第一學期

科目：電子學（一）

班級：電機二甲、電機二乙

授課教師：章國揚林正平

Chapter 1INTRODUCTION

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 materials

Discuss 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 this model, electrons circle the nucleus. Atomic structure of a material determines its ability to conduct or insulate.

Conductors, Insulators, and Semiconductors

The ability of a material to conduct current is based on its atomic structure. The orbit paths of the electrons surrounding the nucleus are called shells.

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

Each shell has a defined number of electrons it will hold. This is a fact of nature and can be determined by the formula, 2n2.

The outer shell is called the valence shell.

The valence shell determines the ability of material to conduct current.

A Copper atom has only 1electron in its valence ring. Thismakes it a good conductor. Ittakes 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 the interaction of their valence electrons.

Certain atoms will combine in this way to form a crystal structure. Silicon and Germanium atoms combine in this way 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 to increase the free electrons.

Other atoms with 3 electrons such as Boron are added to Silicon to create a deficiency of electrons or hole charges.

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

N-type P-type

The Depletion Region

With the formation of the pand n materials combinationof electrons and holes at thejunction takes place.

This creates the depletion region and has a barrier potential. This potential cannot be measured with a voltmeter but it will cause a small voltage drop.

Forward and Reverse Bias

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

Bias must be greater than .3 V for Germanium 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 breakdown voltage.

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 the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small.

Forward Bias Measurements With Applied Voltage Greater Than the Barrier Voltage.

With the applied voltage exceeding the barrier potential the now fully forward-biased diode conducts. Note that the only practical loss is the .7 Volts dropped across the diode.

Ideal Diode Characteristic Curve

In this characteristic curve we do not consider the voltage drop or the resistive properties. Current flow proportionally increases with voltage.

Practical Diode Characteristic Curve

In most cases we consider only the forward bias voltage drop of a diode. Once this voltage is overcome the current increases proportionally with voltage.This drop is particularly important to consider in low voltage applications.

Complex Characteristic Curve of a Diode

The voltage drop is not the only loss of a diode. In some cases we must take into account other factors such as the resistive effects 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 function a specific known voltage is applied from the meter across the diode.

With the diode check function a good diode will show approximately .7 V or .3 V when forward biased.

When checking in reverse bias the full applied testing voltage will be seen on the display. Note some meters show an infinite (blinking) display.

Troubleshooting Diodes

An ohmmeter can be used to check the forward and reverse resistance of a diode if the ohmmeter has enough voltage to force the diode into conduction. Of course, in forward-biased connection, low resistance will be seen and in reverse-biased connection high resistance will be seen.

Troubleshooting Diodes

Open Diode

In the case of an open diode no current flows in either direction which is indicated by the full checking voltage with 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 flows indicated by a 0 V with the diode check function or low resistance with an ohmmeter in both forward and reverse connections.

Diode Packages

Diodes come in a variety of sizes and shapes. The design and structure is determined by what type of 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 a PN junction.

A diode is nothing more than a PN junction.

At the junction a depletion region is formed. This creates barrier that requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place.

Diodes, transistors, and integrated circuits are all made of semiconductor material.

Summary

When reversed-biased, a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage.

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

A diode conducts when forward-biased and does not conduct when reverse biased.

Chapter 2Diode Applications

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

Explain and analyze filters and regulators and their characteristics

Explain and analyze the operation of diode limiting and 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 to convert an AC voltage to a smooth DC voltage.

Half Wave RectifierA half wave rectifier(ideal) allows conduction for only 180° or half of a complete cycle.

The output frequency is the same as the input.

The average VDC or VAVG = Vp/π

Half Wave RectifierPeak inverse voltage is the maximum voltage across the diode when it is in reverse bias.

The diode must be capable of withstanding this amount of voltage.

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

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

The fact that there is no direct connection between the primary and secondary windings prevents shock hazards in the secondary circuit.

