Op-Amp Topologies and Design Considerations
Popular op-amp topologies
P. Bruschi – Design of Mixed Signal Circuits
1
Two stage op-amp with simple differential amplifier in the input
stage and class-A common-source output stage
p-input version
Input range may include
gnd
M
a
y
s
i
n
k
l
a
r
g
e
c
u
r
r
e
n
t
s
n-input version
Input range may include
V
dd
M
a
y
s
o
u
r
c
e
l
a
r
g
e
c
u
r
r
e
n
t
s
Improved topologies
(better performances, but greater complexity)
P. Bruschi – Design of Mixed Signal Circuits
2
Cascode stages
Class AB stages
Larger gain, simpler Rail-
to-Rail input stages
Large output current with
small quiescent power
consumption
Class-AB output stages
P. Bruschi – Design of Mixed Signal Circuits
3
I
D5
does not depend on V
dd
, while I
D6
does.
The output short circuit current depends on V
dd
Poor PSSR
P. Bruschi – Design of Mixed Signal Circuits
4
If we want to save this solution,
we need to adapt both
V
GS5
and
V
GS6
when
V
dd
changes, and
guarantee that:
is always valid
Op-amps with class- AB output stages
Op-amps with class- AB output stages
P. Bruschi – Design of Mixed Signal Circuits
5
Bias chain
Voltage shifter
I
B
P. Bruschi – Design of Mixed Signal Circuits
6
Op-amps with class- AB output stages
G
m
2
=
g
m
5
+
g
m
6
Second stage
Voltage shifter:
additional singularities
Y
m
2
(s)
G
m
2
(s)
Limitations: minimum V
dd
P. Bruschi – Design of Mixed Signal Circuits
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Minimum Vdd: 3V
GS
2.1 V
For the class-A amplifier
, the minimum Vdd was
only V
GS
+2V
DSAT
0.9 V
Reduced V
GS
excursion for the output devices
P. Bruschi – Design of Mixed Signal Circuits
8
•
Both V
GS5
and V
GS6
cannot reach
V
dd
. If large ouput currents are
required, this means that M5 and
M6 should be designed with very
large W.
•
I
ON-MAX
strongly depends on the
input common mode voltage
The Monticelli's class-AB stage
P. Bruschi – Design of Mixed Signal Circuits
9
D. M. Monticelli, “A quad CMOS single-supply op amp with rail-to-rail
output swing,” IEEE J. Solid-Stare Cite., vol. SC-21, pp. 1026-1034, Dec. 1986.
I
h
and
I
k
includes also the
variations due to the input
signal and have a high
output differential resistance
(not shown for simplicity)
I
n
I
p
P. Bruschi – Design of Mixed Signal Circuits
10
The Monticelli's class-AB stage
output devices
M
NM
and M
MP
, are designed to have
same
g
m
in the operating point:
v
a
=
v
b
(variations)
V
B
Here, Ip>>In, then
v
gn
>>v
gp
and V
B
reduces
First stage: folded cascode for improved gain and larger swing
P. Bruschi – Design of Mixed Signal Circuits
11
Folded cascode with rail-to-rail input range
n-input folded cascode
High performance - two-stage CMOS op-amp
P. Bruschi – Design of Mixed Signal Circuits
12
M22-M23 and M33-M34 produce
the gate bias for M24 and M35,
respectively
M24 and M35 form the
Monticelli's cell
In quiescent conditions, we design I
bias
,
M23 and M34 to make: V
GS24
=V
GS23
and
V
GS35
=V
GS34
This simplify
setting of M1,M2
quiescent current
R. Hogervorst, J.P. Tero, R. G. H. Eschauzier, and J. H. Huijsing, “
A
Compact Power-Efficient 3 V CMOS
Rail-to-Rail Input/Output Operational Amplifier for VLSI Cell Libraries”,
IEEE J. SOLID-STATE CIRCUITS. vol. 29, 1994
Commercial products
P. Bruschi – Design of Mixed Signal Circuits
13
This content discusses popular op-amp topologies, including two-stage op-amps, design improvements for better performance, and the use of class-AB output stages. It delves into the complexities and considerations involved in designing op-amps, such as adapting to changing Vdd values and ensuring the validity of parameters. Limitations and issues related to minimum Vdd and reduced VGS excursions are also explored.
