Other Topologies
Hossein Shamsi
1
2
Single-Ended Current-Mirror Opamp
3
Current-Mirror Opamp with Wide-Swing Cascode Current
Sources
4
Fully Differential Current-Mirror Opamp
5
Fully Differential Opamp with Bidirectional Output Drive.
6
Fully Differential Opamp Composed of Two Single-Ended
Output Current-Mirror Opamps
7
Common-Mode Feedback Circuits
Common-mode-feedback (CMFB) circuitry is often the most difficult part of the opamp to design.
There are two types of CMFB circuit:
Continuous-time CMFB
Discrete-time CMFB
8
Continuous-time CMFB Circuit
Vcmo denotes the desired output common-mode voltage. err
V
outV
outV
cmov
2
8 6 14
6 9 10
8 6 6
9 14 10
2 '
2 1 '
ds ds m
m m m
err err
ds ds m out
out m
ctrl err
m m err
ctrl
r g r
g g g
v gain v
loop r
r g R
R v g
v
g g v
v
Small-signal analysis
9
Continuous-time CMFB Circuit
10
Continuous-time CMFB Circuit
tn eff
CM
A V V V
V 10
tn eff
CMO
ref V V V
V 10
8 6 14
6 9 13
8 6 6
9 14 13
4 2
1 2
ds ds m
m m m
err err
ds ds m out
out m
ctrl err
m m err
ctrl
r g r
g g g
v gain v loop
r r g R
R v g
v
g g v
v
Small-signal analysis
cmo out
out
err
V V V
v
2
11
Discrete-time CMFB Circuit
C S
C C C
C 0.25 1
.
0
It is suggested to choose the values of CC and Cs so that the following relation as follows:
Performing a few manipulations, we have:
( ) ( ) ( )
1
2
1 1
z V
z V
z V
C z C
C C z C
C z
V
z V
z z V
V
CM cmo
bias
C S
C C S
S
ctrl
out out
CM
