Diode to Amplifiers

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Chapter 1

INTRODUCTION



Objectives

Discuss the basic structure of atoms

Discuss properties of insulators,

conductors, and semiconductors

Discuss covalent bonding

Describe the properties of both p and

n type materialsDiscuss both forward and reverse biasing of a p-n junction

Discuss basic operation of a diode



Introduction

Forward bias

Current flows

Reverse Bias

No current flows

The basic function of a diode is to restrict current flow

to one direction.



Bohr model of an atom

As seen in thismodel, electronscircle the nucleus. Atomic structureof a materialdetermines itsability to conduct

or insulate.



Conductors, Insulators, and Semiconductors

The ability of a material to conduct current isbased on its atomic structure.

The orbit paths of the electrons surroundingthe nucleus are called shells.

The less complete a shell is filled to capacity themore conductive the material is.

Each shell has a defined number of electrons itwill hold. This is a fact of nature and can bedetermined by the formula, 2n2.

The outer shell is called the valence shell.



The valence shell determines the ability of material toconduct current.

A Copper atom has only 1electron in its valence ring. Thismakes it a good conductor. It

takes 2n2

electrons or in this case32 electrons to fill the valenceshell.

A Silicon atom has 4 electrons inits valence ring. This makes it asemiconductor. It takes 2n2

electrons or in this case or 18electrons to fill the valence shell.

Conductors, Insulators, and Semiconductors



Covalent Bonding

Covalent bonding is a bonding of two or more atoms by theinteraction of their valence electrons.



Certain atoms will combine in this way to form a crystal

structure. Silicon and Germanium atoms combine in thisway in their intrinsic or pure state.

Covalent Bonding



N-type and P-type Semiconductors

Other atoms with 5 electrons such as Antimony are added to Silicon toincrease the free electrons.

Other atoms with 3 electrons such asBoron are added to Silicon to create a

deficiency of electrons or hole charges.

The process of creating N- and P-typematerials is called doping.

N-type P-type



The Depletion Region

With the formation of the p

and n materials combinationof electrons and holes at the junction takes place.

This creates the depletion

region and has a barrierpotential. This potentialcannot be measured with avoltmeter but it will cause asmall voltage drop.



Forward and Reverse Bias

Voltage source or bias connections are+ to the p material and – to the nmaterial.

Bias must be greater than .3 V forGermanium or .7 V for Silicon diodes.

The depletion region narrows.

Voltage source or bias connections are – to the p material and + to the n material.

Bias must be less than the breakdownvoltage.

Current flow is negligible in most cases.

The depletion region widens.

Forward Bias Reverse Bias



Forward Bias Measurements

With Small Voltage Applied

In this case with thevoltage applied is

less than the barrierpotential so thediode for all practicalpurposes is still in anon-conducting

state. Current is verysmall.



Forward Bias Measurements With Applied

Voltage Greater Than the Barrier Voltage.

With the applied voltageexceeding the barrier

potential the now fullyforward-biased diodeconducts. Note that theonly practical loss is the.7 Volts dropped acrossthe diode.



Ideal Diode Characteristic Curve

In this characteristiccurve we do notconsider the voltagedrop or the resistiveproperties. Currentflow proportionallyincreases with

voltage.



Practical Diode Characteristic CurveIn most cases weconsider only theforward bias voltagedrop of a diode. Once

this voltage is overcomethe current increasesproportionally withvoltage.This drop isparticularly important toconsider in low voltageapplications.



Complex Characteristic Curve of a Diode

The voltage drop isnot the only loss of a

diode. In some caseswe must take intoaccount other factorssuch as the resistiveeffects as well as

reverse breakdown.



Troubleshooting DiodesTesting a diode is quite simple, particularly if the multimeter

used has a diode check function. With the diode check functiona specific known voltage is applied from the meter across thediode.

With the diode check

function a good diode willshow approximately .7 V or.3 V when forward biased.

When checking in reverse

bias the full applied testingvoltage will be seen on thedisplay. Note some metersshow an infinite (blinking)display.



Troubleshooting Diodes

An ohmmeter can be used to check theforward and reverse resistance of a diode if

the ohmmeter has enough voltage to force thediode into conduction. Of course, in forward-biased connection, low resistance will be seenand in reverse-biased connection highresistance will be seen.



Troubleshooting Diodes

Open Diode

In the case of an open diode no current flows in eitherdirection which is indicated by the full checking voltagewith the diode check function or high resistance using an

ohmmeter in both forward and reverse connections.

Shorted Diode

In the case of a shorted diode maximum current flowsindicated by a 0 V with the diode check function or lowresistance with an ohmmeter in both forward and reverseconnections.



Diode Packages

Diodes come in a variety of sizes and shapes. Thedesign and structure is determined by what typeof circuit they will be used in.



Summary

P-materials are doped with trivalent impurities

N-materials are doped with pentavalent impurities.

P and N type materials are joined together to form aPN junction.

A diode is nothing more than a PN junction.

