ISU EE C.Y. Lee

Chapter 17Transistors and

Applications

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Objectives

Describe the basic structure and operation of bipolar junction transistors (BJT)Explain the operation of a BJT class A amplifierAnalyze class B amplifiersAnalyze a transistor switching circuitDescribe the basic structure and operation of JFETsand MOSFETsAnalyze two types of FET amplifier configurations

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Bipolar Junction Transistors (BJTs)

The bipolar junction transistor (BJT) is constructed with three doped semiconductor regions separated by two pn junctionsRegions are called emitter, base and collector

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Bipolar Junction Transistors (BJTs)

The term bipolar refers to the use of both holes and electrons as charge carriers in the transistor structureIn order for the transistor to operateproperly, the two junctions must have the correct dc bias voltages the base-emitter (BE) junction is

forward biased the base-collector (BC) junction is

reverse biased

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DC Operation of Bipolar Junction Transistors (BJTs)

Transistor Currents:IE = IC + IB

alpha (DC):DC = IC/IE

beta (DC):DC = IC/IB

DC typically has a value between 20 and 200

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DC Operation of Bipolar Junction Transistors (BJTs)

DC voltages for the biased transistor:Collector voltage:

VC = VCC ICRC

Base voltage:VB = VE + VBE

for silicon transistors, VBE = 0.7 V

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DC Operation of Bipolar Junction Transistors (BJTs)

Example: Determine IB, IC, and VC in below circuit

IB = (10.7)/(22103) = 13.6 A

IC = DCIB = (50)(13.6106) = 680 A

VC = 10(680106)(1.0103)= 9.32 V

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DC Operation of Bipolar Junction Transistors (BJTs)

VBVC

VERIN

The two dc bias sources can be replaced by a single dc sourceInput resistance at the base:

Base voltage:

Emitter voltage:

EDCIN RR

CCB VRRRV

+

21

2

V.VV BE 70=

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DC Operation of Bipolar Junction Transistors (BJTs)

Example: Determine VB, VE, VC, VCE, IB, IE, and IC

VB (10k/32k)(30V) = 9.38 VVE = 9.38V0.7V = 8.68 VIE = VE/RE = 8.68V/1k = 8.68 mAIC = IEIB IE = 8.68 mAIB = IC/DC = 8.68mA/100 = 86.8 AVC = VCCICRC = 30V(8.68mA)(1k)

= 21.3 VVCE = VCVE = 21.3V8.68V = 12.6 V

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BJT Class A AmplifiersGenerally, class A amplifiers are used in low-power applicationsCollector characteristic curves:

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BJT Class A Amplifiers

The transistor is operated in the active (or linear)region, between saturation and cutoff saturation is when both

junctions are forward biased the transistor is in cutoff

when IB = 0The load line is drawn on the collector curves between saturation and cutoff

In a class A amplifier, the transistor conducts for the full cycle of the input signal (360)

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BJT Class A Amplifiers

The base current, IB, is established by the base biasThe point at which the base current curve intersects the dc load line is the quiescent or Q-point for the circuit

(CE)

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BJT Class A Amplifiers

A common-emitter (CE) amplifier voltage gain , current gain, and power gain are all

greater than 1

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BJT Class A Amplifiers

A common-collector (CC) amplifier voltage gain is approximately 1, but current gain is

greater than 1

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BJT Class A Amplifiers

The third configuration is the common-base (CB) the base is the grounded (common) terminal the input signal is applied to the emitter output signal is taken off the collector output is in-phase with the input voltage gain is greater than 1 current gain is always less than 1

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BJT Class B AmplifiersWhen an amplifier is biased such that it operates in the linear region for 180 of the input cycle and is in cutoff for 180, it is a class B amplifier Class B amplifier is more efficient than a class A, you can

get more output power for a given amount of input power

(CC)

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BJT Class B AmplifiersIn order to get a linear reproduction of the input waveform, the class B amplifier is configured in a push-pull arrangement

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BJT Class B AmplifiersIllustration of crossover distortion in a class B push-pull amplifier:

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BJT Class B AmplifiersThe transistors in a class B amplifier must be biased above cutoff to eliminate crossover distortion

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The BJT as a SwitchWhen used as an electronic switch, a transistor normally is operated alternately in cutoff and saturation

A transistor is in cutoff when the base-emitter junction is not forward-biased. VCE is approximately equal to VCC

When the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated

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Field-Effect Transistors (FETs)The junction field-effect transistor (JFET) is operated with a reverse biased junction to control current in a channel the device is identified by the material in the channel,

either n-channel or p-channel

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Field-Effect Transistors (FETs)The channel of JFET is formed between the gate regions controlling the reverse biasing voltage on the gate-to-

source junction controls the channel size and the drain current, ID

I I

ID ID

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Field-Effect Transistors (FETs)The metal-oxide semiconductor field-effect transistor (MOSFET) differs from the JFET in that it has no pn junction; instead, the gate is insulated from the channel by a silicon dioxide (SiO2) layer

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Field-Effect Transistors (FETs)MOSFETs may be depletion mode (D-MOSFET) or enhancement mode (E-MOSFET)

I I

(Normally ON)(D-mode)

p

ID ID

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Field-Effect Transistors (FETs)MOSFETs may be depletion mode (D-MOSFET) or enhancement mode (E-MOSFET)

I

(Normally OFF)(E-mode)

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Field-Effect Transistors (FETs)

D-MOSFET Channel may be

enhanced or restricted by gate voltage

E-MOSFET Channel is created by

gate voltage

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FET AmplifiersVoltage gain of a FET is determined by the transconductance (gm) with units of Siemens (S)

gm = Id / VgAll FETs provide extremely high input resistance

(CD Amplifier)(CS Amplifier)

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Cross-sectional View of IC

TRANSISTORS

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IC Fabrication Process1

2

3 /

5 IC

4

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Summary

A bipolar junction transistor (BJT) consists of three regions: emitter, base, and collectorThe two types of bipolar transistor are the npn and the pnpThe term bipolar refers to two types of current: electron current and hole currentA field-effect transistor (FET) has three regions: source, drain, and gate

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Summary

A junction field-effect transistor (JFET) is operated with a reverse-biased gate-to-source pn junctionJFET current between the drain and source is through a channel whose width is controlled by the amount of reverse bias on the gate-source junctionThe two types of JFETs are n-channel and p-channelMetal-oxide semiconductor field-effect transistors (MOSFETs) differ from JFETs in that the gate of a MOSFET is insulated from the channel

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Summary

The D-MOSFET has a physical channel between the drain and the sourceThe E-MOSFET has no physical channelTwo main types of BJT amplifier configurations are the common-emitter (CE) and common collector (CC). A third type is the common base (CB)Two main types of FET amplifier configurations are common-source (CS) and common-drain (CD)

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SummaryThe class A amplifier conducts for the entire 360of the input cycle and is normally used for low-power applicationsThe class B amplifier conducts for 180 of the input cycle and is normally used for high-power applications

Chapter 17ObjectivesBipolar Junction Transistors (BJTs)Bipolar Junction Transistors (BJTs)DC Operation of Bipolar Junction Transistors (BJTs)DC Operation of Bipolar Junction Transistors (BJTs)DC Operation of Bipolar Junction Transistors (BJTs)DC Operation of Bipolar Junction Transistors (BJTs)DC Operation of Bipolar Junction Transistors (BJTs)BJT Class A AmplifiersBJT Class A AmplifiersBJT Class A AmplifiersBJT Class A AmplifiersBJT Class A AmplifiersBJT Class A AmplifiersBJT Class B AmplifiersBJT Class B AmplifiersBJT Class B AmplifiersBJT Class B AmplifiersThe BJT as a SwitchField-Effect Transistors (FETs)Field-Effect Transistors (FETs)Field-Effect Transistors (FETs)Field-Effect Transistors (FETs)Field-Effect Transistors (FETs)Field-Effect Transistors (FETs)FET AmplifiersCross-sectional View of ICIC Fabrication ProcessSummarySummarySummarySummary