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Introduction To BJT

The document provides an introduction to bipolar junction transistors (BJTs). It describes how BJTs are constructed with three doped semiconductor regions (emitter, base, and collector) separated by two pn junctions. It explains basic BJT operation, characteristics such as current gain, biasing circuits, and modes of operation like active, saturation, and cutoff. Troubleshooting techniques for testing and identifying faults in BJTs are also covered.

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Dave Harvey Villanueva
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75 views38 pages

Introduction To BJT

The document provides an introduction to bipolar junction transistors (BJTs). It describes how BJTs are constructed with three doped semiconductor regions (emitter, base, and collector) separated by two pn junctions. It explains basic BJT operation, characteristics such as current gain, biasing circuits, and modes of operation like active, saturation, and cutoff. Troubleshooting techniques for testing and identifying faults in BJTs are also covered.

Uploaded by

Dave Harvey Villanueva
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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An Introduction

 Introduction
 Invented by a team of three men at Bell
Laboratories in 1947
 beginning of a technological revolution that is
still continuing
 BJT and FET
 BJT
 linear amplifier
 electronic switch
 Bipolar Junction Transistor (BJT)
 constructed with three doped semiconductor
regions separated by two pn junction
 emitter, base and collector
 Base –Emitter Junction – pn junction joining
the base region and the emitter region
 Base-Collector Junction – pn junction
joining the base region and the collector
region
 Base Region – lightly doped and very thin
 Emitter Region – most heavily doped region
 Collector Region – moderately doped region
 Schematic Symbols for the NPN and PNP
BJT
 Bipolar – refers to the use of both holes and
electrons as carriers in the transistor structure
 Basic Transistor Operation
 BE junction is forward-biased and the BC
junction is reverse-biased
 BJT Action
 Transistor Currents
 arrow on the emitter of the transistor symbol points in the direction
of conventional current
 the emitter current (IE) is the sum of the collector current (IC) and the
base current (IB)
 IE = I C + I B
 IB is very small compared to IE and IC
 capital letter subscripts indicate dc values
 Transistor DC Bias Circuits
 Transistor Characteristics and Parameters
 DC Beta (DC)
 the ratio of the dc collector current (IC) to the dc base
current
 dc current gain of a transistor
 DC = IC/IB
 typical values range from less than 20 to 200 or higher
 hfe on data sheets
 DC Alpha (DC)
 ratio of the dc collector current (IC) to the dc emitter
current (IE)
 dc = IC/IE
 values range from 0.95 to 0.99 or greater but is always less
than 1
 Examples
 Determine the current gain  and the
emitter current IE for a transistor where
IB = 50A and IC = 3.65mA.
 A certain transistor has a  of 200. When the
base current is 50A, determine the collector
current.
 BJT Circuit Analysis
 IB : dc base current
 IE : dc emitter current
 IC : dc collector current
 VBE : dc voltage at base with respect to emitter
 VCB :dc voltage at collector with respect to base
 VCE : dc voltage at collector with respect to emitter
 Collector Characteristic Curves
 show how the collector current IC varies with VCE
for specified values of base current IB
 VBB is set to produce a certain value of IB
and VCC is 0
 Saturation region – both junctions are forward
biased
 Saturation – state of a BJT in which the
collector current has reached a maximum and is
independent of the base current
 VCC is increased, VCE increases as the
collector current IC increases
 VCE exceeds 0.7 V
 base-collector junction becomes reverse-biased
 active or linear region of operation
 IC levels off and remains essentially constant for
a given value of IB as VCE continues to increase
 IC increases very slightly as VCE increases
 IC = IB
 VCE reaches a sufficiently high voltage
 reverse-biased base-collector junction goes into
breakdown
 the collector current increase rapidly
 should never be operated in this breakdown
region
 IC versus VCE for several values of IB
 When IB = 0
 transistor is in the cut-off region
 very small collector leakage current
 Cut-off – non-conducting state of a transistor
 Cut-off
 Saturation
 DC Load Line
 More about DC
 varies with collector current and with
temperature
 Maximum Transistor Ratings
 limitations on its operations
 collector to base voltage
 collector to emitter voltage
 emitter to base voltage
 collector current
 power dissipation
 IC = PD(max) / VCE
 VCE = PD(max) / IC
 Derating PD(max)
 PD(max) is usually specified at 25C
 2 mW/C
 Transistor Data Sheet
 Transistor Packages and Terminal
Identification
 Transistor Categories
 general-purpose/ small-signal devices
 low- or medium-power amplifiers or switching
circuits
 power devices
 used to handle large currents (typically more than
1A) and/or large voltages
 RF devices
 designed to operate at extremely high frequencies
 General Purpose Transistors
 Power Transistors
 RF Transistors
 Troubleshooting
 Troubleshooting a Biased Transistor
 Typical Faults and Symptoms
 Typical Faults and Symptoms
 Testing a Transistor with a DMM
 DMM Test for Properly Functioning
Transistor
 DMM Test for Defective Transistor
 Transistor Testers
 Leakage Measurement
 ICEO
 ICBO
 Gain Measurement
 Curve Tracer
 an oscilloscope type of instrument that can
display transistor characteristics

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