At the junction a depletion region is formed. Thiscreates barrier that requires approximately .3 V for a

Germanium and .7 V for Silicon for conduction to takeplace.

Diodes, transistors, and integrated circuits areall made of semiconductor material.



Summary

When reversed-biased, a diode can only withstandso much applied voltage. The voltage at which

avalanche current occurs is called reverse breakdownvoltage.

There are three ways of analyzing a diode. Theseare ideal, practical, and complex. Typically we use a

practical diode model.

A diode conducts when forward-biased and does notconduct when reverse biased.



Chapter 2

Diode Applications



Objectives Explain and analyze the operation of both half andfull wave rectifiers

Explain and analyze filters and regulators andtheir characteristics

Explain and analyze the operation of diode limitingand clamping circuits

Explain and analyze the operation of diode voltage

multipliers Interpret and use a diode data sheet

Troubleshoot simple diode circuits



IntroductionThe basic function of a DC power supply is toconvert an AC voltage to a smooth DC voltage.



Half Wave Rectifier A half waverectifier(ideal)allows conductionfor only 180° orhalf of a complete

cycle.

The outputfrequency is thesame as the

input.

The average VDC or V AVG = Vp /



Half Wave RectifierPeak inversevoltage is themaximumvoltage acrossthe diode when

it is in reversebias.

The diode must

be capable of withstanding thisamount of voltage.



Transformer-Coupled Input Transformers are often used for voltage change and isolation.

The turns ratio of the primary to secondary determines theoutput versus the input.

The fact that there is no direct connection between the primaryand secondary windings prevents shock hazards in the

secondary circuit.



Full-Wave Rectifier A full-wave rectifier allows current to flow during both

the positive and negative half cycles or the full 360º.Note that the output frequency is twice the inputfrequency.

The average V DC or V AVG = 2V p / .



Full-Wave Rectifier

Center-TappedThis method of rectification employs two diodes connectedto a center-tapped transformer.

The peak output is only half of the transformer’speak secondary voltage.



Full-Wave Center TappedNote the current flow

direction during bothalternations. Being thatit is center tapped, thepeak output is about half of the secondary

windings total voltage.

Each diode is subjectedto a PIV of the fullsecondary winding

output minus one diodevoltage drop.

PIV=2V p(out) +0.7V



The Full-Wave Bridge RectifierThe full-wave

bridge rectifiertakes advantageof the full outputof the secondarywinding.

It employs fourdiodes arrangedsuch that current

flows in the samedirection throughthe load duringeach half of thecycle.



The Full-Wave Bridge Rectifier

The PIV for a bridge rectifier is approximately half the PIVfor a center-tapped rectifier.

PIV=V p(out) +0.7V

Note that in most cases we take the diode drop into account.



Power Supply Filters And Regulators

As we have seen, the output of a rectifier is a pulsating DC.With filtration and regulation this pulsating voltage can besmoothed out and kept to a steady value.




A capacitor-inputfilter will chargeand discharge suchthat it fills in the

―gaps‖ betweeneach peak. Thisreduces variationsof voltage. Theremaining voltagevariation is calledripple voltage.



Power Supply Filters And RegulatorsThe advantage of a full-wave rectifier over a half-wave is quite

clear. The capacitor can more effectively reduce the ripple whenthe time between peaks is shorter.




Being that thecapacitor appears as ashort during the initialcharging, the current

through the diodes canmomentarily be quitehigh. To reduce risk of damaging the diodes,a surge current limiting

resistor is placed inseries with the filterand load.



Power Supply Filters And RegulatorsRegulation is the last step in eliminating the remaining ripple

and maintaining the output voltage to a specific value. Typicallythis regulation is performed by an integrated circuit regulator.There are many different types used based on the voltage andcurrent requirements.



Power Supply Filters And RegulatorsHow well the regulation is performed by a regulator ismeasured by it’s regulation percentage. There are twotypes of regulation, line and load. Line and load

regulation percentage is simply a ratio of change involtage (line) or current (load) stated as a percentage.

Line Regulation = ( V OUT / V IN )100%

Load Regulation = (V NL – V FL )/V FL )100%



Diode LimitersLimiting circuits limit the positive or negative amount of an

input voltage to a specific value.

This positive limiter will limit the output to VBIAS + .7V



Diode LimitersThe desired amount of limitation can be attained by a powersupply or voltage divider. The amount clipped can be adjustedwith different levels of VBIAS.

This positive limiter will limitthe output to VBIAS + .7V

The voltage divider provides the VBIAS . VBIAS =(R3/R2+R3)VSUPPLY



Diode Clampers A diode clamper adds a DC level to an AC voltage. The capacitorcharges to the peak of the supply minus the diode drop. Oncecharged, the capacitor acts like a battery in series with the inputvoltage. The AC voltage will ―ride‖ along with the DC voltage. Thepolarity arrangement of the diode determines whether the DCvoltage is negative or positive.



Voltage MultipliersClamping action can be used to increase peak rectified voltage.

