Chapter 3: Bipolar Junction Transistors. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic

Chapter 3:Bipolar Junction Transistors

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Transistor ConstructionTransistor Construction

There are two types of transistors: • pnp • npn

The terminals are labeled: • E - Emitter• B - Base• C - Collector

pnppnp

npnnpn

22



Transistor OperationTransistor OperationWith the external sources, VEE and VCC, connected as shown:

• The emitter-base junction is forward biased• The base-collector junction is reverse biased

33



Currents in a TransistorCurrents in a Transistor

The collector current is comprised of two currents:

BICIEI

minorityCOI

majorityCICI

Emitter current is the sum of the collector and base currents:

44



Common-Base ConfigurationCommon-Base Configuration

The base is common to both input (emitter–base) and output (collector–base) of the transistor.

55



Common-Base AmplifierCommon-Base Amplifier

Input CharacteristicsInput Characteristics

This curve shows the relationship between of input current (IE) to input voltage (VBE) for three output voltage (VCB) levels.

66



This graph demonstrates the output current (IC) to an output voltage (VCB) for various levels of input current (IE).

Common-Base AmplifierCommon-Base Amplifier

Output CharacteristicsOutput Characteristics

77



Operating RegionsOperating Regions

• Active – Operating range of the amplifier.

• Cutoff – The amplifier is basically off. There is voltage, but little current.

• Saturation – The amplifier is full on. There is current, but little voltage.

88



EI

CI

Silicon)(for V 0.7BEV

ApproximationsApproximations

Emitter and collector currents:

Base-emitter voltage:

99



Ideally: = 1 In reality: is between 0.9 and 0.998

Alpha (Alpha ())

Alpha () is the ratio of IC to IE :

EI

CIα dc

Alpha () in the AC modeAC mode:

EI

CIα

Δ

Δac

1010



Transistor AmplificationTransistor Amplification

Voltage Gain:

V 50kΩ 5ma 10

mA 10

10mA20Ω

200mV

))((RL

IL

V

iI

LI

EI

CI

iR

iViIEI

Currents and Voltages:

1111

250200mV

50V

iV

LVvA



Common–Emitter ConfigurationCommon–Emitter Configuration

The emitter is common to both input (base-emitter) and output (collector-emitter).

The input is on the base and the output is on the collector.

1212



Common-Emitter CharacteristicsCommon-Emitter Characteristics

Collector Characteristics Base Characteristics

1313



Common-Emitter Amplifier CurrentsCommon-Emitter Amplifier Currents

Ideal CurrentsIdeal Currents

IE = IC + IB IC = IE

Actual CurrentsActual Currents

IC = IE + ICBO

When IB = 0 A the transistor is in cutoff, but there is some minority current flowing called ICEO.

μA 0

BICBO

CEO α

II

1

where ICBO = minority collector current

1414

ICBO is usually so small that it can be ignored, except in high power transistors and in high temperature environments.



Beta (Beta ())

In DC mode:

In AC mode:

represents the amplification factor of a transistor. ( is sometimes referred to as hfe, a term used in transistor modeling calculations)

B

C

I

Iβ dc

constantac

CEV

B

C

I

I

1515



Determining from a Graph

Beta (Beta ())

108

A 25

mA 2.7β 7.5VDC CE

100

μA 10

mA 1

μA) 20 μA (30

mA) 2.2mA (3.2β

7.5V

AC

CE

1616



Relationship between amplification factors and

1β

βα

1α

αβ

Beta (Beta ())

Relationship Between Currents

BC βII BE 1)I(βI

1717



Common–Collector ConfigurationCommon–Collector Configuration

The input is on the base and the output is on the emitter.

1818



Common–Collector ConfigurationCommon–Collector Configuration

The characteristics are similar to those of the common-emitter configuration, except the vertical axis is IE.

1919



VCE is at maximum and IC is at minimum (ICmax= ICEO) in the cutoff region.

IC is at maximum and VCE is at minimum (VCE max = VCEsat = VCEO) in the saturation region.

The transistor operates in the active region between saturation and cutoff.

Operating Limits for Each ConfigurationOperating Limits for Each Configuration

2020



Power DissipationPower Dissipation

Common-collector:

CCBCmax IVP

CCECmax IVP

ECECmax IVP

Common-base:

Common-emitter:

2121



Transistor Specification SheetTransistor Specification Sheet

2222



Transistor Specification SheetTransistor Specification Sheet

2323



Transistor TestingTransistor Testing• Curve TracerCurve Tracer

Provides a graph of the characteristic curves.

• DMMDMMSome DMMs measure DC or hFE.

• OhmmeterOhmmeter

2424



Transistor Terminal IdentificationTransistor Terminal Identification

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Documents

Chapter 3: Bipolar Junction Transistors. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic