l10_analog(1).pdf

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L10: Analog Building Blocks
OpAmps , A/D, D/A)
Acknowledgement:
Materials in this lecture are courtesy of the following sources and are used with permission.
Dave Wentzloff
L10: 6.111 Spring 2006
Introductory Digital Systems Laboratory
1
( OpAmps
, A/D, D/A)
 
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Introduction to Operational Amplifiers
DC Model
Typically very high input
resistance ~ 300KΩ
+
R
a ⋅
v
R
out
High DC gain (~10 5 )
id
v
v
Output resistance ~75Ω
id
out
−
V
=
a
(
f
)
⋅
V
out
in
LM741 Pinout
a(f)
+10 to +15V
10 5
-20dB/
decade
Reprinted with
permission of
National
Semiconductor
Corporation.
-10 to -15V
10Hz
f
Reprinted with permission of National Semiconductor Corporation.
L10: 6.111 Spring 2006
Introductory Digital Systems Laboratory
2
in
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The Inside of a 741 OpAmp
OpAmp
Reprinted with
permission of
National
Semiconductor
Corporation.
Differential
Input Stage
Current Source
for biasing
Additional
Gain Stage
Output Stage
Output devices
provides large
drive current
Bipolar version
has small input
Bias current
MOS OpAmps
have ~ 0 input
current
Gain is Sensitive to Operating Condition
(e.g., Device, Temperature, Power supply voltage, etc.)
Reprinted with permission of National Semiconductor Corporation.
L10: 6.111 Spring 2006
Introductory Digital Systems Laboratory
3
The Inside of a 741
287137590.005.png 287137590.006.png 287137590.007.png
Simple Model for an OpAmp
OpAmp
i + ~ 0
V CC
v out
V CC = 10V
+
-
+
v id
+
-100 μ V
v id
-
v out
i - ~ 0
-
ε = 100 μ V
Reasonable
approximation
-V CC
-V CC = -10V
Linear Mode
Negative Saturation
Positive Saturation
v id
+
v out
+
v id
+
+
v out
+
v id
+
+
v out
+
- av id
-V CC
+V CC
-
-
-
-
-
-
If -V CC < v out < V CC
v id < - ε
v id > ε
Small input range for “Open” loop Configuration
L10: 6.111 Spring 2006
Introductory Digital Systems Laboratory
4
Simple Model for an
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The Power of (Negative) Feedback
R
-
R
R 1
R 2
v
-
+
v out
v
+
+
out
v
+
v id
- av id
in
in
+
v
in
+
v
id
+
v
out
+
v
id
=
0
v
id =
v
out
v
in
=
−
v
out
⎣
1
+
a
+
1
⎦
R
R
a
R
a
R
R
R
1
2
1
1
2
2
v
out
=
−
R
2
a
≈
−
R
2
(
if
a
>>
1
v
( )
+
a
R
+
R
R
in
1
2
1
Overall (closed loop) gain does not depend on open loop gain
Trade gain for robustness
Easier analysis approach: “virtual short circuit approach”
v + = v - = 0 if OpAmp is linear
L10: 6.111 Spring 2006
Introductory Digital Systems Laboratory
5
⎡
⎤
1
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