Full-Wave RectifierA 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 input frequency.

The average VDC or VAVG = 2Vp/ π.

Full-Wave RectifierCenter-Tapped

This method of rectification employs two diodes connected to a center-tapped transformer.

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

Full-Wave Center TappedNote the current flow direction during both alternations. Being that it is center tapped, the peak output is about half of the secondary windings total voltage.

Each diode is subjected to a PIV of the full secondary winding output minus one diode voltage drop.

PIV=2Vp(out) +0.7V

The Full-Wave Bridge RectifierThe full-wave bridge rectifier takes advantage of the full output of the secondary winding.

It employs four diodes arranged such that current flows in the same direction through the load during each half of the cycle.

The Full-Wave Bridge RectifierThe PIV for a bridge rectifier is approximately half the PIV for a center-tapped rectifier.

PIV=Vp(out) +0.7V

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

Power Supply Filters And RegulatorsAs we have seen, the output of a rectifier is a pulsating DC. With filtration and regulation this pulsating voltage can be smoothed out and kept to a steady value.

Power Supply Filters And Regulators

A capacitor-input filter will charge and discharge such that it fills in the “gaps” between each peak. This reduces variations of voltage. The remaining voltage variation is called ripple 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 when the time between peaks is shorter.

Power Supply Filters And Regulators

Being that the capacitor appears as a short during the initial charging, the current through the diodes can momentarily be quite high. To reduce risk of damaging the diodes, a surge current limiting resistor is placed in series with the filter and load.

Power Supply Filters And RegulatorsRegulation is the last step in eliminating the remaining ripple and maintaining the output voltage to a specific value. Typically this regulation is performed by an integrated circuit regulator. There are many different types used based on the voltage and current requirements.

Power Supply Filters And Regulators

How well the regulation is performed by a regulator is measured by it’s regulation percentage. There are two types of regulation, line and load. Line and load regulation percentage is simply a ratio of change in voltage (line) or current (load) stated as a percentage.

Line Regulation = (∆VOUT/ ∆VIN)100%

Load Regulation = (VNL – VFL)/ VFL)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 power supply or voltage divider. The amount clipped can be adjusted with different levels of VBIAS.

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

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

Diode ClampersA diode clamper adds a DC level to an AC voltage. The capacitor charges to the peak of the supply minus the diode drop. Once charged, the capacitor acts like a battery in series with the input voltage. The AC voltage will “ride” along with the DC voltage. The polarity arrangement of the diode determines whether the DC voltage 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 two batteries in series, effectively doubling the voltage output. The current capacity for voltage multipliers is low.

Voltage MultipliersThe full-wave voltage doubler arrangement of diodes and capacitors takes advantage of both positive and negative peaks to charge the capacitors giving it more current capacity. Voltage triplers and quadruplers utilize three and four diode-capacitor arrangements respectively.

The Diode Data Sheet

The data sheet for diodes and other devices gives detailed information about specific characteristics such as the various maximum current and voltage ratings, temperature range, and voltage versus current curves. It is sometimes a very valuable piece of information, even for a technician. There are cases when you might have to select a replacement diode when the type of diode needed may no longer be available.

Troubleshooting

Our study of these devices and how they work leads more effective troubleshooting. Efficient troubleshooting requires us to take logical steps in sequence. Knowing how a device, circuit, or system works when operating properly must be known before any attempts are made to troubleshoot. The symptoms shown by a defective device often point directly to the point of failure. There are many different methods for troubleshooting. We will discuss a few.

TroubleshootingHere are some helpful troubleshooting techniques:

Power Check: Sometimes the obvious eludes the most proficient troubleshooters. Check for fuses blown, power cords plugged in, and correct battery placement.

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

Component Replacement: Educated guesswork in replacing components is sometimes effective.

TroubleshootingSignal tracing is the most popular and most accurate. We look at signals or voltages through a complete circuit or system to identify the point of failure. This method requires more thorough knowledge of the circuit and what things should look like at the different points throughout.

