T!

29

1!

i

o

v

v


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Wien

Wien--bridge oscillatorbridge oscillator uses a Wienuses a Wien--bridge networkbridge network this produces a phasethis produces a phase--shiftshift

of0of0rr at a single frequency,at a single frequency,and is used with aninvertingand is used with aninvertingamplifieramplifier

the selected frequencyisthe selected frequencyisfr= 1/2TRC

whenthe gainis 1/3whenthe gainis 1/3


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The twin-T utilizes a bandstop arrangement of RC circuits

to block all but the frequency of operation in the negativefeedback loop. The only frequency allowed to effectivelyoscillate is the frequency of resonance.

TwinT Oscillator


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The Colpitts oscillator utilizes a tank circuit (LC) in thefeedback loop. Since the input impedance affects the Q, an

FET is a better choice for the active device.

fr= 1/2TLCT The Clapp is similar to theColpitts with exception tothe additional capacitor in

the tank circuit.


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The Hartley oscillator is similar to the Clapp and Colpitts. The

tank circuit has two inductors and one capacitor. Frequency of oscillation

Cllf

)21(2

1

4

!


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The Armstrong oscillator uses transformer coupling in the

feedback loop. For this reason the Armstrong is not aspopular.


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Crystal oscillatorsCrystal oscillators

AnAnoscillator in whichthein whichthe frequency is controlled by ais controlled by apiezoelectric crystal. A crystal oscillator may require controlledpiezoelectric crystal. A crystal oscillator may require controlledtemperature because its operatingfrequencyis a functionoftemperature because its operatingfrequencyis a functionoftemperaturetemperature

frequency stabilityis determined bythe abilityofthe circuittofrequency stabilityis determined bythe abilityofthe circuittoselect a particular frequencyselect a particular frequency

intuned circuits this is described bythe qualityfactor, Qintuned circuits this is described bythe qualityfactor, Q

piezoelectric crystals act like resonant circuits with a veryhighpiezoelectric crystals act like resonant circuits with a veryhighQQ as high as 100,000as high as 100,000


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UnitUnit--33

Tuned AmplifiersTuned Amplifiers


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Frequency selectivityof resonant circuits allows a radioto beFrequency selectivityof resonant circuits allows a radioto be

tunedtuned toone ofa setoftransmitting stations.toone ofa setoftransmitting stations.

Tuningis usuallyundertaken by varyingthe capacitance ofanTuningis usuallyundertaken by varyingthe capacitance ofanadjustable capacitor.adjustable capacitor.

Resonant circuits are alsoimportantfor tuning and forResonant circuits are alsoimportantfor tuning and fortransmitting signals.transmitting signals.

Tuned amplifiersTuned amplifiers To amplifythe selective range offrequencies , the resistive load ,To amplifythe selective range offrequencies , the resistive load ,

Rc is replaced by a tuned circuit.Rc is replaced by a tuned circuit.

The tuned circuitis capable ofamplifying a signal over a narrowThe tuned circuitis capable ofamplifying a signal over a narrowband offrequencies centered atfr.band offrequencies centered atfr.

Tuned Circuits

CR

LZ

LCf

r

r

!

4! 2

1


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Types Oftuned amplifiersTypes Oftuned amplifiers

Single tuned amplifierSingle tuned amplifier one parallel tuned circuitis used as a loadone parallel tuned circuitis used as a load

Limitation:Limitation: Smaller Bandwidth , smaller gain bandwidth product,Smaller Bandwidth , smaller gain bandwidth product,does not provide flatten response.does not provide flatten response.

Double tuned amplifierDouble tuned amplifier

It provides highgain, high selectivity and required bandwidth.It provides highgain, high selectivity and required bandwidth. Used in IF in radio and TV receivers.Used in IF in radio and TV receivers.

Itgives greater 3db bandwidthhaving steep sides and flattop .Itgives greater 3db bandwidthhaving steep sides and flattop .But alignmentofdouble tuned amplifier is difficultBut alignmentofdouble tuned amplifier is difficult

Stagger tuned amplifierStagger tuned amplifier

Two single tuned amplifier are connected in cascaded form.Two single tuned amplifier are connected in cascaded form.

Resonantfrequency are displaced.Resonantfrequency are displaced.

Tohave better flat , wideband charcteristics with a very sharpTohave better flat , wideband charcteristics with a very sharprejective, narrow band characteristics.rejective, narrow band characteristics.


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Synchronouslytuned amplifierSynchronouslytuned amplifier Toobtain a highover all gain , several identical stages oftunedToobtain a highover all gain , several identical stages oftuned

amplifiers can be used in cascade.amplifiers can be used in cascade.

