Lecture 9: High-Gain Single-Stage Op Amps

52 Pages·2010·0.62 MB·English

EE 435
Lecture 9:
High-Gain Single-Stage Op Amps
- Regulated Folded Cascode Op Amp
- Cu rrent-mirror op amps
- OT A Applications

1
Textbook reference:
Some of the material we have been discussing appears in
Chapter 3, some in Chapter 5, and some in Chapter 6 of the
Martin and Johns text
In particular, the telescopic and folded cascode structures are
referred to as advanced op amps and appear in later chapters of
the text
2
Review from Last Time
What circuit is this?
I
B1 B2 V
OUT
OUT
V
DD
BB 3 I
V M
B1
B1 3
V
IN
1 M
1
IN
V
SS
SS
Folded Cascode Amplifier Biased Folded Cascode
3
Review from Last Time
Folded Cascode Op Amp
(cid:131)Needs CMFB Circuit for V
B4
(cid:131)Either single-ended or differential outputs
(cid:131)Can connect counterpart as current mirror to eliminate CMFB
(cid:131)Folding caused modest deterioration of A and GB energy efficiency
V0
(cid:131)Modest improvement in output swing 4
Review from Last Time
Folded Gain-boosted Telescopic
Cascode Op Amp
V
DD
V
B1
g
M M
− m1 5 6
2
A ≈
o g g
(g +g ) o3 +g o9 M3 A3 A4 M
4
O1 O5 o7
A g A g VB2 VOU
3 m3 1 m9 T
V
B3
M9 A1 A2 M1 CL
g VI VI 0 V
IN
GB = m1 N M1 M2 N
2C M M
L 7 8
V
B4
I I
T T2
VS
VS S
S
(cid:131)Needs CMFB Circuit for V
B4
(cid:131)Either single-ended or differential outputs
(cid:131)Can connect counterpart as current mirror to eliminate CMFB
(cid:131)Folding caused modest deterioration in GB efficiency and gain
(cid:131)Modest improvement in output swing 5
Review from Last Time
Operational Amplifier Structure Comparison
Small Signal Parameter Domain
Reference
1 g g I
A = m1 GB = m1 SR = T
Op Amp
VO 2 g + g 2C 2C
O1 O3 L L
g
Telescopic
m1
2 g I
Cascode A = GB = m1 SR = T
o g g
g o3 +g o5 2C 2C
o1 g o7 g L L
m3 m5
Regulated g
m1 g I
Cascode A ≈ 2 GB = m1 SR = T
o g g 2C 2C
g o3 +g o9
o1 g A o7 g A L L
m3 1 m9 3
Folded g
m1
I
g
Cascode A = 2 GB = m1 SR = T
o g g 2C
(g +g ) o3 +g o9 2C
L
o1 o5 g o7 g L
m3 m9
Folded g
m1 I
g
Regulated A = 2 GB = m1 SR = T
o g g 2C
Cascode (g +g ) o3 +g o9 2C
L
o1 o5 g A o7g A L
m3 3 m9 9
6
Review from Last Time
Summary of Folded Amplifier Performance
• + Modest improvement in output signal
s wing (from 5 V to 4V )
DS SAT DS SAT

• - Deterioration in A (maybe 30% or
V0
m ore)
• - Deterioration in GB power efficiency
( can be significant)
• - Minor increase in circuit size

7
Folded Gain-boosted
Cascode Amplifier
− g
A ≈ m1
o g
( )
g o3
o1
A g
m3
g
GB = m1
2C
L
(cid:131) with ideal current source bias
(cid:131) modest improvement in output swing
8
Folded Gain-boosted
Cascode Amplifier
V − g
OUT ≈ m1
( )
V g + g g
IN sC + o1 o5 o3
L
g A
m3
− g g A
A ≈ m1 m3
0 ( )
g + g g
o1 o5 o3
g
GB = m1
C
L modest improvement in output swing
9
Basic Amplifier Structure Comparisons
Small Signal Parameter Domain
Common Source
g g
A = m GB = m
VO g C
O L
Cascode g g g
A = m1 m3 GB = m1
VO
g g C
O1 o3 L
g
Regulated g g
GB = m1
A ≈ m1 m3 A
Cascode VO g g C
O1 o3 L
g
Folded Cascode
g g
GB = m1
A = m1 m3
VO ( ) C
g +g g
O1 O5 o3 L
Folded Regulated g g g
A = m1 m3 A GB = m1
Cascode VO ( )
g +g g C
O1 O5 o3
L
10