chapter 3: bipolar junction transistors - centurion...
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Chapter 3:Bipolar Junction Transistors
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Transistor ConstructionTransistor Construction
There are two types of transistors: • pnp• npn
The terminals are labeled: • E - Emitter
pnppnp
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• E - Emitter• B - Base• C - Collector
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
BICIEI ++++====
Emitter current is the sum of the collector and base currents:
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
The collector current is comprised of two currents:
minorityCOI
majorityCICI +=
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CommonCommon--Base ConfigurationBase Configuration
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The base is common to both input (emitter–base) and output (collector–base) of the transistor.
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CommonCommon--Base AmplifierBase Amplifier
Input CharacteristicsInput Characteristics
This curve shows the relationship between of input current (IE) to input voltage (VBE) for three output voltage
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voltage (VBE) for three output voltage (VCB) levels.
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This graph demonstrates the output current (I C) to an output voltage (VCB) for
CommonCommon--Base AmplifierBase Amplifier
Output CharacteristicsOutput Characteristics
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an output voltage (VCB) for various levels of input current (I E).
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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.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
current.
•• Saturation – The amplifier is full on. There is current, but little voltage.
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EI
CI ≅
ApproximationsApproximations
Emitter and collector currents:
Base-emitter voltage:
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Silicon)(for V 0.7BEV =
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Ideally: αααα = 1
Alpha (Alpha (αααααααα))
Alpha (αααα) is the ratio of IC to IE :
EICI
α =dc
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Ideally: αααα = 1In reality: αααα is between 0.9 and 0.998
Alpha (αααα) in the AC modeAC mode:
EICI
α∆
∆
ac =
1010
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Transistor AmplificationTransistor Amplification
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Voltage Gain:
V 50kΩ 5ma 10
mA 10
10mA20Ω
200mV
===
=≅
≅
====
))((RL
IL
V
iI
LI
EI
CI
iRiV
iIEI
Currents and Voltages:
1111
250200mV
50V===
iVLV
vA
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CommonCommon––Emitter ConfigurationEmitter Configuration
The emitter is common to both input (base-emitter) and output (collector-emitter).
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The input is on the base and the output is on the collector.
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CommonCommon--Emitter CharacteristicsEmitter Characteristics
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Collector Characteristics Base Characteristics
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CommonCommon--Emitter Amplifier CurrentsEmitter Amplifier Currents
Ideal CurrentsIdeal Currents
IE = IC + IB IC = αααα IE
Actual CurrentsActual Currents
IC = αααα IE + ICBO where ICBO = minority collector current
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IC = αααα IE + ICBO
When IB = 0 µµµµA the transistor is in cutoff, but there is some minority current flowing called ICEO.
µA 0=−
=BI
CBOCEO
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.
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Beta (Beta (ββββββββ))
In DC mode:
ββββ represents the amplification factor of a transistor. (ββββ is sometimes referred to as hfe, a term used in transistor modeling calculations)
CIβ =dc
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In AC mode:
BIβ =dc
constantac =∆
∆=
CEVB
C
II
β
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Determining ββββ from a Graph
Beta (Beta (ββββββββ))
100
µA 10mA 1
µA) 20 µA (30mA) 2.2mA (3.2
β
7.5V
AC
CE
====
====
−−−−
−−−−====
====
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
108
A 25mA 2.7
β 7.5VDC CE
====
µµµµ==== ====
100====
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Relationship between amplification factors ββββ and αααα
1β
βα
++++====
1α
αβ
−−−−====
Beta (Beta (ββββββββ))
Relationship Between Currents
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Relationship Between Currents
BC βII ==== BE 1)I(βI ++++====
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CommonCommon––Collector ConfigurationCollector Configuration
The input is on the base and the output is on the emitter.
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CommonCommon––Collector ConfigurationCollector Configuration
The characteristics are similar to those of the common-emitter
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common-emitter configuration, except the vertical axis is IE.
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VCE is at maximum and IC is at minimum (I Cmax= ICEO) in the cutoff region.
IC is at maximum and VCE is at minimum (V CE max = VCEsat = VCEO) in
Operating Limits for Each ConfigurationOperating Limits for Each Configuration
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minimum (V CE max = VCEsat = VCEO) in the saturation region.
The transistor operates in the active region between saturation and cutoff.
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Power DissipationPower Dissipation
CCBCmax IVP ====
CCECmax IVP ====
Common-base:
Common-emitter:
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Common-collector:
ECECmax IVP ====
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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 ββββDC or hFE.
•• OhmmeterOhmmeter
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Transistor Terminal IdentificationTransistor Terminal Identification
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