All amplifiers stages are assumed to be identical and to be tunedAll amplifiers stages are assumed to be identical and to be tunedtothe same frequency w0.tothe same frequency w0.

Q factor:Q factor:

Ratioof reactance to resistance.Ratioof reactance to resistance.

Measure ofefficiency with whichinductor can store the energy.Measure ofefficiency with whichinductor can store the energy.

Unloaded Q oftuned circuitUnloaded Q oftuned circuit

Ratioofstored energyto dissipated energyin a reactor orRatioofstored energyto dissipated energyin a reactor orresonator.resonator.

Loaded QLoaded Q Itis determined byhow tightlythe resonator is coupled toitsItis determined byhow tightlythe resonator is coupled toits

terminations.terminations.


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Instabilityoftuned circuitsInstabilityoftuned circuits Transistor are used atthe frequencies nearer totheir unitygainTransistor are used atthe frequencies nearer totheir unitygain

BWto amplifynarrow band ofhighfrequencies.BWto amplifynarrow band ofhighfrequencies.

Atthis RF frequencies , inter capacitance between base andAtthis RF frequencies , inter capacitance between base andcollector becomes dominant.collector becomes dominant.

Some feedback signal manages to reachthe inputfrom output ,Some feedback signal manages to reachthe inputfrom output ,so circuit becomes unstable and generating the oscillation andso circuit becomes unstable and generating the oscillation andcan stop working as amplifier.can stop working as amplifier.

Stabilizationtechniques:Stabilizationtechniques:

1.1. Hazeltine neutralizationHazeltine neutralization

2.2. Neutrodyne neutralizationNeutrodyne neutralization

3.3. Neutralizationusing coilNeutralizationusing coil

4.4. Rice neutralizationRice neutralization


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Class C amplifiersClass C amplifiers

Class CClass C amplifiersamplifiers cannotcannot be used in audio circuitry because ofbe used in audio circuitry because oftheir high distortion.their high distortion.

Class C amplifiers can be used as tuned rfamplifiers where theClass C amplifiers can be used as tuned rfamplifiers where theundesired harmonic frequencies can be filtered out.undesired harmonic frequencies can be filtered out.

A class C amplifier is more efficientthan either a class A or class BA class C amplifier is more efficientthan either a class A or class Bamplifier; its efficiency approaches 100%.amplifier; its efficiency approaches 100%.

The input coupling capacitor, baseresistor, and base-emitter junctionform a negative clamper.Because ofthe clamping action,onlythe positive peaks ofthe input

signal drive the transistor, Q1, intoconduction. The RBC time constantis madelong with respecttothe period ofthe input waveform to provide theproper clamping action.


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Atthe resonantfrequency, fr,the impedance ofthe tunedLC circuitis maximum. The tank impedance, Ztank, is

purely resistive atfr.Application: used in radioreceivers ad mixer circuits.

Equivalentinput circuitofclass c TUNED AMPLIFIER


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UnitUnit--44

Blocking OscillatorBlocking Oscillator


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Linear wave shaping:Process by whichthe shape ofa nonLinear wave shaping:Process by whichthe shape ofa nonsinusoidal signal is changed by passingthe signal throughsinusoidal signal is changed by passingthe signal through

the network consistingof linear elementsthe network consistingof linear elements Diodes can be used in wave shaping circuits.Diodes can be used in wave shaping circuits.

EitherEither

limitor clip signal portionlimitor clip signal portion------ clipperclipper

shiftthe dc voltage level ofthe signalshiftthe dc voltage level ofthe signal ------ clampersclampers

Types ofnon sinusoidal inputTypes ofnon sinusoidal input

step, pulse ,square, Ramp inputstep, pulse ,square, Ramp input


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RC circuits (Step Response)

High-Pass Low-Pass

Vin

Vout Vout

Vin

Voltage on capacitor cannot change

instantaneously. So Vout = Vin initially.Voltage on capacitor cannot change

instantaneously. So Vout = 0 initially.

Vin Vout Vin Vout


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RL circuitRL circuit

~

L=1 mH

R VoutVin

RL circuitis used for small time constants.

Toget a large time constantthe inductance value has to bechosenhigh

Higher inductance value are provided byiron core inductorswhich are bigger in size, heavy and costly.