TroubleshootingThis is just one example of troubleshooting that illustrates the 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 an idea of what would be seen in the case of an open diode in a full-wave rectifier. Note the ripple frequency is now half of what it was normally. Imagine the effects of a shorted diode.

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

Half-wave rectifiers only utilize half of the cycle to produce a DC voltage. Transformer Coupling allows voltage manipulation through its windings ratio. Full-Wave rectifiers efficiently make use of the whole cycle. This makes it easier to filter. The full-wave bridge rectifier allows use of the full secondary winding output whereas the center-tapped full wave uses only half.

Summary

Summary

Filtering and Regulating the output of a rectifier helps keep the DC voltage smooth and accurate. Limiters are used to set the output peak(s) to a given value.

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

Voltage Multipliers allow a doubling, tripling, or quadrupling of rectified DC voltage for low current applications.

Summary The Data Sheet gives us useful information and characteristics of device for use in replacement or designing circuits.

Troubleshooting requires use of common sense along with proper troubleshooting techniques to effectively determine the point of failure in a defective circuit or system.

Chapter 3Special-Purpose

Diodes

Objectives Describe the characteristics of a zener diode and analyze its operation Explain how a zener is used in voltage regulation and limiting Describe the varactor diode and its variable capacitance characteristics Discuss the operation and characteristics of LEDs and photodiodes Discuss the basic characteristics of the current regulator 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 specific voltage across its terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power supplies and other equipment.

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

Zener DiodesA zener diode is much like a normal diode, the exception being is that it is placed in the circuit in reverse bias and operates in reverse breakdown. This typical characteristic curve illustrates the operating range for a zener. Note that its forward characteristics are just like a normal diode.

Zener DiodesThe zener diode’s breakdown characteristics are determined by the doping process. Low voltage zeners less than 5V operate in the zener breakdown range. Those designed to operate more than 5 V operate mostly in avalanche breakdown range. Zeners are available with voltage breakdowns of 1.8 V to 200 V.

This curve illustrates the minimum and maximum ranges of current operation that the zener can effectively maintain its voltage.

Zener Diodes

As with most devices, zener diodes have given characteristics such as temperature coefficients and power ratings that have to be considered. The data sheet provides this information.

Zener Diode ApplicationsRegulation

In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage. Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation.

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

Varactor DiodesA varactor diode is best explained as a variable capacitor. Think of the depletion region a variable dielectric. The diode 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 as visible light. Its purpose is for indication and other intelligible displays. Various impurities are added during the doping process to vary the color output.

Optical DiodesThe seven segment display is an example of 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. As with most diodes when in reverse bias, no current flows when in reverse bias, but when light strikes the exposed junction through a tiny window, reverse current increases proportional to light intensity.

Other Diode TypesCurrent regulator diodes keeps a constant current value over a specified range of forward voltages ranging from about 1.5 V to 6 V.

Other Diode TypesThe Schottky diode’s significant characteristic is its fast switching speed. This is useful for high frequencies and digital applications. It is not a typical diode in that it does not 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 microwave frequency applications. Its variable forward series resistance characteristic is used for attenuation, modulation, and switching. In reverse bias it exhibits a nearly constant capacitance.

Other Diode Types

The step-recovery diode is also used for fast switching applications. This is achieved by reduced doping at the junction.

Other Diode TypesThe tunnel diode has negative resistance. It will actually conduct well with low forward bias. With further increases in bias it reaches the negative resistance range where current 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 stimulated emission of radiation) produces a monochromatic (single color) light. Laser diodes in conjunction with photodiodes are used to retrieve data from compact discs.

TroubleshootingAlthough precise power supplies typically use IC type regulators, zener diodes can be used alone as a voltage regulator. As with all troubleshooting techniques we must know what is normal.

