Microelectronics
Circuit Analysis and Design
Donald A. Neamen
Chapter 6
Basic BJT Amplifiers
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-1
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
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-2
Common Emitter
with Time-Varying Input
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-3
IB Versus VBE
Characteristic
iB  I BQ (1 
vbe
)  I B  ib
VT
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-4
ac Equivalent Circuit
for Common Emitter
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-5
Small-Signal Hybrid p Model for npn BJT
gm 
rp 
I CQ
VT
VT
I CQ
g m rp  
Phasor signals are shown in parentheses.
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-6
Small-Signal Equivalent Circuit Using
Common-Emitter Current Gain
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-7
Small-Signal Equivalent Circuit for
npn Common Emitter circuit
rp
Av  ( g m RC )(
)
rp  RB
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-8
Problem-Solving Technique:
BJT AC Analysis
1. Analyze circuit with only dc sources to find Q
point.
2. Replace each element in circuit with smallsignal model, including the hybrid p model for
the transistor.
3. Analyze the small-signal equivalent circuit
after setting dc source components to zero.
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-9
Transformation of Elements
Element
DC Model
AC Model
Resistor
R
R
Capacitor
Open
C
Inductor
Short
L
+Vg, rf –
rd = VT/ID
+ VS -
Short
Diode
Independent Constant
Voltage Source
Independent Constant
Current Source
Neamen
14 February 2012
IS
Microelectronics
McGraw-Hill
Open
Chapter 6-10
Hybrid p Model for npn with Early Effect
VA
ro 
I CQ
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-11
Hybrid p Model for pnp with Early Effect
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-12
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-13
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-14
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-15
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-16
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-17
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-18
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-19
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-20
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-21
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-22
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-23
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-24
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-25
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-26
Expanded Hybrid p Model for npn
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-27
h-Parameter Model for npn
hie  rb  rp r
h fe  
Neamen
14 February 2012
Microelectronics
McGraw-Hill
rp
hre 
r
1  1
hoe 

r
ro
Chapter 6-28
T-Model of an npn BJT
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-29
4 Equivalent 2-port Networks
Voltage Amplifier
Current Amplifier
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-30
4 Equivalent 2-port Networks
Transconductance
Amplifier
Transresistance
Amplifier
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-31
Common Emitter with Voltage-Divider
Bias and a Coupling Capacitor
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-32
Small-Signal Equivalent Circuit –
Coupling Capacitor Assumed a Short
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-33
npn Common Emitter
with Emitter Resistor
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-34
Small-Signal Equivalent Circuit:
Common Emitter with RE
Rib  rp  (1   ) RE
Ri  R1 R 2 Rib
 RC
Ri
Av 
(
)
rp  (1   ) RE Ri  RS
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-35
RE and Emitter Bypass Capacitor
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-36
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
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-37
Common-Collector
or Emitter-Follower Amplifier
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-38
Small-Signal Equivalent Circuit:
Emitter Follower
Rib  rp  (1   )(ro RE )
Ri  R1 R2 R ib
(1   )(ro RE )
Ri
Av 
(
)
rp  (1   )(ro RE ) Ri  RS
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-39
Output Resistance:
Emitter Follower
rp
Ro 
RE ro
1 
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-40
Common-Base Amplifier
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-41
Small-Signal Equivalent Circuit:
Common Base
Av  g m ( RC RL )
RC
rp
Ai  g m (
)[
RE ]
RC  RL 1  
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-42
Input Resistance:
Common Base
Rie = rp/(1+)
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-43
Output Resistance:
Common Base
RO = RC
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-44
Common Emitter Cascade Amplifier
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-45
Small-Signal Equivalent Circuit:
Cascade Amplifier
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-46
Darlington Pair
Ai  1 2
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-47
Cascode Amplifier
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-48
Small-Signal Equivalent Circuit:
Cascode Amplifier
Av   g m1 ( RC RL )
Neamen
14 February 2012
Microelectronics
McGraw-Hill
Chapter 6-49