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X ! RC

Fall Time

Vout

time

1.0

0.9

0.1 10%

90%

100%

1/e~37%

e

t

RC

Fall Time & Time Constant ( )XR

elationship Between Time Constant T &Rise-Time orFall-Time

T = RC or L/R

Rise-Time (Fall-Time) = T X ln9 = 2.2T


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ClippersClippers

Clipping removes partoftheClipping removes partofthepositive or negative peaks ofapositive or negative peaks ofasignal or both. Silicon diodessignal or both. Silicon diodesdonot conductuntil the applieddonot conductuntil the appliedvoltage exceeds about 0.6voltage exceeds about 0.6

volts and only whenthe anodevolts and only whenthe anodeis positive with respecttotheis positive with respecttothecathode.cathode.

The circuitis like a potentialThe circuitis like a potentialdivider withthe diode partdivider withthe diode partbeinghigh resistance forbeinghigh resistance for

voltages below 0.6 volts andvoltages below 0.6 volts andlow resistance above.low resistance above.


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Diode :Diode :-- ClamperClamperPositive ClamperPositive Clamper

Duringthe negative half cycle oftheinput signal, the diode conducts andacts like a short circuit.

The output voltage Vo 0 volts .The capacitor is charged tothe peakvalue ofinput voltage Vm. and itbehaves like a battery.

Duringthe positive halfofthe inputsignal, the diode does not conductand acts as anopen circuit.

Hence the output voltage

VoVm+ Vm This gives a positivelyclamped voltage.

Vo Vm+ Vm = 2 Vm


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Negative ClamperNegative ClamperDuringthe positive half cycle thediode conducts and acts like ashort circuit.The capacitor chargesto peak value ofinput voltage Vm.

Duringthis interval the outputVo

whichis taken across the shortcircuit will be zero

Duringthe negative half cycle,the diode is open.The outputvoltage can be found by applyingKVL.


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Bistable (FlipBistable (Flip -- Flop) MultivibratorFlop) Multivibrator

+VBB

-VCC

R5

R2

R3 R4

R1C1 C2

Q2Q1

0

0

-

-

INPUT

OUTPUT

2

OUTPUT

1 R6

-

0

C3 C4

Bistable Multivibrator


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Physical DescriptionPhysical Description

Multivibrator thatfunctions inone oftwo stable states asMultivibrator thatfunctions inone oftwo stable states assynchronized by aninputtrigger pulse.synchronized by aninputtrigger pulse.

Operational CharacteristicsOperational Characteristics

Circuitis turned on.Circuitis turned on. One ofthe twotransistors will conductharder and therebyOne ofthe twotransistors will conductharder and thereby

reach saturationfirst. (Assume Q2)reach saturationfirst. (Assume Q2)

The 0V atthe collector ofQ2 is coupled tothe base ofQ1The 0V atthe collector ofQ2 is coupled tothe base ofQ1which drives Q1 into cutoff.which drives Q1 into cutoff.

TheThe --VVCCCC atthe collector ofQ1 is coupled tothe base ofQ2atthe collector ofQ1 is coupled tothe base ofQ2holding Q2 in saturation.holding Q2 in saturation.

Aninputtrigger pulse is applied tothe bases ofboth Q1 andAninputtrigger pulse is applied tothe bases ofboth Q1 andQ2 simultaneously. Since Q2 is alreadyin saturation, there isQ2 simultaneously. Since Q2 is alreadyin saturation, there isno effecton Q2.no effecton Q2.

The trigger pulse turns on Q1 and drives the transistor intoThe trigger pulse turns on Q1 and drives the transistor intosaturation.saturation.

The 0Vonthe collector ofQ1 is coupled tothe base ofQ2The 0Vonthe collector ofQ1 is coupled tothe base ofQ2driving Q2 into cutoff.driving Q2 into cutoff.

TheThe --VVCCCC onthe collector ofQ2 is coupled tothe base ofQ1onthe collector ofQ2 is coupled tothe base ofQ1holding Q1 in saturation.holding Q1 in saturation.

This process will continue as long as there are trigger pulsesThis process will continue as long as there are trigger pulsesapplied tothe circuit.applied tothe circuit.

The outputfrequencyofthe waveforms will be determined byThe outputfrequencyofthe waveforms will be determined bythe frequencyofthe inputtrigger pulses.the frequencyofthe inputtrigger pulses.


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cc .) Astable (Free.) Astable (Free -- Running) MultivibratorRunning) Multivibrator

-VCC

R1

Q2Q1

0

-

OUTPUT2

OU

TPU

T1 R4

-

0

C2

R2 R3

C1

Astable Multivibrator


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Physical DescriptionPhysical Description

Circuithas twooutputs butnoinputs.Circuithas twooutputs butnoinputs.

