Neamen Microelectronics Chapter 6-1 14 February 2012 McGraw-Hill
Microelectronics Circuit Analysis and Design
Donald A. Neamen
Chapter 6
Basic BJT Amplifiers
Neamen Microelectronics Chapter 6-2 14 February 2012 McGraw-Hill
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. Analyze the common-emitter amplifier.
Understand the ac load line & determine the maximum symmetrical swing of the output.
Analyze the emitter-follower amplifier. Analyze the common-base amplifier.
Analyze multitransistor or multistage amplifiers. Understand the concept of signal power gain in an
amplifier circuit.
Neamen Microelectronics Chapter 6-3 14 February 2012 McGraw-Hill
Common Emitter with Time-Varying Input
Neamen Microelectronics Chapter 6-4 14 February 2012 McGraw-Hill
IB Versus VBE Characteristic
bB
T
beBQB iI
V
vIi )1(
Neamen Microelectronics Chapter 6-5 14 February 2012 McGraw-Hill
ac Equivalent Circuit for Common Emitter
Neamen Microelectronics Chapter 6-6 14 February 2012 McGraw-Hill
Small-Signal Hybrid p Model for npn BJT
p
p
rg
I
Vr
V
Ig
m
CQ
T
T
CQ
m
Phasor signals are shown in parentheses.
Neamen Microelectronics Chapter 6-7 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit Using Common-Emitter Current Gain
Neamen Microelectronics Chapter 6-8 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit for npn Common Emitter circuit
))((B
CmvRr
rRgA
p
p
Neamen Microelectronics Chapter 6-9 14 February 2012 McGraw-Hill
Problem-Solving Technique: BJT AC Analysis
1. Analyze circuit with only dc sources to find Q point.
2. Replace each element in circuit with small-signal model, including the hybrid p model for the transistor.
3. Analyze the small-signal equivalent circuit after setting dc source components to zero.
Neamen Microelectronics Chapter 6-10 14 February 2012 McGraw-Hill
Transformation of Elements
Element DC Model AC Model
Resistor R R
Capacitor Open C
Inductor Short L
Diode +Vg, rf –
rd = VT/ID
Independent Constant Voltage Source
+ VS -
Short
Independent Constant Current Source
IS
Open
Neamen Microelectronics Chapter 6-11 14 February 2012 McGraw-Hill
Hybrid p Model for npn with Early Effect
CQ
AoI
Vr
Neamen Microelectronics Chapter 6-12 14 February 2012 McGraw-Hill
Hybrid p Model for pnp with Early Effect
Neamen Microelectronics Chapter 6-13 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-14 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-15 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-16 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-17 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-18 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-19 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-20 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-21 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-22 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-23 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-24 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-25 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-26 14 February 2012 McGraw-Hill
Neamen Microelectronics Chapter 6-27 14 February 2012 McGraw-Hill
Expanded Hybrid p Model for npn
Neamen Microelectronics Chapter 6-28 14 February 2012 McGraw-Hill
h-Parameter Model for npn
p
fe
bie
h
rrrh
o
oe
re
rrh
r
rh
11
p
Neamen Microelectronics Chapter 6-29 14 February 2012 McGraw-Hill
T-Model of an npn BJT
Neamen Microelectronics Chapter 6-30 14 February 2012 McGraw-Hill
4 Equivalent 2-port Networks
Voltage Amplifier
Current Amplifier
Neamen Microelectronics Chapter 6-31 14 February 2012 McGraw-Hill
4 Equivalent 2-port Networks
Transconductance Amplifier
Transresistance Amplifier
Neamen Microelectronics Chapter 6-32 14 February 2012 McGraw-Hill
Common Emitter with Voltage-Divider Bias and a Coupling Capacitor
Neamen Microelectronics Chapter 6-33 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit – Coupling Capacitor Assumed a Short
Neamen Microelectronics Chapter 6-34 14 February 2012 McGraw-Hill
npn Common Emitter with Emitter Resistor
Neamen Microelectronics Chapter 6-35 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit: Common Emitter with RE
)()1(
)1(
21
Si
i
E
Cv
ibi
Eib
RR
R
Rr
RA
RRRR
RrR
p
p
Neamen Microelectronics Chapter 6-36 14 February 2012 McGraw-Hill
RE and Emitter Bypass Capacitor
Neamen Microelectronics Chapter 6-37 14 February 2012 McGraw-Hill
Problem-Solving Technique: Maximum Symmetrical Swing
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 Chapter 6-38 14 February 2012 McGraw-Hill
Common-Collector or Emitter-Follower Amplifier
Neamen Microelectronics Chapter 6-39 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit: Emitter Follower
)())(1(
))(1(
))(1(
21
Si
i
Eo
Eo
v
ibi
Eoib
RR
R
Rrr
RrA
RRRR
RrrR
p
p
Neamen Microelectronics Chapter 6-40 14 February 2012 McGraw-Hill
Output Resistance: Emitter Follower
oEo rRr
R
p
1
Neamen Microelectronics Chapter 6-41 14 February 2012 McGraw-Hill
Common-Base Amplifier
Neamen Microelectronics Chapter 6-42 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit: Common Base
]1
)[(
)(
E
LC
Cmi
LCmv
Rr
RR
RgA
RRgA
p
Neamen Microelectronics Chapter 6-43 14 February 2012 McGraw-Hill
Input Resistance: Common Base
Rie = rp/(1+)
Neamen Microelectronics Chapter 6-44 14 February 2012 McGraw-Hill
Output Resistance: Common Base
RO = RC
Neamen Microelectronics Chapter 6-45 14 February 2012 McGraw-Hill
Common Emitter Cascade Amplifier
Neamen Microelectronics Chapter 6-46 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit: Cascade Amplifier
Neamen Microelectronics Chapter 6-47 14 February 2012 McGraw-Hill
Darlington Pair
21iA
Neamen Microelectronics Chapter 6-48 14 February 2012 McGraw-Hill
Cascode Amplifier
Neamen Microelectronics Chapter 6-49 14 February 2012 McGraw-Hill
Small-Signal Equivalent Circuit: Cascode Amplifier
)(1 LCmv RRgA