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Presentation Transcript
Popular op-amp topologies Two stage op-amp with simple differential amplifier in the input stage and class-A common-source output stage p-input version Input range may include gnd May sink large currents n-input version Input range may include Vdd May source large currents P. Bruschi Design of Mixed Signal Circuits 1
Improved topologies (better performances, but greater complexity) Class AB stages Cascode stages Large output current with small quiescent power consumption Larger gain, simpler Rail- to-Rail input stages P. Bruschi Design of Mixed Signal Circuits 2
Class-AB output stages ( V 1 2 = 5 7 I I ID5does not depend on Vdd, while ID6does. The output short circuit current depends on Vdd 5 D bias 3 8 ) 2 = = 6 I V V V V V Poor PSSR 6 6 D GS tp 6 5 GS DD batt GS 2 P. Bruschi Design of Mixed Signal Circuits 3
Op-amps with class- AB output stages If we want to save this solution, we need to adapt both VGS5and VGS6when Vddchanges, and guarantee that: I I = 5 6 D D is always valid P. Bruschi Design of Mixed Signal Circuits 4
Op-amps with class- AB output stages = 5 I I 5 3 D D Bias chain Voltage shifter 3 = = I I I 11 I 10 11 7 D D D = 22 B 7 I 7 7 D IB = V V V V 6 10 3 GS DD GS GS = V V V V 22 21 20 GS DD GS GS I I = 20 B = 21 B = I I V V I I = 6 6 D 6 22 GS GS 3 3 D 10 10 D 7 7 D P. Bruschi Design of Mixed Signal Circuits 5
Op-amps with class- AB output stages Second stage Gm2=gm5+gm6 Voltage shifter: additional singularities Ym2(s) Gm2(s) P. Bruschi Design of Mixed Signal Circuits 6
Limitations: minimum Vdd Minimum Vdd: 3VGS 2.1 V For the class-A amplifier , the minimum Vdd was only VGS+2VDSAT 0.9 V P. Bruschi Design of Mixed Signal Circuits 7
Reduced VGSexcursion for the output devices ( ) 2 = 6 I V V OP MAX 6 GS MAX tp 2 ( ) 2 = 5 I V V ON MAX 5 GS MAX tn 2 Both VGS5and VGS6cannot reach Vdd. If large ouput currents are required, this means that M5 and M6 should be designed with very large W. ION-MAXstrongly depends on the input common mode voltage = + V V V DSAT V 5 GS MAX iC GS = V V V 6 10 GS MAX dd GS P. Bruschi Design of Mixed Signal Circuits 8
The Monticelli's class-AB stage Ihand Ikincludes also the variations due to the input signal and have a high output differential resistance (not shown for simplicity) p n InIp = I I const h k + + 0 i = i I = I const v n p p n i g i g v n mMN gn p mMP gp = g v g v mMN gn mMP gp v v g g gn = mMP gp mMN D. M. Monticelli, A qu d C OS in l -supply op amp with rail-to-rail ou pu win , IEEE J. Solid-Stare Cite., vol. SC-21, pp. 1026-1034, Dec. 1986. P. Bruschi Design of Mixed Signal Circuits 9
The Monticelli's class-AB stage Here, Ip>>In, then vgn>>vgp and VBreduces p VB n output devices MNMand MMP, are designed to have same gmin the operating point: va=vb (variations) v v g g gn = mMP = I 2 g m D gp mMN P. Bruschi Design of Mixed Signal Circuits 10
First stage: folded cascode for improved gain and larger swing n-input folded cascode Folded cascode with rail-to-rail input range P. Bruschi Design of Mixed Signal Circuits 11
High performance - two-stage CMOS op-amp 3 High gain: ???? Class-AB output stage Rail-to-Rail input range M24 and M35 form the Monticelli's cell M22-M23 and M33-M34 produce the gate bias for M24 and M35, respectively In quiescent conditions, we design Ibias, M23 and M34 to make: VGS24=VGS23and VGS35=VGS34 This simplify setting of M1,M2 quiescent current = = R. Hogervorst, J.P. Tero, R. G. H. Eschauzier, and J. H. Huijsing, A Compact Power-Efficient 3 V CMOS Rail-to-R il Inpu /Ou pu Op r ion l Amplifi r for VLSI C ll Libr ri , IEEE J. SOLID-STATE CIRCUITS. vol. 29, 1994 V V V V 1 22 GS GS 2 33 GS GS P. Bruschi Design of Mixed Signal Circuits 12
Commercial products P. Bruschi Design of Mixed Signal Circuits 13
