11/18/2013
1
Neamen Microelectronics, 4e Chapter 6-1McGraw-Hill
Microelectronics Circuit Analysis and Design
Donald A. Neamen
Chapter 6
Basic BJT Amplifiers
Neamen Microelectronics, 4e Chapter 6-2McGraw-Hill
In this chapter, we will:
� Understand the principle of a linear amplifier.
� Discuss and compare the three basic transistor amplifier configurations.� the common-emitter amplifier.� the emitter-follower amplifier.� the common-base amplifier.
� Analyze multi-transistor amplifiers.
Neamen Microelectronics, 4e Chapter 6-3McGraw-Hill
Common Emitter with Time-Varying Input
Neamen Microelectronics, 4e Chapter 6-4McGraw-Hill
ac Equivalent Circuit for Common Emitter
11/18/2013
2
Neamen Microelectronics, 4e Chapter 6-5McGraw-Hill
IB Versus VBE
Characteristic
bB
T
beBQB iI
V
vIi +=+≅ )1(
�� � ���exp��
��� �
� � ≅ 1 + x +..
��� � ��� � ��
Neamen Microelectronics, 4e Chapter 6-6McGraw-Hill
ac Equivalent Circuit
Neamen Microelectronics, 4e Chapter 6-7McGraw-Hill
ac Equivalent Circuit
Neamen Microelectronics, 4e Chapter 6-8McGraw-Hill
ac Equivalent Circuit
11/18/2013
3
Neamen Microelectronics, 4e Chapter 6-9McGraw-Hill
ac Equivalent Circuit
Neamen Microelectronics, 4e Chapter 6-10McGraw-Hill
Small-Signal Equivalent Circuit Using Common-Emitter Current Gain
Neamen Microelectronics, 4e Chapter 6-11McGraw-Hill
ac Equivalent Circuit
Neamen Microelectronics, 4e Chapter 6-12McGraw-Hill
Small-Signal Hybrid π Model for npn BJT
β
β
π
π
=
=
=
rg
I
Vr
V
Ig
m
CQ
T
T
CQ
m
Phasor signals are shown in parentheses.
11/18/2013
4
Neamen Microelectronics, 4e Chapter 6-13McGraw-Hill
Small-Signal Equivalent Circuit for npn Common Emitter circuit
))((B
CmvRr
rRgA
+−=
π
π
Neamen Microelectronics, 4e Chapter 6-14McGraw-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 π model for the transistor.
3. Analyze the small-signal equivalent circuit after setting dc source components to zero.
Neamen Microelectronics, 4e Chapter 6-15McGraw-Hill
Transformation of Elements
Element DC Model AC Model
Resistor R R
Capacitor Open C
Inductor Short L
Diode +Vγ, rf – rd = VT/ID
Independent Constant Voltage Source
+ VS - Short
Independent Constant Current Source
IS Open
Neamen Microelectronics, 4e Chapter 6-16McGraw-Hill
Hybrid π Model for npn with Early Effect
CQ
Ao
I
Vr =
11/18/2013
5
Neamen Microelectronics, 4e Chapter 6-17McGraw-Hill
Hybrid p Model for pnp with Early Effect
All voltage and current polarities are reversed.Gain equation are the SAME.
