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Lecture 3 Electronic Devices 1
ETU 07103LECTURE 3
Lecture 3 Electronic Devices 2
Lecture 3Coverage
• Bipolar Junction Transistor (BJT) Structure and principals of operationBJT configurationsBJT characteristic curvesBJT DC biasing Analysis under various biasing conditions
References • Electronic devices and circuit theory, 7th Ed. by Robert
L. Boylestad & Louis Nashelsky, pg. 112 to 166, • Electronic devices and circuits, by Theodore F.
Bogart, Jr., page 85 to 120
Lecture 3 Electronic Devices 3
Bipolar Junction Transistor (BJT)Introduction
• BJT is a three layers semiconductor device with either NPN or PNP structure
• BJT has three terminals described as follow Base – the control terminal Emitter – the source of majority carriers Collector – the collector of majority carriers
• Applications include the use as amplifier and switch• There are two types of BJT namely as
PNP transistorNPN transistor
• BJT is a current controlled device
Lecture 3 Electronic Devices 4
BJT continue…Structure
Figure 3.1 Structure and schematic symbols of NPN and PNP transistors
Lecture 3 Electronic Devices 5
BJT continue…Mode of operation
• The basic operation of BJT will be described using the PNP transistor of Fig. 3.2
• For linear operation The base-emitter junction is always forward biased, while The base-collector junction is always reversed biased
• A small base-emitter current controls a much large collector-emitter current
Lecture 3 Electronic Devices 6
BJT continue…
Figure 3.2 Majority and minority carriers flow of a PNP transistor
Lecture 3 Electronic Devices 7
BJT continue...
From Fig. 3.2 • Base-Emitter junction (Forward biased)
(1) Holes diffusing from E into the B (2) Electrons diffusing from the B into the E
• At the Base region (3) Recombination of holes injected into the base (4) Most holes reach the C
• Base-Collector junction (Reverse biased) (5) Electron minority carrier current from C to B (6) Hole minority carrier current from B to C
• In most practical purposes, current (5) and (6) can be neglected
Lecture 3 Electronic Devices 8
BJT continue…• For both NPN and PNP transistors
IE = IB + IC ……………………………………….......................(3.1)
• The IC is comprised of two components, the majority and minority carriers as indicated in Fig. 3.2
• The minority current component is called the leakage current (ICO)
• The IC, therefore is determined in total by Eqn. 3.2
IC = ICmajority + ICOminority ……………………………………………(3.2)
• Alpha is an important transistor parameter defined asα = ICmajority/IE = IC/IE ……………………………………............(3.3)
• α measures the portion of the IE that managed to cross the base and become IC (0.90 < α < 1)
Lecture 3 Electronic Devices 9
BJT Configurations Introduction
• A transistor can be connected in three configurations Common Base Configuration (CB) Common Emitter Configuration (CE) Common Collector Configuration (CC)
Common base configuration• The base terminal is common to both input and output
as shown in Fig. 3.3
• IC and IE are related by Eqn. 3.3 (i.e. α = IC/IE )
Lecture 3 Electronic Devices 10
BJT config. Continue…
Figure 3.3 Common base configuration
Common emitter configuration• The emitter terminal is common to both input and
output terminals as shown in Fig. 3.4
• The IC is related to the IB by a factor known as beta (β) as shown in Eqn. 3.4
β = IC/IB……………………………………………..(3.4)
Lecture 3 Electronic Devices 11
BJT config. Continue…• β is known as common-emitter, forward-current,
amplification factor
Figure 3.4 Common emitter configuration
Common collector configuration• Collector terminal is common to both input and output
as shown in Fig. 3.5
• Eqn. 3.5 shows on how IB and IE relates in CC
IE/IB = β / α ……………………………………………………..(3.5)
Lecture 3 Electronic Devices 12
BJT config. Continue…
Figure 3.5 Common collector configuration
BJT Characteristic curve• The behavior of BJT can be described using two sets
of characteristic curves namely Input characteristics: To show the relation between input I
and V Output characteristics: To show the relation between output
I and V
Lecture 3 Electronic Devices 13
BJT char. curve continue…Consider the CB configuration• Input characteristics
They are shown in Fig. 3.6a They relate an input IE to an input VBE for various
levels of output VCB
• Output characteristics The characteristics are as shown in Fig. 3.6b They relate an output IC to an output VCB for
various levels of input IE
Lecture 3 Electronic Devices 14
BJT char. curve continue…
(a) (b)
Figure 3.6 (a) Input characteristics for CB amplifier; (b) Output characteristics for CB amplifier
Lecture 3 Electronic Devices 15
BJT DC Biasing
Introduction• BJT is used for different purposes like amplifier,
oscillator, etc.• BJT must be DC biased for the most purposes • DC biasing is that process of introducing a fixed level
of IDC and VDC at the output circuit of BJT
• For transistor amplifiers, IDC and VDC establish an operating point (Q- point) on the characteristic curve
• Q-point define the region to be used for amplification of input signal
Lecture 3 Electronic Devices 16
BJT DC bias continue…
Biasing Circuits (CE)• Most commonly used biasing circuits are
Collector to base bias circuit Fixed bias circuit Emitter stabilized bias circuit Voltage divider bias circuit
• Fixed Bias Circuit Fig. 3.7 shows the fixed bias circuit Applying KVL to the input loop
VCC - IBRB - VBE = 0 ………………………………………(3.6)
Applying KVL to the collector-emitter loopVCE + ICRC – VCC = 0 …………………………………(3.7)
Lecture 3 Electronic Devices 17
BJT DC bias continue…
Figure 3.7 Fixed Bias Circuit Figure 3.8 Emitter stabilized bias circuit
• Emitter stabilized bias circuitIt is a modified version of a fixed bias circuit as shown in Fig.
3.8 Consider base-emitter loop
IB(RB + ( β +1)RE) = VCC – VBE ………………………(3.8)
Lecture 3 Electronic Devices 18
BJT DC bias continue… Consider collector-emitter loop
VCE – VCC + IC(RC + RE) = 0 ………………………(3.9)
• Voltage divider bias circuit The name voltage divider comes from voltage divider formed
by the resistor R1 and R2 in Fig. 3.9
Figure 3.9 Voltage divider bias circuit
Lecture 3 Electronic Devices 19
BJT DC bias continue…Consider the input side of Fig. 3.9
The output side of Fig. 3.9 will make use of an Eqn. 3.9
Lecture 3 Electronic Devices 20
THE END OF LECTURE 3