Once C1 and C2 charges to the peak voltage they act like twobatteries in series, effectively doubling the voltage output. Thecurrent capacity for voltage multipliers is low.



Voltage MultipliersThe full-wave voltage doubler arrangement of diodes and

capacitors takes advantage of both positive and negativepeaks to charge the capacitors giving it more currentcapacity. Voltage triplers and quadruplers utilize three andfour diode-capacitor arrangements respectively.



The Diode Data SheetThe data sheet for diodes and other devices givesdetailed information about specific characteristicssuch as the various maximum current and voltage

ratings, temperature range, and voltage versuscurrent curves. It is sometimes a very valuablepiece of information, even for a technician. Thereare cases when you might have to select areplacement diode when the type of diode neededmay no longer be available.



Troubleshooting

Our study of these devices and how they work leads more effective troubleshooting. Efficienttroubleshooting requires us to take logicalsteps in sequence. Knowing how a device,

circuit, or system works when operatingproperly must be known before any attemptsare made to troubleshoot. The symptomsshown by a defective device often pointdirectly to the point of failure. There are manydifferent methods for troubleshooting. We willdiscuss a few.



TroubleshootingHere are some helpful troubleshooting techniques:

Power Check: Sometimes the obvious eludes themost proficient troubleshooters. Check for fuses

blown, power cords plugged in, and correct batteryplacement.

Sensory Check: What you see or smell may leadyou directly to the failure or to a symptom of a

failure. Component Replacement: Educated guesswork inreplacing components is sometimes effective.



TroubleshootingSignal tracing is the most popular and most accurate. Welook at signals or voltages through a complete circuit orsystem to identify the point of failure. This method requiresmore thorough knowledge of the circuit and what thingsshould look like at the different points throughout.



TroubleshootingThis is just one example of troubleshooting that illustratesthe effect of an open diode in this half-wave rectifier circuit.Imagine what the effect would be if the diode were shorted.



TroubleshootingThis gives us anidea of whatwould be seen inthe case of an

open diode in afull-wave rectifier.Note the ripplefrequency is nowhalf of what it was

normally. Imaginethe effects of ashorted diode.



The basic function of a power supply to give us a smoothripple free DC voltage from an AC voltage.

Half-wave rectifiers only utilize half of the cycle toproduce a DC voltage.

Transformer Coupling allows voltage manipulationthrough its windings ratio.

Full-Wave rectifiers efficiently make use of thewhole cycle. This makes it easier to filter.

The full-wave bridge rectifier allows use of the fullsecondary winding output whereas the center-tapped

full wave uses only half.

Summary



Summary

Filtering and Regulating the output of a rectifier helpskeep the DC voltage smooth and accurate.

Limiters are used to set the output peak(s) to a givenvalue.

Clampers are used to add a DC voltage to an ACvoltage.

Voltage Multipliers allow a doubling, tripling, orquadrupling of rectified DC voltage for low current

applications.



Summary The Data Sheet gives us useful information andcharacteristics of device for use in replacement ordesigning circuits.

Troubleshooting requires use of common sense along with

proper troubleshooting techniques to effectively determine thepoint of failure in a defective circuit or system.



Chapter 3

Special-Purpose Diodes



Objectives Describe the characteristics of a zener diode andanalyze its operation

Explain how a zener is used in voltage regulation andlimiting

Describe the varactor diode and its variablecapacitance characteristics

Discuss the operation and characteristics of LEDsand photodiodes

Discuss the basic characteristics of the currentregulator diode, the pin diode, the step-recovery

diode, the tunnel diode, and the laser diode.



IntroductionThe basic function of zener diode is to maintain a specificvoltage across its terminals within given limits of line orload change. Typically it is used for providing a stablereference voltage for use in power supplies and otherequipment.

This particular zener circuit will work to maintain 10 V across the load.



Zener Diodes A zener diode is muchlike a normal diode, theexception being is that it isplaced in the circuit inreverse bias and operates

in reverse breakdown.This typical characteristiccurve illustrates theoperating range for azener. Note that its

forward characteristics are just like a normal diode.



Zener DiodesThe zener diode’s

breakdown characteristicsare determined by thedoping process. Lowvoltage zeners less than5V operate in the zenerbreakdown range. Thosedesigned to operate morethan 5 V operate mostlyin avalanche breakdown

range. Zeners areavailable with voltagebreakdowns of 1.8 V to200 V.

This curve illustrates the minimum andmaximum ranges of current operation that thezener can effectively maintain its voltage.



Zener Diodes

As with most devices, zener diodes have givencharacteristics such as temperature coefficients andpower ratings that have to be considered. The data sheetprovides this information.



Zener Diode ApplicationsRegulation

In this simple illustration of zener regulation circuit, the zenerdiode will ―adjust‖ its impedance based on varying inputvoltages and loads (R L) to be able to maintain its designatedzener voltage. Zener current will increase or decrease directlywith voltage input changes. The zener current will increase or

decrease inversely with varying loads. Again, the zener has afinite range of operation.