)||( oCmo rRvgv π=
)||( oCmo rRgv =
)(B
sRr
rvv
+−=
π
ππ
)||( oCbo rRBiv =
)(B
sb
Rr
vi
+
−=
π
Neamen Microelectronics, 4e Chapter 6-18McGraw-Hill
h-Parameter Model for npn
loopoutputatKCLVhIhI
loopinputatKVLVhIhV
ceoebfec
cerebiebe
+=
+=
Neamen Microelectronics, 4e Chapter 6-19McGraw-Hill
h-Parameter Model for npn
β
µπ
=
+=
fe
bie
h
rrrh
o
oe
re
rrh
r
rh
11+
+=
≅
µ
µ
π
β
Neamen Microelectronics, 4e Chapter 6-20McGraw-Hill
Common Emitter with Voltage-Divider Bias and a Coupling Capacitor
11/18/2013
6
Neamen Microelectronics, 4e Chapter 6-21McGraw-Hill
Small-Signal Equivalent Circuit – Coupling Capacitor assumed a Short
)(
21
Si
iomv
coo
i
RR
RRgA
RrR
rRRR
+−=
=
= π
Neamen Microelectronics, 4e Chapter 6-22McGraw-Hill
npn Common Emitter with Emitter Resistor
Neamen Microelectronics, 4e Chapter 6-23McGraw-Hill
Small-Signal Equivalent Circuit:Common Emitter with RE
)()1(
)1(/
21
Si
i
E
Cv
ibi
Ebib
RR
R
Rr
RA
RRRR
RrIVinR
+++
−=
=
++==
β
β
β
π
π
�� � −�����
��� � �� �� � ���������
Neamen Microelectronics, 4e Chapter 6-24McGraw-Hill
RE and Emitter Bypass Capacitor
11/18/2013
7
Neamen Microelectronics, 4e Chapter 6-25McGraw-Hill
DC and AC Load Lines: RE and Emitter Bypass Capacitor
Neamen Microelectronics, 4e Chapter 6-26McGraw-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, 4e Chapter 6-27McGraw-Hill
Common-Collector or Emitter-Follower Amplifier
Neamen Microelectronics, 4e Chapter 6-28McGraw-Hill
Small-Signal Equivalent Circuit:Emitter Follower
11/18/2013
8
Neamen Microelectronics, 4e Chapter 6-29McGraw-Hill
Small-Signal Equivalent Circuit:Emitter Follower
Typically a large value due to the reflected impedance
Neamen Microelectronics, 4e Chapter 6-30McGraw-Hill
S-S Equivalent :Emitter Follower
1)())(1(
))(1(≅
+++
+=
Si
i
Eo
Eo
vRR
R
Rrr
RrA
β
β
π
Neamen Microelectronics, 4e Chapter 6-31McGraw-Hill
Output Resistance: Emitter Follower
oEo rRr
Rβ
π
+=
1Typically a low value due to the reflected impedance
x
xo
I
VR =
Neamen Microelectronics, 4e Chapter 6-32McGraw-Hill
Emitter Follower Current Gain
11/18/2013
9
Neamen Microelectronics, 4e Chapter 6-33McGraw-Hill
Emitter Follower Current Gain
Neamen Microelectronics, 4e Chapter 6-34McGraw-Hill
Common-Base Amplifier
Neamen Microelectronics, 4e Chapter 6-35McGraw-Hill
Small-Signal Equivalent Circuit:Common Base
]1
)[(
)(
E
LC
Cmi
LCmv
Rr
RR
RgA
RRgA
βπ
++=
≅
Neamen Microelectronics, 4e Chapter 6-36McGraw-Hill
Small-Signal Equivalent Circuit:Common Base
11/18/2013
10
Neamen Microelectronics, 4e Chapter 6-37McGraw-Hill
Small-Signal Equivalent Circuit:Common Base
Neamen Microelectronics, 4e Chapter 6-38McGraw-Hill
Common Base
Neamen Microelectronics, 4e Chapter 6-39McGraw-Hill
Common Base
Neamen Microelectronics, 4e Chapter 6-40McGraw-Hill
Common Base
11/18/2013
11
Neamen Microelectronics, 4e Chapter 6-41McGraw-Hill
Neamen Microelectronics, 4e Chapter 6-42McGraw-Hill
Common Emitter Cascade Amplifier
Neamen Microelectronics, 4e Chapter 6-43McGraw-Hill
Small-Signal Equivalent Circuit:Cascade Amplifier
Neamen Microelectronics, 4e Chapter 6-44McGraw-Hill
Darlington Pair
21ββ≅iA
11/18/2013
12
Neamen Microelectronics, 4e Chapter 6-45McGraw-Hill
Cascode Amplifier
Neamen Microelectronics, 4e Chapter 6-46McGraw-Hill
Small-Signal Equivalent Circuit:Cascode Amplifier
)(1 LCmv RRgA −≅