R1 = R4, R2 = R3, C1 = C2, Q1 & Q2 are as close as is possibleR1 = R4, R2 = R3, C1 = C2, Q1 & Q2 are as close as is possibleintheir operating characteristics.intheir operating characteristics.

Operational CharacteristicsOperational Characteristics Circuitis turned on.Circuitis turned on.

Assume that Q2 conducts harder than Q1 and goes intoAssume that Q2 conducts harder than Q1 and goes intosaturationfirst.saturationfirst.

The 0V atthe collector ofQ2 is coupled tothe base ofQ1 whichThe 0V atthe collector ofQ2 is coupled tothe base ofQ1 whichdrives Q1 into cutoff.drives Q1 into cutoff.

C2 begins to charge. C1 is atC2 begins to charge. C1 is at--VVCCCC and this voltage is applied toand this voltage is applied to

the base ofQ2 tohold Q2 in saturation.the base ofQ2 tohold Q2 in saturation. After a finite period oftime, (as set bythe RC time constantofAfter a finite period oftime, (as set bythe RC time constantof

C2 and R3), C2 reaches a voltage value sufficientto snap Q1 on.C2 and R3), C2 reaches a voltage value sufficientto snap Q1 on.

Q1 quicklygoes into saturation.The change in voltage fromQ1 quicklygoes into saturation.The change in voltage from --VVCCCCto 0Vcauses C1 to discharge.to 0Vcauses C1 to discharge.

This voltage is coupled tothe base ofQ2 Placing / holding Q2 inThis voltage is coupled tothe base ofQ2 Placing / holding Q2 incutoff.cutoff.

C1 begins to charge and will snap Q2 on when a sufficientC1 begins to charge and will snap Q2 on when a sufficientvoltage value is reached.voltage value is reached.

In Summary, whenever a transistor saturates, its VIn Summary, whenever a transistor saturates, its VCC will changewill changefromfrom --VVCCCC to 0V.This voltage will then be coupled tothe base ofto 0V.This voltage will then be coupled tothe base ofthe other transistor which will drive the other transistor intothe other transistor which will drive the other transistor intocutoff.The frequencyofthe output waveform will depend onthecutoff.The frequencyofthe output waveform will depend ontheRC time constants established at C1R2 and C2R3.RC time constants established at C1R2 and C2R3.


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ee .).) After a finite period of time, (as set by the RCAfter a finite period of time, (as set by the RC

time constant ofC

2 and R3),C

2 reaches a voltagetime constant ofC

2 and R3),C

2 reaches a voltagevalue sufficient to snap Q1 on.value sufficient to snap Q1 on.

ff .) Q1 quickly goes into saturation. The change in.) Q1 quickly goes into saturation. The change involtage fromvoltage from --VVCCCC to 0Vcauses C1 to discharge.to 0Vcauses C1 to discharge.

gg .) This voltage is coupled to the base ofQ2 Placing.) This voltage is coupled to the base ofQ2 Placing/ holding Q2 in cutoff./ holding Q2 in cutoff.

hh .) C1 begins to charge and will snap Q2 on when a.) C1 begins to charge and will snap Q2 on when asufficient voltage value is reached.sufficient voltage value is reached.

ii .) In Summary, whenever a transistor saturates,.) In Summary, whenever a transistor saturates,its Vits VCC will change fromwill change from --VVCCCC to 0V. This voltage willto 0V. This voltage willthen be coupled to the base of the other transistorthen be coupled to the base of the other transistorwhich will drive the other transistor into cutoff.which will drive the other transistor into cutoff.

The frequency of the output waveform willThe frequency of the output waveform willdepend on the RC time constants established atdepend on the RC time constants established atC1R2 and C2R3.C1R2 and C2R3.

Astable Multivibrator


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Monostable (One Shot) MultivibratorMonostable (One Shot) Multivibrator

+VBB

-VCC

R1 R2R3

R4

R5

C1

C2Q2

Q1

0

0-

-INPUT

OUTPUT

Multivibrators


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UsesUses

Used for pulse stretchingUsed for pulse stretching Used in computer logic systems and Communication /Used in computer logic systems and Communication /Navigation systems.Navigation systems.

Operational CharacteristicsOperational Characteristics

VVBBBB is connected tothe base ofQ1 which places Q1 inis connected tothe base ofQ1 which places Q1 incutoff.cutoff.

Q2 is saturated byQ2 is saturated by--VVCCCC applied toits base throughR2.applied toits base throughR2.

C1 is fully charged maintaining approximatelyC1 is fully charged maintaining approximately--VVCCCC ononthe base ofQ2.the base ofQ2.