Zener LimitingZener diodes can used for limiting just as normal diodes.Recall in previous chapter studies about limiters. Thedifference to consider for a zener limiter is its zenerbreakdown characteristics.



Varactor Diodes A varactor diode is best explained as a variable capacitor.Think of the depletion region a variable dielectric. Thediode is placed in reverse bias. The dielectric is ―adjusted‖ by bias changes.



Varactor DiodesThe varactor diode can be useful in filter

circuits as the adjustable component.



Optical DiodesThe light-emitting diode (LED) emits photons asvisible light. Its purpose is for indication and otherintelligible displays. Various impurities are addedduring the doping process to vary the color output.



Optical DiodesThe seven segment display is an exampleof LEDs use for display of decimal digits.



Optical DiodesThe photodiode is used to vary current by the amount of light that strikes it. It is placed in the circuit in reverse bias. Aswith most diodes when in reverse bias, no current flows whenin reverse bias, but when light strikes the exposed junctionthrough a tiny window, reverse current increases proportional

to light intensity.



Other Diode TypesCurrent regulator diodes keeps a constantcurrent value over a specified range of forwardvoltages ranging from about 1.5 V to 6 V.



Other Diode TypesThe Schottky diode’s significant characteristic is its fastswitching speed. This is useful for high frequencies anddigital applications. It is not a typical diode in that it doesnot have a p-n junction. Instead, it consists of a heavily-doped n-material and metal bound together.



Other Diode TypesThe pin diode is also used in mostly microwavefrequency applications. Its variable forward seriesresistance characteristic is used for attenuation,modulation, and switching. In reverse bias itexhibits a nearly constant capacitance.



Other Diode Types

The step-recovery diode is also used for fast

switching applications. This is achieved by reduceddoping at the junction.



Other Diode TypesThe tunnel diode has negative resistance. It will actuallyconduct well with low forward bias. With further increasesin bias it reaches the negative resistance range wherecurrent will actually go down. This is achieved by heavily-doped p and n materials that creates a very thin depletion

region.



Other Diode TypesThe laser diode (light amplification by stimulatedemission of radiation) produces a monochromatic(single color) light. Laser diodes in conjunctionwith photodiodes are used to retrieve data fromcompact discs.



Troubleshooting Although precise power supplies typically use IC type

regulators, zener diodes can be used alone as a voltageregulator. As with all troubleshooting techniques wemust know what is normal.

A properly functioning zener will work to maintain the output voltagewithin certain limits despite changes in load.



TroubleshootingWith an open zener diode, the full unregulated

voltage will be present at the output without aload. In some cases with full or partial loading anopen zener could remain undetected.



TroubleshootingWith excessive zener impedance the voltage would be

higher than normal but less than the full unregulatedoutput.



Summary The zener diode operates in reverse breakdown.

A zener diode maintains a nearly constant voltageacross its terminals over a specified range of currents.

Line regulation is the maintenance of a specific

voltage with changing input voltages. Load regulation is the maintenance of a specificvoltage for different loads.

There are other diode types used for specific RF

purposes such as varactor diodes (variablecapacitance), Schottky diodes (high speed switching),and PIN diodes (microwave attenuation andswitching).



Summary Light emitting diodes (LED) emit either infrared orvisible light when forward-biased.

Photodiodes exhibit an increase in reverse currentwith light intensity.

The laser diode emits a monochromatic light



Chapter 4Bipolar Junction

Transistors



Objectives Describe the basic structure of the bipolar junctiontransistor (BJT)

Explain and analyze basic transistor bias andoperation

Discuss how a transistor can be used as anamplifier or a switch

Troubleshoot various failures typical of transistorcircuits

Discuss the parameters and characteristics of atransistor and how they apply to transistor circuits



Introduction A transistor is a device that can be used as either anamplifier or a switch. Let’s first consider its operationin a simpler view as a current controlling device.



Basic Transistor OperationLook at this one circuit as two separate circuits, the base-

emitter(left side) circuit and the collector-emitter(rightside) circuit. Note that the emitter leg serves as aconductor for both circuits.The amount of current flow inthe base-emitter circuit controls the amount of currentthat flows in the collector circuit. Small changes in base-

emitter current yields a large change in collector-current.



Transistor StructureWith diodes there is one p-n junction. With bipolar junction transistors (BJT), there are three layersand two p-n junctions. Transistors can be either pnp or npn type.

Transistor Characteristics and




Parameters As previouslydiscussed, base-emitter currentchanges yield

large changes incollector-emittercurrent. Thefactor of thischange is called

beta ().

= I C /I B




ParametersThere are three key dc voltages and three key dc currents to

be considered. Note that these measurements are importantfor troubleshooting.

I B: dc base current

I E: dc emitter current

I C: dc collector current

V BE: dc voltage across

base-emitter junction

V CB: dc voltage acrosscollector-base junction

V CE: dc voltage from

collector to emitter

Transistors Characteristics and



ParametersFor proper operation, the base-emitter junction is forward-

biased by VBB and conducts just like a diode.The collector-base junction is reverse biased by VCC andblocks current flow through it’s junction just like a diode.

Remember that

current flow throughthe base-emitter junction will helpestablish the pathfor current flowfrom the collector toemitter.

rans s or arac er s cs anP



Parameters Analysis of this transistor circuit to predict the dc voltages andcurrents requires use of Ohm’s law, Kirchhoff’s voltage law and

the beta for the transistor.

Application of these laws begins with the base circuit to determinethe amount of base current. Using Kirchhoff’s voltage law,subtract the .7 VBE and the remaining voltage is dropped across

R B. Determining the current for the base with this information is amatter of applying of Ohm’s law. V RB/RB = IB

The collectorcurrent is

determined bymultiplying thebase currentby beta.

.7 VBE will be used in most analysis examples.





ParametersWhat we ultimatelydetermine by use of Kirchhoff’s voltage lawfor series circuits is thatin the base circuit VBB is

distributed across thebase-emitter junctionand R B in the basecircuit. In the collectorcircuit we determine

that VCC is distributedproportionally across R Cand the transistor(VCE).




ParametersCollector characteristic

curves give a graphicalillustration of therelationship of collectorcurrent and VCE withspecified amounts of base

current. With greaterincreases of VCC , VCE continues to increase untilit reaches breakdown, butthe current remains aboutthe same in the linear region from .7V to thebreakdown voltage.





ParametersWith no IB the transistor is in the cutoff region and justas the name implies there is practically no current flowin the collector part of the circuit. With the transistor ina cutoff state the the full VCC can be measured acrossthe collector and emitter(VCE)




Parameters

Current flow in the collector part of thecircuit is, as stated previously, determined by

I B multiplied by . However, there is a limitto how much current can flow in thecollector circuit regardless of additionalincreases in I B .

rans s or arac er s cs anP t



ParametersOnce this maximum is reached, the transistor is said tobe in saturation. Note that saturation can be

determined by application of Ohm’s law. I C(sat)=V CC/R CThe measured voltage across the now ―shorted‖ collectorand emitter is 0V.




ParametersThe dc load line graphically illustrates I C(sat) and cutoff for a

transistor.




Parameters

The beta for a transistor is not always constant.Temperature and collector current both affect beta,not to mention the normal inconsistencies during themanufacture of the transistor.

There are also maximum power ratings to consider.

The data sheet provides information on thesecharacteristics.

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Transistor Amplifier Amplification of a relatively small ac voltage can be had byplacing the ac signal source in the base circuit.

Recall that small changes in the base current circuit causes largechanges in collector current circuit.

The small ac voltage causes the base current to increase anddecrease accordingly and with this small change in current thecollector current will mimic the input only with greater amplitude.



Transistor Switch A transistor when used as a switch is simply being biased sothat it is in cutoff (switched off) or saturation (switched on).Remember that the VCE in cutoff is VCC and 0 V in saturation.

T bl h i



Troubleshooting

Troubleshooting a live transistor circuitrequires us to be familiar with known goodvoltages, but some general rules do apply.Certainly a solid fundamental understandingof Ohm’s law and Kirchhoff’s voltage andcurrent laws is imperative. With live circuits itis most practical to troubleshoot with voltagemeasurements.

T bl h i



Troubleshooting

Internal opens within the transistoritself could also cause transistoroperation to cease.

Erroneous voltage measurementsthat are typically low are a result of point that is not ―solidly connected‖.This called a floating point. This is

typically indicative of an open. More in-depth discussion of typicalfailures are discussed within thetextbook.

Opens in the external resistors or connections of the base or the

circuit collector circuit would cause current to cease in the collectorand the voltage measurements would indicate this.



TroubleshootingTesting a transistor can be viewed more simply if you view itas testing two diode junctions. Forward bias having lowresistance and reverse bias having infinite resistance.



TroubleshootingThe diode test function of a multimeter is more reliable thanusing an ohmmeter. Make sure to note whether it is an npn orpnp and polarize the test leads accordingly.



Troubleshooting

In addition to the traditional DMMs there are alsotransistor testers. Some of these have the ability

to test other parameters of the transistor, such asleakage and gain. Curve tracers give us even moredetailed information about a transistorscharacteristics.



Summary The bipolar junction transistor (BJT) is constructed of

three regions: base, collector, and emitter. The BJT has two pn junctions, the base-emitter junction and the base-collector junction.

The two types of transistors are pnp and npn.

For the BJT to operate as an amplifier, the base-emitter junction is forward-biased and the collector-base junction isreverse-biased.

Of the three currents IB is very small in comparison to IE

and IC. Beta is the current gain of a transistor. This the ratio of IC /IB.

S



Summary A transistor can be operated as an electronics switch.

When the transistor is off it is in cutoff condition (nocurrent).

When the transistor is on, it is in saturation condition(maximum current).

Beta can vary with temperature and also varies fromtransistor to transistor.



