Tài liệu Circuits & Electronics P11 pdf

6.002 Fall 2000 Lecture
1
11
6.002
CIRCUITS
AND
ELECTRONICS
Small Signal Circuits
6.002 Fall 2000 Lecture
2
11

Small signal notation

v
A
= V
A
+ v
a
total
operating
point
small
signal
()
i
Vv
I
I
out

IOUT
vvf
dv
d
v
v
fv
II
⋅=
=
=
)(

S
V
L
R
oOO
vVv
+=
I
V
+
–
+
–
iII
vVv
+=
i

v
Review:
6.002 Fall 2000 Lecture
3
11
I Graphical view
(using transfer function)
behaves linear
for small
perturbations
I
v
O
v
Review:
6.002 Fall 2000 Lecture
4
11
II Mathematical view
()
L
TI
SO
R
VvK
Vv
2
2
−
−=

()
i
Vv
LTIS
I
o
v
RVv
K
V
dv
d
v
II
⋅






−−
=
=
2
2
related to V
I
constant for fixed
DC bias

()
iLTIo
vRVVKv ⋅−−=
g
m
Review:
6.002 Fall 2000 Lecture
5
11
Demo
Choosing a bias point:
DS
i
O
v
L
SL
TI
KR
VKR
Vv
211 ++−
+=
TI
Vv =
2
ODS
v
2
K

i <
load line
L
O
L
S
DS
R
v
R
V
i −=
How to choose the bias point,
using yet another graphical view
based on the load line
O
V
I
V
input signal
response
ILm
VRg ∝
1. Gain
2. Input valid operating range for amp.
3. Bias to select gain and input swing.