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

TroubleshootingWith an open zener diode, the full unregulated voltage will be present at the output without a load. In some cases with full or partial loading an open zener could remain undetected.

TroubleshootingWith excessive zener impedance the voltage would be higher than normal but less than the full unregulated output.

Summary The zener diode operates in reverse breakdown.

A zener diode maintains a nearly constant voltage across 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 specific voltage for different loads. There are other diode types used for specific RF purposes such as varactor diodes (variable capacitance), Schottky diodes (high speed switching), and PIN diodes (microwave attenuation and switching).

Summary Light emitting diodes (LED) emit either infrared or visible light when forward-biased. Photodiodes exhibit an increase in reverse current with light intensity. The laser diode emits a monochromatic light

Chapter 4Bipolar Junction

Transistors

Objectives Describe the basic structure of the bipolar junction transistor (BJT) Explain and analyze basic transistor bias and operation

Discuss how a transistor can be used as an amplifier or a switch

Troubleshoot various failures typical of transistor circuits

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

IntroductionA transistor is a device that can be used as either an amplifier or a switch. Let’s first consider its operation in 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(right side) circuit. Note that the emitter leg serves as a conductor for both circuits.The amount of current flow in the base-emitter circuit controls the amount of current that 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 layers and two p-n junctions. Transistors can be either pnpor npn type.

Transistor Characteristics and Parameters

As previously discussed, base-emitter current changes yield large changes in collector-emitter current. The factor of this change is called beta(β).

β = IC/ IB

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

be considered. Note that these measurements are important for troubleshooting. IB: dc base current

IE: dc emitter current

IC: dc collector current

VBE: dc voltage across base-emitter junction

VCB: dc voltage across collector-base junction

VCE: 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 and blocks current flow through it’s junction just like a diode.

Remember that current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter.

Transistor Characteristics and ParametersAnalysis of this transistor circuit to predict the dc voltages and

currents 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 determine the amount of base current. Using Kirchhoff’s voltage law, subtract the .7 VBE and the remaining voltage is dropped across RB. Determining the current for the base with this information is a matter of applying of Ohm’s law. VRB/RB = IB

The collector current is determined by multiplying the base current by beta.

.7 VBE will be used in most analysis examples.

Transistor Characteristics and ParametersWhat we ultimately

determine by use of Kirchhoff’s voltage law for series circuits is that in the base circuit VBB is distributed across the base-emitter junction and RB in the base circuit. In the collector circuit we determine that VCC is distributed proportionally across RC and the transistor(VCE).

Transistor Characteristics and ParametersCollector characteristic

curves give a graphical illustration of the relationship of collector current and VCE with specified amounts of base current. With greater increases of VCC , VCEcontinues to increase until it reaches breakdown, but the current remains about the same in the linearregion from .7V to the breakdown voltage.

Transistor Characteristics and ParametersWith no IB the transistor is in the cutoff region and just

as the name implies there is practically no current flow in the collector part of the circuit. With the transistor in a cutoff state the the full VCC can be measured across the collector and emitter(VCE)

Transistor Characteristics and Parameters

Current flow in the collector part of the circuit is, as stated previously, determined by IB multiplied by β. However, there is a limit to how much current can flow in the collector circuit regardless of additional increases in IB.

Transistor Characteristics and ParametersOnce this maximum is reached, the transistor is said to

be in saturation. Note that saturation can be determined by application of Ohm’s law. IC(sat)=VCC/RC The measured voltage across the now “shorted” collector and emitter is 0V.

Transistor Characteristics and ParametersThe dc load line graphically illustrates IC(sat) and cutoff for a

transistor.

Transistor Characteristics and Parameters

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

There are also maximum power ratings to consider.

The data sheet provides information on these characteristics.

Transistor AmplifierAmplification of a relatively small ac voltage can be had by placing the ac signal source in the base circuit.

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

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

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

Troubleshooting

Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm’s law and Kirchhoff’s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.

