Lecture 26
Analyzing Complex Amplifiers
Outline
•
•
•
6.012 Spring 2009
Two­port hand analysis
Examples
What’s Next?
Lecture 26
1
Multi­Stage Amplifier Analysis
• Draw circuit such that signal stages and
biasing devices can be easily identified.
• Identify signal path and establish
amplifier parameters.
• Determine function of all other
transistors­usually current or voltage
sources.
• Find high impedance nodes to estimate
frequency response.
6.012 Spring 2009
Lecture 26
2
Can now understand more complex
circuits?
Examples:
NMOS
CD
NMOS CS ­ PMOS CD
6.012 Spring 2009
Lecture 26
3
Can now understand more complex
circuits?
PNP CE
PNP CC ­ PNP CE
6.012 Spring 2009
Lecture 26
4
BiCMOS Voltage Amplifier
5V
M7
M7B
M5
R
35 kΩ
M6B
Q4
M6
X
100 µA
Q2B
M8
M3
Q2
M9
Vs
+
−
VBIAS
+
−
vOUT
M1
M10
Qualitative View
• Identify signal path and establish amplifier parameters
• CS­CB­CD­CC ­ Good voltage amplifier
• Determine function of all other transistors­usually current
or voltage sources
• Find high impedance nodes to estimate frequency response
6.012 Spring 2009
Lecture 26
5
Small Signal Voltage Gain
Cascode+Voltage Buffer
• Cascode­CS­CB
• Rin − −− > ∞
I2
RS
Vs +
−
Iout
+
+
Vgs1
gm1Vgs1
−
ro1
−I2
1/gm2
βo2ro2
gm6ro6ro7
Vout
−
Rout = βo 2 ro2 gm6 ro6 ro7
v out2
v out
Avo =
= −g m1 βo2 ro2 g m6 ro6 ro7 ≈
vs
vs
• Voltage buffer CD­CC
(
)
1
1
gm4 + β (g + g )
o4 m3
mb3
1
(gm3 + gmb3)
+
vin3
+
−
+
+
vin3
+
−
rπ4 + βo4(RL ⎢⎢ro ⎢⎢roc)
vin4
−
−
vin4 vout
RL
−
CD − CC
Rin′ → ∞
Rout =
6.012 Spring 2009
1
g m4
+
Av ≈ 1
1
βo4 (g m3 + gmb3 )
Lecture 26
6
Frequency Response
X
β o2 ro2
roc
Cµ 2
Cgd 6
Cdb6 + Ccs 2
Cgd 3
(1 − Av 3 )C gs 3
v out 2
v out
Avo =
= −gm1 (β o2 ro2 g m 6 ro6 ro7 ) ≈
vs
vs

1
ω3dB = 
β r g r r
 o2 o2 m6 o6 o7
(




)


1



(C + C
+
C
+
1−
A
C
+
C
+
C
gd6
gd3
vCgs3 gs3
db6
cs2 
 µ2
(
6.012 Spring 2009
)
Lecture 26
7
Bode Plot
V
out
V
s
≈ gm1βo2ro2
1
ωunity
ω
3dB

1
ω3dB = 
β r g r r
 o2 o2 m6 o6 o7
(
ω�
log scale




)


1



(C + C
+
C
+
1−
A
C
+
C
+
C
gd6
gd3
vCgs3
gs3
db6
cs2
µ
2


(
)
ωunity = Avo * ω3dB
=
g m1
(Cµ 2 + Cgd6 + Cgd3 + (1 − AvCgs3 )Cgs3 + Cdb6 + Ccs2)
6.012 Spring 2009
Lecture 26
8
Large Signal DC Analysis
• AssumeVBE = 0.7V
VGS = 1.5V
5V
3.5 V
M7B
M7
M5
4.5 V
3.5 V
3.0 V
R
35 kΩ
2.0 V
M6B
M6
100 µA
2.0 V
Q2B
3.0 V
2.0 V
1.5 V
M3
3.8 V
2.8 V
2.3 V
Q2
Q4
vOUT
RS
vs
M8
6.012 Spring 2009
+
−
M1
1.5 V
M9
M10
+
V
− BIAS
Lecture 26
9
Wrap­up of 6.012
6.012: Introductory subject to microelectronic
devices and circuits
• MICROELECTRONIC DEVICES
– Semiconductor physics: electrons/holes and
drift/diffusion, carrier concentration controlled
by doping or electrostatically
– Metal­oxide­semiconductor field­effect transistors (MOSFETs): drift of carriers in inversion layer
– Bipolar junction transistors (BJTs): minority carrier
diffusion
• MICROELECTRONIC CIRCUITS
•
– Digital circuits (mainly CMOS): no static power
dissipation; power ↓, delay ↓ & density ↑ as W &
L↓
– Analog circuits (BJT and CMOS): fτ ↑ and gm ↑ as L
↓: however, Avomax ↓ as L ↓
Follow­on Courses
•
•
•
•
•
6.152J — Microelectronics Processing Technology
6.720J — Integrated Microelectronic Devices
6.301 — Solid State Circuits
6.374 — Analysis and Design of Digital ICs
6.775 — Design of Analog MOS ICs
6.012 Spring 2009
Lecture 26
10
MIT OpenCourseWare
http://ocw.mit.edu
6.012 Microelectronic Devices and Circuits
Spring 2009
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