Chapter 5' Bipolar Junction Transistors BJTs

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Chapter 5. Bipolar Junction Transistors (BJTs)
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Bipolar Junction Transistor

• Three terminal device
• Voltage between two terminals to control current
  flow in third terminal
• Invented in 1948 at Bell Telephone Laboratories
• Dominant until late 1980s
• Reliable under harsh operating conditions
• High frequency applications
• High speed designs
• High power applications

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• npn transistor
• n-type emitter (E) region, p-type base (B)
  region, n-type collector (C) region
• Two pn junctions (naming basis for bipolar
  junction transistor)
• Modes
• Active used for amplifier design
• Cutoff
• Saturation used for logic design
• Reverse active limited operation

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pnp transistor dual of npn transistor
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Active Mode of npn Transistor
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Circuit Models for Active Mode npn Transistor
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Practical Implementation
E and C are not symmetrical.
pnp transistors works dual to npn transistors
much in the same way PMOSFET works dual to
NMOSFET. (In this class, we will concentrate on
npn transistors.)
Skip Sections 5.1.4, 5.1.5, and 5.1.6 (pages 387
392)
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Circuit Symbols for npn Transistors
Biasing in active mode Directions of current flow
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iC vBE Characteristics
Temperature Dependence
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Common Base Characteristics
In active region, vCB - 0.4 V
Base voltage is fixed at zero.
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Dependence of iC on Collector Voltage
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Circuit Models with Output Resistance ro
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Common Emitter Configuration
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Common Emitter Saturation Model
Skip Section 5.2.5 (pages 406 407).
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Designing Linear Amplifiers (Active Region)
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Amplifier Gain
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Graphical Analysis
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To determine iB, iC and vCE, you need to use both
graphs.
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Quiescent point must be selected to give a
symmetric output swing.
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Input part
10 V
RB1
RBB
RB1

VBB


10 V
RB2
_
_
VBB
VBB
_
RB2
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Biasing BJT

• Determining a quiescent point for linearization
• Active mode operation
• Considerations
• Stable with respect to manufacturing parameters
  (e.g., ro, ß)
• Desired gains
• Acceptable output swing

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Biasing with Single Power Supply

• Fix VBE or IB.
• Output directly depends on ß
• Unstable with respect to temperature variation

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Addition of Degeneration Resistor
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Biasing with Two Power Supplies
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Biasing with Feedback Resistor
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Biasing with Current Source
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Small Signal Analysis

• A quiescent point has been determined by biasing.
• Active mode operation
• Forward biasing for base-emitter junction by VBE
• Reverse biasing for collector-base junction by RC
  and VCC

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The transistor performs as a voltage controlled
current source with gain gm when input varies by
10 mV or less.
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Hybrid p Model

• Short circuit voltage sources
• Open circuit current sources
• Short circuit capacitors

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T Model
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Hybrid p Model with Early Effect
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Structure of Single Stage Amplifier
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Common Emitter Amplifier
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Common Emitter Amplifier with RE
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Common Base Amplifier
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Common Collector Amplifier
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Skip Sections 5.8, 5.9 (Pages 485 503)
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Digital Logic Inverter
Logic 1 vI VCC ? vO VCEsat 0.2 V Logic 0
vI 0 ? vO VCC
Transistor is in saturation mode.
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vI vO Transfer Function
VCC 5 V RB 10 kO RC 1 kO ß 50
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Skip Sections 5.10.2, 5.11 (Pages 505 515)