Troubleshooting

Internal opens within the transistor itself could also cause transistor operation to cease.

Erroneous voltage measurements that 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 typical failures are discussed within the textbook.

Opens in the external resistors or connections of the base or the circuit collector circuit would cause current to cease in the collector and the voltage measurements would indicate this.

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

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

Troubleshooting

In addition to the traditional DMMs there are also transistor testers. Some of these have the ability to test other parameters of the transistor, such as leakage and gain. Curve tracers give us even more detailed information about a transistors characteristics.

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 is reverse-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.

Summary A transistor can be operated as an electronics switch.

When the transistor is off it is in cutoff condition (no current). When the transistor is on, it is in saturation condition (maximum current).

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

Chapter 5 Transistor Bias

Circuits

Objectives

Discuss the concept of dc biasing of a transistor for linear operation Analyze voltage-divider bias, base bias, and collector-feedback bias circuits.

Basic troubleshooting for transistor bias circuits

Introduction

For the transistor to properly operate it must be biased. There are several methods to establish the DC operating point. We will discuss some of the methods used for biasing transistors as well as troubleshooting methods used for transistor bias circuits.

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 saturation and cutoff is called the Q-point. The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied.

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 curves for this particular transistor we see that 30 mA of collector current is best for maximum amplification, giving equal amount above and below the Q-point. Note that this is three different scenarios of collector current being viewed simultaneously.

The DC Operating PointWith a good Q-point established, let’s look at the effect a superimposed ac voltage has on the circuit. Note the collector current swings do not exceed the limits of operation(saturation and cutoff). However, as you might already know, applying too much ac voltage to the base would result in driving the collector current into saturation or cutoff resulting in a distorted or clipped waveform.

Voltage-Divider Bias

Voltage-divider bias is the most widely used type of bias circuit. Only one power supply is needed and voltage-divider bias is more stable(β independent)than other bias types. For this reason it will be the primary focus for study.

Voltage-Divider BiasApply your knowledge of voltage-dividers to understand how R1 and R2are used to provide the needed voltage to point A(base). The resistance to ground from the base is not significant enough to consider in most cases. Remember, the basic operation of the transistor has not changed.

Voltage-Divider BiasIn the case where base to ground resistance(input resistance) is low enough to consider, we can determine it by the simplified equation RIN(base) = βDCRE

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.

Voltage-Divider Bias

For this circuit we will not take the input resistance into consideration. Essentially we are determining the voltage across R2(VB) by the proportional method.

VB = (R2/R1 + R2)VCC

Voltage-Divider BiasWe now take the known base voltage and subtract VBE to find out what is dropped across RE. Knowing the voltage across REwe can apply Ohm’s law to determine the current in the collector-emitter side of the circuit. Remember the current in the base-emitter circuit is much smaller, so much in fact we can for all practical purposes we say that IE approximately equals IC.

IE≈ IC

Voltage-Divider Bias

Although we have used npn transistors for most of this discussion, 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 β changes with temperature and collector current. Base biasing circuits are mainly limited to switching applications.

Emitter BiasThis type of circuit is independent of β making it as stable as the voltage-divider type. The drawback is that it requires two power supplies.

Two key equations for analysis of this type of bias circuit are shown below. With these two currents known we can apply Ohm’s law and Kirchhoff's law to solve for the voltages. IB ≈ IE/β

IC ≈ IE ≈ -VEE-VBE/RE + RB/βDC

Collector-Feedback BiasCollector-feedback bias is kept stable with negativefeedback, although it is not as stable as voltage-divider or emitter. With increases of IC, less voltage is applied to the base. With less IB ,IC comes down as well. The two key formulas are shown below.

IB = VC - VBE/RB

IC = VCC - VBE/RC + RB/βDC

TroubleshootingShown is a typical voltage divider circuit with correct voltage readings. Knowing these voltages is a requirement before logical troubleshooting can be applied. We will discuss some of the faults and symptoms.

