Funtion Transistor

7/27/2019 Funtion Transistor

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TRANSISTORS

A bipolar transistor consists of a three-layer "sandwich" of doped (extrinsic)semiconductor materials, either P-N-P

or N-P-N. Each layer forming thetransistor has a specific name (Emitter,Base and Collector), and each layer is

provided with a wire contact forconnection to a circuit


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PNP transistor

construction


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construction

NPN transistor


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FORWARD & REVERSE BIASED


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(a) The majority carriers in the emitter p-type material are holes

(b) The base-emitter junction is forward biased to the majority

carriers and the holes cross the junction and appear in the baseregion

(c) The base region is very thin and is only lightly doped withelectrons so although some electron-hole pairs are formed, many

holes are left in the base region

(d) The base-collector junction is reverse biased to electrons in thebase region and holes in the collector region, but forward biasedto holes in the base region; these holes are attracted by the

negative potential at the collector terminal

(e) A large proportion of the holes in the base region cross the basecollector junction into the collector region, creating a collectorcurrent; conventional current flow is in the direction of holemovement.


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(a) The majority carriers in the n-type emitter material are electrons

(b) The base-emitter junction is forward biased to these majoritycarriers and electrons cross the junction and appear in the baseregion

(c) The base region is very thin and only lightly doped with holes, sosome recombination with holes occurs but many electrons are left

in the base region

(d) The base-collector junction is reverse biased to holes in the baseregion and electrons in the collector region, but is forward biasedto electrons in the base region; these electrons are attracted by

the positive potential at the collector terminal

(e) A large proportion of the electrons in the base region cross thebase collector junction into the collector region, creating acollector current.


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TRANSISTOR AS A SWITCH


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Any sufficient source of DC current may beused to turn the transistor on, and thatsource of current need only be a fraction ofthe amount of current needed


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Construction Stage


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Operation

The base of theNPN transistor

must be positive

with respect to

the emitter,

And the

collector must

be more positivethan the base.


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Operation


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Biasing


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Biasing


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METER CHECK

OF ATRANSISTOR


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Testing of Transistor

TESTING A TRANSISTOR to determine if it is good orbad can be done with an ohmmeter or transistor tester orby the substitution method.

PRECAUTIONS should be taken when working with

transistors since they are susceptible to damage byelectrical overloads, heat, humidity, and radiation.

TRANSISTOR LEAD IDENTIFICATION plays animportant part in transistor maintenance because before atransistor can be tested or replaced, its leads must be

identified. Since there is NO standard method ofidentifying transistor leads, check some typical leadidentification schemes or a transistor manual beforeattempting to replace a transistor.


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Multimeter without diode check


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Meter readings will beexactly opposite, of

course, for an NPNtransistor, with both PNjunctions facing the

other way


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If a multimeter with a Diode Check" function is

used in this test, it will be found that theemitter-base junction possesses a slightlygreater forward voltage drop than the

collector-base junction. This forward voltage

difference is due to the disparity in dopingconcentration between the emitter and collectorregions of the transistor: the emitter is a much

more heavily doped piece of semiconductor

material than the collector, causing its junctionwith the base to produce a higher forward

voltage drop.


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Meter touching wire 1 (+) and 2 (-): "OL"Meter touching wire 1 (-) and 2 (+): "OL"Meter touching wire 1 (+) and 3 (-): 0.655volts*

Meter touching wire 1 (-) and 3 (+): "OL"Meter touching wire 2 (+) and 3 (-): 0.621 volts*Meter touching wire 2 (-) and 3 (+): "OL"

Using multimeter with Diode Check, the dataobtained:

* Indicating forward biasing of the emitter-to-basejunction and the collector-to-base junction


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In both those sets of meterreadings (*), the black(-) meter

test lead was touching wire 3,which tells us that the base of thistransistor is made of N-typesemiconductor material (black =

negative). Thus, the transistor isan PNP type with base on wire 3,emitter on wire 1 and collector onwire 2.