A negative gate signal is applied tothe base oftransistorA negative gate signal is applied tothe base oftransistorQ1 whichturns Q1 on and drives itinto saturation.Q1 whichturns Q1 on and drives itinto saturation.

The voltage atthe collector ofQ1 is then attached totheThe voltage atthe collector ofQ1 is then attached tothebase ofQ2 whichturns Q2 off.base ofQ2 whichturns Q2 off.

C1 is discharged to attemptto keep VC1 is discharged to attemptto keep VCC at Q2 constant.at Q2 constant.This maintains Q2 off.This maintains Q2 off.


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When C1 is discharged, it canno longer keep Q2 off.When C1 is discharged, it canno longer keep Q2 off.

Q2 turns on and saturates which causes its VQ2 turns on and saturates which causes its VCC togototogoto

approximately 0V.approximately 0V. This 0Vis applied tothe base ofQ1 whichturns Q1 off.This 0Vis applied tothe base ofQ1 whichturns Q1 off.

Q1s VQ1s VCC goes togoes to--VVCCCC and C1 charges toand C1 charges to--VVCCCC..

The multivibrator will remaininthis original state untilThe multivibrator will remaininthis original state untilanother gate triggeringpulse is received.another gate triggeringpulse is received.

Outputfrom the circuitis takenfrom Q2s collector.Outputfrom the circuitis takenfrom Q2s collector. Onlyone trigger pulse is required togenerate a completeOnlyone trigger pulse is required togenerate a complete

cycle ofoutput.cycle ofoutput.

SchmittTrigger:SchmittTrigger:

Used for wave shaping circuit.Used for wave shaping circuit.

Used togenerate square wave from a sine wave I/p.Used togenerate square wave from a sine wave I/p.Trigger is not pulse transform but slowly varying acTrigger is not pulse transform but slowly varying ac

signalsignal

Switches attwotrigger level :upper & lower triggerSwitches attwotrigger level :upper & lower trigger

levellevel


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Unit 5Unit 5

Blockingoscillators &Blockingoscillators &

Time base generatorsTime base generators


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Pulse transformerPulse transformer

A transformer which couples a source ofpulses ofelectricalA transformer which couples a source ofpulses ofelectricalenergytothe load.energytothe load.

Keepingthe shape and other properties ofpulses unchanged.Keepingthe shape and other properties ofpulses unchanged.

Characteristics:Characteristics:

Leakage inductance is minimumLeakage inductance is minimum

Iron cored and small in sizeIron cored and small in size

Inter winding capacitance is lowInter winding capacitance is low

High permeabilityHigh permeability

High magnetizinginductanceHigh magnetizinginductance

Uses:Uses:

Used in Blocking Oscillator, pulse signal and digital signalUsed in Blocking Oscillator, pulse signal and digital signal

transmission, Polarityinversion, coupling and to provide DC isolation.transmission, Polarityinversion, coupling and to provide DC isolation.


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Constant current chargingConstant current charging

A capacitor is charged with constant current source.A capacitor is charged with constant current source.

As it charged with constant current, itis charged linearly.As it charged with constant current, itis charged linearly.

Miller circuit:Miller circuit:

Integrator is used to convert a step waveform to rampIntegrator is used to convert a step waveform to rampwaveform.waveform.

Bootstrap circuitsBootstrap circuits

A constant current source is obtained by maintainingnearlyA constant current source is obtained by maintainingnearlyconstant voltage across the fixed resistor in series withconstant voltage across the fixed resistor in series withcapacitor.capacitor.

CompensatingnetworkCompensatingnetwork is used toimprove the linearityofis used toimprove the linearityofbootstrap and miller time base generatorbootstrap and miller time base generator


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Relaxationoscillators make use ofanRC timing and a device thatchanges states togenerate a periodic waveform.

This triangular-wave oscillator makes use ofa comparator andintegrator to actually produce both a triangle-wave and square-wave.

Output levels are set bythe ratioofR2 and R3 times themaximum outputofthe comparator.The frequencyofoutput canbe determined bythe formula fr = 1/4R1C(R2/R3)


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The voltage-controlled sawtooth oscillators frequency can bechanged by a varied by a given dc control voltage. One possibletype uses a programmable unijunctiontransistor.

A square wave oscillator relaxationoscillator use the charging anddischargingofthe capacitor to cause the op-amp to switch statesrapidly and produce a square wave.The RC time constant

determines the frequency.


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Relaxation OscillatorsRelaxation Oscillators

The forwardvoltage of thePUT (VF)determines thefrequency of theoutput. Theformula belowshows therelationship.

f = VIN/RiC(1/Vp-VF)

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EC2251- Electronic Circuits II

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