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In this chapter, we will:
Microelectronics
Discuss the physical structure and operation of
the bipolar junction transistor.
Circuit Analysis and Design
Donald A. Neamen
Understand the dc analysis and design
techniques of bipolar transistor circuits.
Chapter 5
Examine three basic applications of bipolar
transistor circuits.
The Bipolar Junction Transistor
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-1
Neamen
Cross Section of Integrated Circuit
npn Transistor
Microelectronics, 4e
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Chapter 5-2
Modes of Operation
Forward-Active
B-E junction is forward biased
B-C junction is reverse biased
Saturation
B-E and B-C junctions are forward biased
Cut-Off
B-E and B-C junctions are reverse biased
Inverse-Active (or Reverse-Active)
B-E junction is reverse biased
B-C junction is forward biased
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-3
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Microelectronics, 4e
McGraw-Hill
Chapter 5-4
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npn BJT in Forward-Active
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Microelectronics, 4e
McGraw-Hill
Chapter 5-5
Electrons and Holes in npn BJT
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Microelectronics, 4e
McGraw-Hill
Chapter 5-6
Circuit Symbols and
Current Conventions
Electrons and Holes in npn BJT
With a + potential across the C-E terminals.
If a positive voltage is applied to the base
(>0.6V), the B-E pn junction is forward biased.
The E side electrons cross the pn junction and
many electrons are swept to the positive C side
voltage (since the p base material is thin). This
results in electron flow from E to C.
(Conventional current flow from C to E).
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Microelectronics, 4e
McGraw-Hill
Chapter 5-7
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Microelectronics, 4e
McGraw-Hill
Chapter 5-8
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Current Relationships
Common-Emitter Configurations
iE = iC + iB
iC = βiB
iE = (1 + β iB )
iC = αiE
β=
Neamen
α
1−α
Microelectronics, 4e
McGraw-Hill
Chapter 5-9
Neamen
Current-Voltage Characteristics of a
Common-Emitter Circuit
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-11
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Chapter 5-10
Early Voltage/Finite Output
Resistance
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Chapter 5-12
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DC Equivalent Circuit for
npn Common Emitter
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Microelectronics, 4e
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DC Equivalent Circuit for
pnp Common Emitter
Chapter 5-13
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Load Line
Microelectronics, 4e
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Chapter 5-14
Problem-Solving Technique:
Bipolar DC Analysis
1. Assume that the transistor is biased in
forward active mode
a. VBE = VBE(on), IB > 0, & IC = βIB
2. Analyze ‘linear’ circuit.
3. Evaluate the resulting state of transistor.
a. If VCE > VCE(sat), assumption is correct
b. If IB < 0, transistor likely in cutoff
c. If VCE < 0, transistor likely in saturation
4. If initial assumption is incorrect, make new
assumption and return to Step 2.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-15
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Microelectronics, 4e
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Chapter 5-16
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Voltage Transfer Characteristic for
npn Circuit
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Microelectronics, 4e
McGraw-Hill
Chapter 5-17
Voltage Transfer Characteristic for
pnp Circuit
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Common Emitter with Voltage
Divider Biasing and Emitter Resistor
Microelectronics, 4e
McGraw-Hill
Chapter 5-18
Microelectronics
Circuit Analysis and Design
Donald A. Neamen
Chapter 6
Basic BJT Amplifiers
VTH = [ R2 /( R1 + R2 )]VCC
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Microelectronics, 4e
McGraw-Hill
Chapter 5-19
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Microelectronics, 4e
McGraw-Hill
Chapter 5-20
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Common Emitter
with Time-Varying Input
In this chapter, we will:
Understand the concept of an analog signal and
the principle of a linear amplifier.
Investigate how a transistor circuit can amplify a
small, time-varying input signal.
Discuss and compare the three basic transistor
amplifier configurations.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-21
Neamen
Neamen
Chapter 5-22
ac Equivalent Circuit
for Common Emitter
IB Versus VBE
Characteristic
iB ≅ I BQ (1 +
Microelectronics, 4e
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vbe
) = I B + ib
VT
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Chapter 5-23
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Microelectronics, 4e
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Chapter 5-24
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Small-Signal Hybrid π Model for npn BJT
gm =
rπ =
Small-Signal Equivalent Circuit Using
Common-Emitter Current Gain
I CQ
VT
βVT
I CQ
g m rπ = β
Phasor signals are shown in parentheses.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-25
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-26
Problem-Solving Technique:
BJT AC Analysis
Small-Signal Equivalent Circuit for
npn Common Emitter circuit
1. Analyze circuit with only dc sources to find Q
point.
2. Replace each element in circuit with smallsignal model, including the hybrid π model for
the transistor.
3. Analyze the small-signal equivalent circuit
after setting dc source components to zero.
Av = −( g m RC )(
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Microelectronics, 4e
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rπ
)
rπ + RB
Chapter 5-27
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Microelectronics, 4e
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Chapter 5-28
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Hybrid π Model for npn with Early Effect
ro =
Neamen
Hybrid p Model for pnp with Early Effect
VA
I CQ
Microelectronics, 4e
McGraw-Hill
Chapter 5-29
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-30
h-Parameter Model for npn
Expanded Hybrid π Model for npn
ܸ௕௘ = ℎ௜௘ ‫ܫ‬௕ + ℎ௥௘ ܸ௖௘
‫ܫ‬௖ = ℎ௙௘ ‫ܫ‬௕ + ℎ௢௘ ܸ௖௘
hie = rb + rπ rµ
h fe = β
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-31
Neamen
Microelectronics, 4e
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hre ≅
rπ
rµ
hoe =
1+ β 1
+
rµ
ro
Chapter 5-32
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Common Emitter with Voltage-Divider
Bias and a Coupling Capacitor
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-33
npn Common Emitter
with Emitter Resistor
Small-Signal Equivalent Circuit –
Coupling Capacitor Assumed a Short
Neamen
Microelectronics, 4e
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Chapter 5-34
Small-Signal Equivalent Circuit:
Common Emitter with RE
ܸ௢ = −ߚ‫ܫ‬௕ ܴ௖
ܸ
‫ܫ‬௕ = ௜௡൘(‫ ݎ‬+ ߚ + 1 ܴ )
గ
ா
ܴ௜
ܸ௜௡ = ܸ௦ (
)
ܴ௜ + ܴ௦
Rib = rπ + (1 + β ) RE
Ri = R1 R 2 Rib
Av =
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-35
Neamen
Microelectronics, 4e
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− βRC
Ri
(
)
rπ + (1 + β ) RE Ri + RS
Chapter 5-36
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RE and Emitter Bypass Capacitor
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Microelectronics, 4e
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Chapter 5-37
dc AND ac Load Lines:
RE and Emitter Bypass Capacitor
Neamen
Problem-Solving Technique:
Maximum Symmetrical Swing
Microelectronics, 4e
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Chapter 5-38
Common-Collector
or Emitter-Follower Amplifier
1. Write dc load line equation that relates ICQ
and VCEQ.
2. Write ac load line equations that relates ic
and vce
3. In general, ic = ICQ – IC(min), where IC(min)
is zero or other minimum collector current.
4. In general, vce = VCEQ – VCE(min), where
VCE(min) is some specified minimum
collector-emitter voltage.
5. Combine above 4 equations to find optimum
ICQ and VCEQ.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-39
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-40
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Small-Signal Equivalent Circuit:
Emitter Follower
Output Resistance:
Emitter Follower
Ro =
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 5-41
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rπ
RE ro
1+ β
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Chapter 5-42
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