9. transistors.rr

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TRANSISTORS

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History of Transistors

If cells are the building blocks of life, transistors are the building blocks of the digital revolution. Without transistors, the technological wonders you use every day -- cell phones, computers, cars -- would be vastly different, if they existed at all.

Before transistors, product engineers used vacuum tubes and electromechanical switches to complete electrical circuits.

In the late 1920's, Polish American physicist Julius Lilienfeld filed patents for a three-electrode device made from copper sulfide.

1947-John Bardeen, Walter Brattain and William Shockley, used the element germanium to create an amplifying circuit, also called a point-contact transistor. Soon afterward, Shockley improved on their idea by developing a junction transistor.

In 1952, transistorized hearing aids hit the market.

In 1954, George Teal, a scientist at Texas Instruments, created the first silicon transistor.

With smart engineering, transistors helped computers power through huge numbers of calculations in a short time.

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History of TransistorsThe simple switch operation of transistors is what enables your computer to complete massively complex tasks. One computer chip can have millions of transistors continually switching, helping complete complex calculations.

In a computer chip, the transistors aren't isolated, individual components. They're part of what's called an integrated circuit (also known as a microchip), in which many transistors work in concert to help the computer complete calculations. An integrated circuit is one piece of semiconductor material loaded with transistors and other electronic components.

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A transistor is a semiconductor device used to amplify and switch electronic signals.

It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit.

A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals.

Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal.

Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Transistor Construction

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Transistor Construction• A transistor has three doped regions.• For both types, the base is a narrow region sandwiched

between the larger collector and emitter regions.

Fig. 28-1

The emitter region is heavily doped and its job is to emit carriers into the base.

The base region is very thin and lightly doped. Most of the current carriers injected into the base pass on to the collector.

The collector region is moderately doped and is the largest of all three regions.

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For a transistor to function properly as an amplifier, the emitter-base junction must be forward-biased and the collector-base junction must be reverse-biased.

The common connection for the voltage sources are at the base lead of the transistor.The emitter-base supply voltage is designated VEE and the collector-base supply voltage is designated VCC.

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Proper Transistor Biasing

Fig. 28-4 shows transistor biasing for the common-base connection.

Proper biasing for an npn transistor is shown in (a).

The EB junction is forward-biased by the emitter supply voltage, VEE.

VCC reverse-biases the CB junction.

Fig. 28-4 (b) illustrates currents in a transistor.

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Proper Transistor Biasing

the electrons in the n-type emitter are repelled into the base by the negative terminal of the emitter supply voltage VEE.Since the base is very thin, only a few electrons combine with holes in the base.The small current flowing out of the base lead, IB,is called recombination current because free electrons injected into the base must fall into a hole before they can flow out of the base lead.Most of the emitter-injected electrons pass through the base region and into the collector region.Only a small voltage is needed in the collector-based junction to collect almost all free electrons injected into the base.If the voltage across the collector-base junction is too large, the breakdown voltage may be exceeded, which could destroy the transistor.IE = IB + IC

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Common-Base (CB) Connection

dc alpha- dc, a characteristic that describes how closely the emitter

and collector currents are in a common base;

dc = IC/ IE

The base lead is common to both the input and output sides of the circuit.

In most cases, dc alpha is 0.99 or greater.The thinner and lightly doped the base, the closer alpha is to unity.

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Common-Emitter (CE) Connection

n

p

n

IC

IC

IE = IB + IC

IC

IB

IB

VBB

VCC

The emitter lead is common to both the input and output sides of the circuit.

dc beta- dc, a characteristic that describes the dc current gain of a transistor in the CE connection;

dc = IC/ IB

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Transistor as Amplifier

When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly.

The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.

When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off.

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Transistor Ratings

• A transistor, like any other device, has limitations on its operations.

• These limitations are specified in the manufacturer’s data sheet.

• Maximum ratings are given for – Collector-base voltage, VCB

– Collector-emitter voltage, VCE

– Emitter-base voltage, VEB

– Collector current, IC

– Power dissipation, Pd(max) = VCE x IC

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Derating Factor

The power dissipation rating of a transistor is usually given at 25 oC.

At higher temperatures, the power dissipation rating is less.

For most silicon transistors, the maximum allowable junction temperature is between 150 oC to 200 oC.

The transistor’s power dissipation rating must be kept to less than its rated value so that the junction temperature will not reach its destructive levels.

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Derating Factor

Manufacturers supply the derating factor for determining the power dissipation rating at any temperature above 25 oC.the derating factor is given in watts per degree celsius, W/ oC

e.g. 3 mW/oC means in every 1 oC increase in junction temp.,the power rating is reduced by 3 mW.

Pd = (T )(derating factor)

Where Pd = change in power rating Pd initial - Pd = new Power rating T = change in temperature

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Checking a Transistor with an Ohmmeter

An analog ohmmeter can be used to check a transistor because the emitter-base and collector-base junctions are p-n junctions. This is illustrated in Fig. 28-8 where the npn transistor is replaced by its diode equivalent circuit.

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Checking a Transistor with an Ohmmeter

Fig. 28-9

To check the base-emitter junction of an npn transistor, first connect the ohmmeter as shown in Fig. 28-9 (a) and then reverse the ohmmeter leads as shown in (b). For a good p-n junction made of silicon, the ratio RR/RF should be equal to or greater than 1000:1.

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Checking a Transistor with an Ohmmeter

Fig. 28-10

To check the collector-base junction, first connect the ohmmeter as shown in Fig. 28-10 (a) and then reverse the ohmmeter leads as shown in (b). For a good p-n junction made of silicon, the ratio RR/RF should be equal to or greater than 1000:1. The resistance measured between the collector and emitter should read high or infinite for both connections of the meter leads.

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Checking a Transistor with an Ohmmeter

Low resistance across the junctions in both directions: transistor is shorted.

High resistance on both directions: transistor is open.

In these cases, the transistor is defective and must be replaced.

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Example Problems:

1. Solve for the unknown transistor current in each of the ff. cases:

a.) IE = 1 mA, IB = 5A, IC=?b.) IB = 50A, IC= 2.25 mA, IE=?c.) IC = 40m A, IE= 40.5 mA, IB=?d.) IE = 2.7A, IB= 30 mA, IC=?e.) IC = 3.65 mA, IE= 3.75 mA, IB=?

2. Calculate the dc alpha and the dc beta for each set of current values listed in problem #1.

3. Calculate the power dissipation, Pd, in a

transistor for each of the ff. values of VCE and IC:

a.) VCE = 5 V, IC = 20 mAb.) VCE = 20 V, IC = 50 mAc.) VCE = 24 V, IC = 300 mA

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Example Problems:

4. A transistor has a power rating of 1.5 W at an ambient temperature, TA, of 25oC. If the derate factor is 12 mW/oC, what is the transistor’s power rating at each of the ff. temperatures?

a.) 50oCb.) 75oCc.) 100oCd.) 125oC

5. A transistor has a power rating of 2 W. Calculate the maximum allowable collector current, IC(max),

for each of the following values of VCE :a.) 5 Vb.) 12 Vc.) 25 V

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