TroubleshootingR1 Open

With no bias the transistor is in cutoff.

Base voltage goes down to 0 V.

Collector voltage goes up to 10 V(VCC).

Emitter voltage goes down to 0 V.

TroubleshootingResistor RE Open:

Transistor is in cutoff.

Base reading voltage will stay approximately the same.

Collector voltage goes up to 10 V(VCC).

Emitter voltage will be approximately the base voltage + .7 V.

TroubleshootingBase Open Internally:

Transistor is in cutoff.

Base voltage stays approximately the same.

Collector voltage goes up to 10 V(VCC).

Emitter voltage goes down to 0 V.

TroubleshootingOpen BE Junction:

Transistor is in cutoff.

Base voltage stays approximately the same.

Collector voltage goes up to 10 V(VCC)

Emitter voltage goes down to 0 V.

TroubleshootingOpen BC Junction:

Base voltage goes down to 1.11 V because of more base current flow through emitter.

Collector voltage goes up to 10 V(VCC).

Emitter voltage will drop to .41 V because of small current flow from forward-biased base-emitter junction.

TroubleshootingRC Open:

Base voltage goes down to 1.11 V because of more current flow through the emitter.

Collector voltage will drop to .41 V because of current flow from forward-biased collector-base junction.

Emitter voltage will drop to .41 V because of small current flow from forward-biased base-emitter junction.

TroubleshootingR2 Open:

Transistor pushed close to or into saturation.

Base voltage goes up slightly to 3.83V because of increased bias.

Emitter voltage goes up to 3.13V because of increased current.

Collector voltage goes down because of increased conduction of transistor.

Summary

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

The Q-point is the best point for operation of a transistor for a given collector current. The dc load line helps to establish the Q-point for a given 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 is stable and uses only one voltage supply.

Base bias is very unstable because it is β dependent.

Emitter bias is stable but require two voltage supplies.

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

Chapter 6BJT 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 to amplify ac signals. This could be an audio signal or perhaps some high frequency radio signal. It has to be able to do this without distorting 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 relate the 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 into saturation 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 terms of just its ac component.

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

Amplifier OperationThe boundary between cutoff and saturation is called the linear region. A transistor which operates in the linear region is called a linear amplifier. Note that only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of phase with input.

Transistor Equivalent CircuitsWe can view transistor circuits by use of resistance or r parameters for better understanding. Since the base resistance, rb is small it normally is not considered and since the collector resistance, rc is fairly high we consider it as an open. The emitter resistance, rc is the main parameter that is viewed.

You can determine rc from this simplified equation.

rc = 25 mV/IE

Transistor Equivalent CircuitsThe two graphs best illustrate the difference between βDC and βac. The two only differ slightly.

Transistor Equivalent CircuitsSince r parameters are used throughout the rest of the textbook we will not go into deep discussion about hparameters. However, since some data sheets include or exclusively provide h parameters these formulas can be used to convert them to r parameters.

r’e = hre/ hoe

r’c = hre + 1/ hoe

r’b = hie - (1+ hfe)

The Common-Emitter AmplifierThe common-emitter amplifier exhibits high voltage and

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

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

The Common Emitter Amplifier

DC Analysis The dc component of the circuit “sees” only the part of the circuit that is within the boundaries of C1, C2, and C3 as the dc will not pass through these components. The equivalent circuit for dc analysis is shown.

The methods for dc analysis are just are the same as dealing with a voltage-divider circuit.

Common Emitter AmplifierAC Equivalent Circuit

The ac equivalent circuit basically replaces the capacitors with shorts, being that ac passes through easily through them. The power supplies are also effectively shorts to ground for ac analysis.

Common Emitter AmplifierAC Equivalent Circuit

We can look at the input voltage in terms of the equivalent base circuit (ignore the other components from the previous diagram). Note the use of simple series-parallel analysis skills for determining Vin.