Chapter 5

Transistor Bias Circuits



Objectives

Discuss the concept of dc biasing of a transistor forlinear operation

Analyze voltage-divider bias, base bias, andcollector-feedback bias circuits.

Basic troubleshooting for transistor bias circuits



Introduction

For the transistor to properly operate it must bebiased. There are several methods to establishthe DC operating point. We will discuss some of

the methods used for biasing transistors as wellas troubleshooting methods used for transistorbias circuits.

Th DC O i P i



The DC Operating PointThe goal of amplification in most cases is to increase the

amplitude of an ac signal without altering it.



The DC Operating PointFor a transistor circuit to amplify it must be properly biased

with dc voltages. The dc operating point between saturationand cutoff is called the Q-point. The goal is to set the Q-pointsuch that that it does not go into saturation or cutoff when ana ac signal is applied.

Th DC O ti P i t



The DC Operating PointRecall that the collector characteristic curves graphically show the

relationship of collector current and VCE for different base currents.With the dc load line superimposed across the collector curvesfor this particular transistor we see that 30 mA of collector currentis best for maximum amplification, giving equal amount above andbelow the Q-point. Note that this is three different scenarios of

collector current being viewed simultaneously.

The DC Operating Point



The DC Operating PointWith a good Q-point established, let’s look at the effect a superimposedac voltage has on the circuit. Note the collector current swings do not

exceed the limits of operation(saturation and cutoff). However, as youmight already know, applying too much ac voltage to the base wouldresult in driving the collector current into saturation or cutoff resultingin a distorted or clipped waveform.

V lt Di id Bi



Voltage-Divider Bias

Voltage-divider bias isthe most widely usedtype of bias circuit. Onlyone power supply isneeded and voltage-

divider bias is morestable( independent)than other bias types.For this reason it will bethe primary focus for

study.



Voltage-Divider Bias Apply your knowledge of voltage-dividers tounderstand how R 1 and R 2 are used to provide theneeded voltage to point

A(base). The resistance toground from the base isnot significant enough toconsider in most cases.Remember, the basic

operation of the transistorhas not changed.

V lt Di id Bi



Voltage-Divider BiasIn the case where base to ground resistance(input resistance) is

low enough to consider, we can determine it by the simplifiedequation R IN(base) = DCR E

We can view the voltage at point A of the circuit in two ways,with or without the input resistance(point A to ground)considered.

V lt Di id Bi




For this circuit we will nottake the input resistanceinto consideration.

Essentially we aredetermining the voltageacross R2(VB) by theproportional method.

V B = (R 2/R 1 + R 2)V CC



Voltage-Divider BiasWe now take the known basevoltage and subtract VBE to findout what is dropped across R E.Knowing the voltage across R E we can apply Ohm’s law to

determine the current in thecollector-emitter side of thecircuit. Remember the current inthe base-emitter circuit is muchsmaller, so much in fact we can

for all practical purposes we saythat IE approximately equals IC.

IE≈ IC

Voltage Di ider Bias




Although we have used npn transistors for most of thisdiscussion, there is basically no difference in its operation

with exception to biasing polarities. Analysis for each part of the circuit is no different than npn transistors.



Base BiasThis type of circuit is very unstable since its changes withtemperature and collector current. Base biasing circuits aremainly limited to switching applications.

Emitter Bias



Emitter BiasThis type of circuit isindependent of making it asstable as the voltage-dividertype. The drawback is that itrequires two power supplies.

Two key equations for analysis

of this type of bias circuit areshown below. With these twocurrents known we can applyOhm’s law and Kirchhoff's lawto solve for the voltages.

IB ≈ IE/

IC ≈ IE ≈ -V EE-V BE/R E + R B/DC



Collector-Feedback BiasCollector-feedback bias iskept stable with negative feedback , although it is notas stable as voltage-divideror emitter. With increases of

I C, less voltage is applied tothe base. With less I B ,I Ccomes down as well. Thetwo key formulas are shownbelow.

I B = V C - V BE/R B

I C = V CC - V BE/R C + R B/DC



TroubleshootingShown is a typical voltage divider circuit with correctvoltage readings. Knowing these voltages is arequirement before logical troubleshooting can beapplied. We will discuss some of the faults andsymptoms.

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TroubleshootingR1 Open

With no bias thetransistor is incutoff.

Base voltage goesdown to 0 V.

Collector voltagegoes up to

10 V(V CC ).

Emitter voltagegoes down to 0 V.



TroubleshootingResistor R E Open:

Transistor is in cutoff.

Base reading voltage willstay approximately thesame.

Collector voltage goes upto 10 V(V CC).

Emitter voltage will be

approximately the basevoltage + .7 V.



TroubleshootingBase Open Internally:

Transistor is in cutoff.

Base voltage staysapproximately the

same.

Collector voltage goesup to 10 V(V CC).

Emitter voltage goes

down to 0 V.



TroubleshootingOpen BE Junction: Transistor is in cutoff.

Base voltage staysapproximately the

same.

Collector voltage goesup to 10 V(V CC)

Emitter voltage goes

down to 0 V.