Wire 3 is common to both sets of conductive readings.Thus it must be the base connection of the transistor,

because the base is the only layer of the three-layerdevice common to both sets of PN junctions.


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Identification

An easy way to identify a specific transistorconfiguration is to follow three simple steps:

Identify the element (emitter, base, or

collector) to which the input signal is applied. Identify the element (emitter, base, or

collector) from which the output signal is

taken. The remaining element is the common

element, and gives the configuration its name.


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TRANSISTOR RATINGSPower dissipation

Transistors are rated in terms ofhow many watts theycan safely dissipate without sustaining damage. Hightemperature is the mortal enemy of all semiconductordevices, and bipolar transistors tend to be moresusceptible to thermal damage than most.

Reverse voltagesAs with diodes, bipolar transistors are rated formaximum allowable reverse-bias voltage across theirPN junctions.

Collector currentA maximum value for collector current will be given bythe manufacturer in amps.


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Saturation voltages

Ideally, a saturated transistor acts as a closed switchcontact between collector and emitter, dropping zerovoltage at full collector current.

Beta: The ratio of collector current to base current, isthe fundamental parameter characterizing theamplifying ability of a bipolar transistor.


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Transistor as Amplifier The key to understanding how

amplifiers can exist withoutviolating the Law of Conservation

of Energy lies in the behavior of

active devices.

The result is a device that appears

to magically magnify the power

of a small electrical signal

(usually an AC voltagewaveform) into an identically-

shaped waveform of larger

magnitude.


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Perfect or Imperfect Machine

There does exist, however, a class of machines known asamplifiers, which are able to take in small-power signals andoutput signals of much greater power.

The Law of Conservation of Energy is not violated because :

- The additional power is supplied by an external source, usuallya DC battery or equivalent.

- The power output of a machine can approach, but never exceed,

the power input for100% efficiency as an upper limit.-A realistic machine most often loses some of its input energy as

heatin transforming it into the output energy stream.

-Hypothetical perpetual motion machinepowers itself?


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Amplifier

Amplifier can scale a small input signal to large

output, its energy source is an external power supply.

Amplifiers, like all machines, are limited in efficiency

to a maximum of 100 percent. Usually, electronic amplifiers are far less efficient

than that, dissipating considerable amounts of energy

in the form of waste heat.

Because the efficiency of an amplifier is always 100

percent or less, one can never be made to function as

a perpetual motion device.


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Amplifier

The requirement of an external source of poweris common toall types of amplifiers, electrical and non-electrical.

A common analogy of a non-electrical amplification systemwould be power steering in an automobile, amplifying the

power of the driver's arms in turning the steering wheel tomove the front wheels of the car. The source of powernecessary for the amplification comes from the engine. Theactive device controlling the driver's input signal is ahydraulic valve shuttling fluid power from a pump attached to

the engine to a hydraulic piston assisting wheel motion. If theengine stops running, the amplification system fails to amplifythe driver's arm power and the car becomes very difficult toturn.


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Connection


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COMMON-BASE AMPLIFIER

Both the signal source and the load share thebase lead as a common connection point


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Gain

The term hfe used in place of b. The terms hfe and b areequivalent and may be used interchangeably. This is because"hfe" means:

h = hybrid (meaning mixture)

f = forward current transfer ratio

e = common emitter configuration

The resistance gain of the common emitter can be found in amethod similar to the one used for finding beta:

Once the resistance gain is known, the voltage gain is easy to

calculate since it is equal to the current gain multiplied by theresistance gain (E = bR).

And, the power gain is equal to the voltage gain multiplied bythe current gain b (P = bE).


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Example: Measurements at several points of interestusing oscilloscope


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COMMON-EMITTER AMPLIFIER

Both the signal source and the load share theemitter lead as a common connection point


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Example: Measurements at several points of interestusing oscilloscope


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COMMON-COLLECTOR

AMPLIFIER

Both the signal source and the load share thecollector lead as a common connection point


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Example: Measurements at several points ofinterest using oscilloscope


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Transistor's Characteristic Curves


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ACTIVE MODE OPERATIONWhen a transistor is inthe fully-off state (likean open switch), it issaid to be cutoff.

