B stad_BJT

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

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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

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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:

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Common-Base ConfigurationCommon-Base Configuration

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

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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.

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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

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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.

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EI

CI ≅

Silicon)(for V 0.7BEV =

ApproximationsApproximations

Emitter and collector currents:

Base-emitter voltage:

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

Alpha (Alpha (αα ))

Alpha (α) is the ratio of IC to IE :

EI

CIα =dc

Alpha (α) 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.

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Common-Emitter CharacteristicsCommon-Emitter Characteristics

Collector Characteristics Base Characteristics

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Common-Emitter Amplifier CurrentsCommon-Emitter Amplifier Currents

Ideal CurrentsIdeal Currents

IE = IC + IB IC = α IE

Actual CurrentsActual Currents

IC = α IE + ICBO

When IB = 0 µA 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

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



Beta (Beta (ββ ))

In DC mode:

In AC mode:

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

B

C

I

Iβ =dc

constantac =∆∆=

CEVB

C

I

Iβ

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Determining β from a Graph

Beta (Beta (ββ ))

108

A 25

mA 2.7β 7.5VDC CE

=µ

= =

100

μA 10

mA 1

μA) 20 μA (30

mA) 2.2mA (3.2β

7.5V

AC

CE

=

=

−−=

=

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Relationship between amplification factors β and α

1β

βα

+=

1α

αβ

−=

Beta (Beta (ββ ))

Relationship Between Currents

BC βII = BE 1)I(βI +=

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Common–Collector ConfigurationCommon–Collector Configuration

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

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Common–Collector ConfigurationCommon–Collector Configuration

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

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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

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Power DissipationPower Dissipation

Common-collector:

CCBCmax IVP =

CCECmax IVP =

ECECmax IVP =

Common-base:

Common-emitter:

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Transistor Specification SheetTransistor Specification Sheet

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Transistor Specification SheetTransistor Specification Sheet

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Transistor TestingTransistor Testing• Curve TracerCurve Tracer

Provides a graph of the characteristic curves.

• DMMDMMSome DMMs measure βDC or hFE.

• OhmmeterOhmmeter

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Transistor Terminal IdentificationTransistor Terminal Identification

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B stad_BJT

Engineering

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