Troubleshooting



TroubleshootingOpen BC Junction:

Base voltage goesdown to 1.11 Vbecause of more basecurrent flow through

emitter.Collector voltage goesup to 10 V(V CC).

Emitter voltage will

drop to .41 V becauseof small current flowfrom forward-biasedbase-emitter junction.

Troubleshooting



TroubleshootingRC Open:

Base voltage goes down to1.11 V because of morecurrent flow through theemitter.

Collector voltage will dropto .41 V because of currentflow from forward-biasedcollector-base junction.

Emitter voltage will drop to

.41 V because of smallcurrent flow from forward-biased base-emitter junction.

Troubleshooting



TroubleshootingR 2 Open:

Transistor pushed close toor into saturation.

Base voltage goes upslightly to 3.83V because

of increased bias.Emitter voltage goes up to3.13V because of increased current.

Collector voltage goesdown because of increased conduction of transistor.

Summary



Summary

The purpose of biasing is to establish a stable operatingpoint (Q-point).

The Q-point is the best point for operation of a transistorfor a given collector current.

The dc load line helps to establish the Q-point for agiven collector current.

The linear region of a transistor is the region of operation within saturation and cutoff.



Summary Voltage-divider bias is most widely used because it isstable and uses only one voltage supply.

Base bias is very unstable because it is dependent.

Emitter bias is stable but require two voltage supplies.

Collector-back is relatively stable when compared to basebias, but not as stable as voltage-divider bias.



Chapter 6

BJT Amplifiers



Objectives

Understand the concept of amplifiers

Identify and apply internal transistor parameters

Understand and analyze common-emitter,common-base, and common-collector amplifiers

Discuss multistage amplifiers

Troubleshoot amplifier circuits



Introduction

One of the primary uses of a transistor is toamplify ac signals. This could be an audiosignal or perhaps some high frequency radio

signal. It has to be able to do this withoutdistorting the original input.



Amplifier OperationRecall from the previous chapter that the purpose of dc biasing was to establish the Q-point for operation.The collector curves and load lines help us to relatethe Q-point and its proximity to cutoff and saturation.The Q-point is best established where the signal

variations do not cause the transistor to go intosaturation or cutoff.

What we are most interested in is the ac signal itself.Since the dc part of the overall signal is filtered out in

most cases, we can view a transistor circuit in termsof just its ac component.



Amplifier OperationFor the analysis of transistor circuits from both dc and acperspectives, the ac subscripts are lower case and italicized.Instantaneous values use both italicized lower case lettersand subscripts.

Amplifier Operation



Amplifier OperationThe boundary between cutoff and saturation is called thelinear region. A transistor which operates in the linearregion is called a linear amplifier. Note that only the accomponent reaches the load because of the capacitivecoupling and that the output is 180º out of phase withinput.

Transistor Equivalent Circuits



Transistor Equivalent CircuitsWe can view transistor circuits by use of resistance

or r parameters for better understanding. Sincethe base resistance, r b is small it normally is notconsidered and since the collector resistance, r c isfairly high we consider it as an open. The emitterresistance, r c is the main parameter that is viewed.

You can determine r c from this simplifiedequation.

r c = 25 mV/I E



Transistor Equivalent CircuitsThe two graphs best illustrate the differencebetween DC and ac. The two only differ slightly.

Transistor Equivalent Circuits



Transistor Equivalent CircuitsSince r parameters are used throughout the rest of the

textbook we will not go into deep discussion about h parameters. However, since some data sheets include orexclusively provide h parameters these formulas can beused to convert them to r parameters.

r’ e = h re /h oe

r’ c = h re + 1/h oe

r’ b

= hi e

- (1+ h fe

)

The Common-Emitter

Amplifier



AmplifierThe common-emitter amplifier exhibits high voltage and

current gain. The output signal is 180º out of phase with theinput.

Now let’s use our dc and ac analysis methods to view this typeof transistor circuit.

The Common Emitter Amplifier



DC AnalysisThe dc component of thecircuit ―sees‖ only the partof the circuit that is withinthe boundaries of C1, C2,and C3 as the dc will not

pass through thesecomponents. The equivalentcircuit for dc analysis isshown.

The methods for dc analysisare just are the same asdealing with a voltage-divider circuit.

Common Emitter AmplifierAC Equivalent Circuit



AC Equivalent Circuit

The ac equivalentcircuit basicallyreplaces thecapacitors with

shorts, being that acpasses througheasily through them.The power suppliesare also effectively

shorts to ground forac analysis.

Common Emitter Amplifier




We can look at the input voltage in terms of the equivalent

base circuit (ignore the other components from the previousdiagram). Note the use of simple series-parallel analysis skillsfor determining V in .





AC Equivalent CircuitThe input resistance as seen by the input voltagecan be illustrated by the r parameter equivalent circuit.The simplified formula below is used.

R in(base) = ac r’ e

The outputresistance isfor all practicalpurposes the

value of R C.





Voltage gain can beeasily determined bydividing the ac outputvoltage by the ac inputvoltage.

Av = V out /Vi n = V c /V b

Voltage gain can also be

determined by thesimplified formula below.

Av = R C/r’ e





AC Equivalent CircuitTaking the attenuation from the ac supply internalresistance and inputresistance into considerationis included in the overall

gain.

A’ v = (V b/V s)Av

or

A’ v = R in(total)/ R s + R in(total)

The Common-EmitterAmplifier



AmplifierThe emitter bypass capacitor helps increase the gainby allowing the ac signal to pass more easily.

The X C(bypass) should be about ten times less than R E .

h l f



The Common-Emitter AmplifierThe bypasscapacitor makes thegain unstable sincetransistor amplifier

becomes moredependent on IE.This effect can beswamped orsomewhat alleviated

by adding anotheremitter resistor(R E1).

The Common-Collector

Amplifier



AmplifierThe common-collector amplifier is usually referred

to as the emitter follower because there is no phaseinversion or voltage gain. The output is taken from theemitter. The common-collector amplifier’s mainadvantages are its high current gain and high inputresistance.


lifi



AmplifierBecause of its high inputresistance the common-collector amplifier used as abuffer to reduce the loadingeffect of low impedanceloads. The input resistancecan be determined by thesimplified formula below.

R in(base) ac (r’ e + R e )


A lifi



Amplifier

The output resistance is very low. This makesit useful for driving low impedance loads.

The current gain( A i ) is approximately ac.

The power gain is approximately equal tothe current gain( A i ).

The voltage gain is approximately 1.

The Common-Collector AmplifierThe darlington pair is



e Co o Co ecto p eThe darlington pair isused to boost the input

impedance to reduceloading of high outputimpedance circuits. Thecollectors are joinedtogether and the emitterof the input transistor isconnected to the base of the output transistor. Theinput impedance can be

determined the formulabelow.

R in = ac1 ac2 R e

The Common-Base AmplifierThe common-base amplifier has high voltage gain with a



pThe common-base amplifier has high voltage gain with acurrent gain no higher than 1. It has a low input resistance

making it ideal for low impedance input sources. The acsignal is applied to the emitter and the output is taken fromthe collector.

The Common-Base Amplifier



p

The common-base voltage gain(A v ) isapproximately equal to R c /r’ e

The current gain is approximately 1.

The power gain is approximately equal to the voltage gain.The input resistance is approximately equal to r’ e .

The output resistance is approximately equal to R C .

M lti t A lifi



Multistage AmplifiersTwo or more amplifiers can be connected to increase thegain of an ac signal. The overall gain can be calculated bysimply multiplying each gain together.

A’ v = Av1 Av2 Av3 ……

M lti t A lifi



Multistage AmplifiersGain can be expressed in decibels(dB).The formula below can be used toexpress gain in decibels.

A v(dB) = 20logAv

Each stage’s gain can now can besimply added together for the total.

M lti t A lifi



Multistage AmplifiersThe capacitive coupling keeps dc bias voltages separatebut allows the ac to pass through to the next stage.

M lti t A lifi



Multistage AmplifiersThe output of stage 1 is loaded by input of stage 2. Thislowers the gain of stage 1. This ac equivalent circuit helpsgive a better understanding how loading can effect gain.

Multistage Amplifiers



Multistage AmplifiersDirect coupling between stage improves low frequency gain.

The disadvantage is that small changes in dc bias fromtemperature changes or supply variations becomes morepronounced.

Troubleshooting



Troubleshooting techniques for transistoramplifiers is similar to techniques covered inChapter 2. Usage of knowledge of how anamplifier works, symptoms, and signal tracing areall valuable parts of troubleshooting. Needless to

say experience is an excellent teacher but havinga clear understanding of how these circuits work makes the troubleshooting process more efficientand understandable.

Troubleshooting



gThe following slide is a diagram for a two stage common-

emitter amplifier with correct voltages at various points.Utilize your knowledge of transistor amplifiers andtroubleshooting techniques and imagine what the effectswould be with various faulty components —for example, openresistors, shorted transistor junctions or capacitors. Moreimportantly, how would the output be affected by these faults?In troubleshooting it is most important to understand theoperation of a circuit.

What faults could cause low or no output?

What faults could cause a distorted output signal?

T bl h ti



Troubleshooting

S mmar



Summary

Transistor circuits can be view in terms of its ac equivalentfor better understanding.

The common-emitter amplifier has high voltage andcurrent gain.

The common-collector has a high current gain andvoltage gain of 1. It has a high input impedance and low

output impedance.

Most transistors amplifiers are designed to operatein the linear region.

Summary



Summary

Multistage amplifiers are amplifier circuits cascaded to

increased gain. We can express gain in decibels (dB).

Troubleshooting techniques used for individualtransistor circuits can be applied to multistage amplifiersas well.

The common-base has a high voltage gain and acurrent gain of 1. It has a low input impedance and

high output impedance

Documents

Diode to Amplifiers