Conversely, when it isfully conductivebetween emitter andcollector (passing as

much current throughthe collector as thecollector power supplyand load will allow), it issaid to besaturated.


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CLASS A AMPLIFIERClass A operation is

where the entireinput waveform isfaithfullyreproduced.

Class A operationcan only be obtainedwhen the transistorspends its entire

time in the activemode, neverreaching eithercutoff or saturation


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CLASS B AMPLIFIER

Class B operation isthe transistor spenthalf its time inactive mode and theother half in cutoff

with the inputvoltage too low (oreven of the wrong

polarity!) toforward-bias itsbase-emitterjunction.


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Class Input and Output

FIDELITY and EFFICIENCYare two terms used inconjunction with amplifiers.

Fidelity is the faithful

reproduction of a signal, while Efficiency is the ratio ofoutput

signal power compared to thetotal input power.

The class A amplifier has thehighest degree of fidelity, butthe class C amplifier has thehighest efficiency.


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Typical Configuration This illustration is a class A amplifier

configured as a common emitter using

fixed bias. From this, you should beable to conclude the following:

Because of its fixed bias, theamplifier is thermally unstable.

Because of its class A operation, theamplifier has low efficiency but goodfidelity.

Because it is configured as a commonemitter, the amplifier has goodvoltage, current, and power gain.

In conclusion, the type of bias, classof operation, and circuit configuration

are all clues to the function andpossible application of the amplifier.


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Example

If the input current (IB) in a common emitter changes

from 75 mA to 100 mA and the output current (IC)

changes from 1.5 mA to 2.6 mA, the current gain (b)

will be 44.

This simply means that a change in base current

produces a change in collector current which is 44

times as large.


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Amplifier Rating

Because amplifiers have the ability to increase themagnitude of an input signal, it is useful to be able torate an amplifier's amplifying ability in terms of anoutput/input ratio.

The technical term for an amplifier's output/inputmagnitude ratio isgain.

As a ratio of equal units (power out / power in,voltage out / voltage in, or current out / current in),gain is naturally a unitless measurement.

Mathematically, gain is symbolized by the capitalletter A.


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Example If an amplifier takes in

an AC voltage signal

measuring 2 volts RMS

and outputs an AC

voltage of 30 voltsRMS, it has an AC

voltage gain of 30

divided by 2, or 15:

If an amplifier with an

AC current gain of 3.5

is given an AC input

signal of 28 mA RMS,

the output will be 3.5times 28 mA, or 98 mA:


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Rating

Amplifiers often

amplify changes or

variations in input

signal magnitude(AC) at a different

ratio thansteady

input signal

magnitudes (DC).


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Rating


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Multistage Amplifier

If multiple amplifiers are staged, their respectivegains form an overall gain equal to the product(multiplication) of the individual gains. (Figure

below)

A 1 V signal were applied to the input of the gain of

3 amplifier in Figure above, a 3 V signal out of thefirst amplifier would be further amplified by a gain of5 at the second stage yielding 15 V at the final output.


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Heat Sink


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Application of Electronics From electric to electronic

Active versus passive devices

Diode

Transistor

MICROELECTRONICS is a broad term used to describe the use of integratedcircuits to miniaturize electronic equipment.

A PRINTED CIRCUIT BOARD (PCB) is a flat, insulating surface upon whichprinted wiring and miniaturized components are connected in apredetermined design and attached to a common base.

MODULAR CIRCUITRY is an assembly technique in which printed circuitboards are stacked and connected together to form a module. This techniqueincreases the packaging density of circuit components and results in aconsiderable reduction in the size of electronic equipment.

An INTEGRATED CIRCUIT is a device that integrates (combines) bothactive components (transistors, diodes, etc.) and passive components (resistors,capacitors, etc.) of a complete electronic circuit in a single chip.

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Funtion Transistor