Common Emitter Amplifier AC Equivalent Circuit

The input resistance as seen by the input voltage can be illustrated by the r parameter equivalent circuit. The simplified formula below is used.

R in(base) = βacr’e

The output resistance is for all practical purposes the value of RC.

Common Emitter AmplifierAC Equivalent Circuit

Voltage gain can be easily determined by dividing the ac output voltage by the ac input voltage.

Av = Vout/ Vin = Vc/ Vb

Voltage gain can also be determined by the simplified formula below.

Av = RC/r’e

Common Emitter AmplifierAC Equivalent Circuit

Taking the attenuationfrom the ac supply internal resistance and input resistance into consideration is included in the overall gain.

A’v = (Vb/Vs)Av

or

A’v = Rin(total)/ Rs + Rin(total)

The Common-Emitter Amplifier

The emitter bypass capacitor helps increase the gain by allowing the ac signal to pass more easily.

The XC(bypass) should be about ten times less than RE.

The Common-Emitter Amplifier

The bypass capacitor makes the gain unstable since transistor amplifier becomes more dependent on IE. This effect can be swamped or somewhat alleviated by adding another emitter resistor(RE1).

The Common-Collector AmplifierThe common-collector amplifier is usually referred

to as the emitter follower because there is no phase inversion or voltage gain. The output is taken from the emitter. The common-collector amplifier’s main advantages are its high current gain and high input resistance.

The Common-Collector Amplifier

Because of its high input resistance the common-collector amplifier used as a buffer to reduce the loading effect of low impedance loads. The input resistance can be determined by the simplified formula below.

R in(base) ≅ βac(r’e + Re)

The Common-Collector Amplifier

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

The current gain(Ai) is approximately βac.

The power gain is approximately equal to the current gain(Ai).

The voltage gain is approximately 1.

The Common-Collector AmplifierThe darlington pair is used to boost the input impedance to reduce loading of high output impedance circuits. The collectors are joined together and the emitter of the input transistor is connected to the base of the output transistor. The input impedance can be determined the formula below.

R in = βac1βac2Re

The Common-Base AmplifierThe common-base amplifier has high voltage gain with a current gain no higher than 1. It has a low input resistance making it ideal for low impedance input sources. The ac signal is applied to the emitter and the output is taken from the collector.

The Common-Base AmplifierThe common-base voltage gain(Av) is approximately equal to Rc/r’eThe 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 RC.

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

A’v = Av1Av2Av3 ……

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

A v(dB) = 20logAv

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

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

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

Multistage AmplifiersDirect coupling between stage improves low frequency gain. The disadvantage is that small changes in dc bias from temperature changes or supply variations becomes more pronounced.

Troubleshooting

Troubleshooting techniques for transistor amplifiers is similar to techniques covered in Chapter 2. Usage of knowledge of how an amplifier works, symptoms, and signal tracing are all valuable parts of troubleshooting. Needless to say experience is an excellent teacher but having a clear understanding of how these circuits work makes the troubleshooting process more efficient and understandable.

TroubleshootingThe following slide is a diagram for a two stage common-emitter amplifier with correct voltages at various points.

Utilize your knowledge of transistor amplifiers and troubleshooting techniques and imagine what the effects would be with various faulty components—for example, open resistors, shorted transistor junctions or capacitors. More importantly, how would the output be affected by these faults? In troubleshooting it is most important to understand the operation of a circuit.

What faults could cause low or no output?

What faults could cause a distorted output signal?

Troubleshooting

Summary

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

The common-emitter amplifier has high voltage and current gain.

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

Most transistors amplifiers are designed to operate in the linear region.

Summary

Multistage amplifiers are amplifier circuits cascaded to increased gain. We can express gain in decibels (dB).

Troubleshooting techniques used for individual transistor circuits can be applied to multistage amplifiers as well.

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