advanced opamp topologies - cppsim · m.h. perrott summary of single-ended versus fully...
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M.H. Perrott
Analysis and Design of Analog Integrated CircuitsLecture 19
Advanced Opamp Topologies
Michael H. PerrottApril 11, 2012
Copyright © 2012 by Michael H. PerrottAll rights reserved.
M.H. Perrott 2
Opamps Are Utilized in a Wide Range of Applications
Each application comes with different opamp requirements- How are the input common-mode range requirements
different among the above applications?- How are the output range requirements different?- How are the bandwidth requirements different?
C1
R1Vin
Vref
Vout
C2
Vin
Vref
VoutC1
Vref
Rref
Iref
Analog Filters Current References Switched Capacitor Circuits
VinVout
Analog Buffers
Integrated opamps are typically custom designedfor their specific application
M.H. Perrott
Single-Ended Versus Fully Differential Topologies
Analog circuits are sensitive to noise from the power supply and other coupling mechanisms
Fully differential topologies can offer rejection of common-mode noise (such as from supplies)- Information is encoded as the difference between two
signals- More complex implementation than single-ended
designs3
C1
R1Vin
Vref
Vout
Single-EndedC1a
R1aVin+ Vout+
R1b
Vin- Vout-
C1b
Fully Differential
M.H. Perrott
Key Focus of Lecture
Examine fully differential version of basic two stage opamp
Examine more advanced opamp topologies and the advantages/disadvantages they present
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M.H. Perrott
Fully Differential Version of Basic Two Stage Opamp
We can separate this into differential and common mode circuits, similar to a single differential amplifier- Differential behavior same as the single-ended opamp
Note that we have twice the effective range in input/output swing due to the differential signaling
- Common mode setting needs to be dealt with5
M7a
M6a
IrefM1 M2
M3
M8
Vout+
CLa
CcaRca
M4
M5
Vin+Vin-
M7b
M6b
Vout-
CLb
Ccb RcbVbias
M.H. Perrott
Illustration of Common Mode Influence
Maximum swing for fully differential signals requires- Accurate setting of the common mode value- Suppression of common mode noise
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C1a
R1aVin+ Vout+
R1b
Vin- Vout-
C1b
Vcm
Vout+ Vout-
Vdd
Gnd
Vcm
Vout+ Vout-
Vdd
Gnd
Vcm
Vout+ Vout-
Vdd
Gnd
Common-Mode Too High Common-Mode Too Low Common-Mode Just Right
M.H. Perrott
Common-Mode Gain From Input
Analysis is same as for single-ended design- Can be simplified to common-mode “half-circuit”
- Common-mode output is sensitive to common-mode input7
M1 M2
ro3
2ro5 2ro5
ro4
vic vic
M7a
M6a
Iref
M8
Vout+
CLa
CcaRca
M7b
M6b
Vout-
CLb
Ccb Rcb
Common-Mode Half Circuit
avc =ro4
1/gm2 + 2ro5gm6a (ro6a||ro7a)
M.H. Perrott
Common-Mode Gain From Input Bias
Common mode “half circuit can still be used
- Common-mode output is extremely sensitive to Vbias!8
M1 M2
2ro5 2ro5
vic vic
M7a
M6a
Iref
M8
Vout+
CLa
CcaRca
M7b
M6b
Vout-
CLb
Ccb Rcb
Common-Mode Half Circuit
M3 M4
Vbias
avbias ≈ (gm4ro4) gm6a (ro6a||ro7a)
M.H. Perrott
Common Mode Feedback Biasing (CMFB)
Use an auxiliary circuit to accurately set the common mode output value to a controlled value Vref- Need to be careful not to load the outputs with the
common mode sensing circuit (Rlarge in this case)- Need to design CMFB to be stable
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M1 M2
2ro5 2ro5
vic vic
M7a
M6a
Iref
M8
Vout+
CLa
CcaRca
M7b
M6b
Vout-
CLb
Ccb Rcb
Common-Mode Half Circuit
M3 M4
Vbias
M10 M11
M12 M13
M9
RlargeVref
Rlarge
M.H. Perrott
Parasitic Pole/Zero Pair of Current Mirrors
Single-ended signal travels through current mirror- Introduces an extra parasitic pole/zero
Fully differential signal does not see this pole/zero pair10
M1 M2
M4
M5
Vin+Vin-
Vbias
M1 M2
M4
M5
Vin+Vin-
isc
Single-Ended Fully-Differential
M3M3
isc+isc-
wp par =gm3
Cgs3 + Cgs4wz par = 2wp par
M.H. Perrott
Closer Examination of Pole/Zero Relationship
Note that signal at V2 is composed of the sum of paths (a) and (b) shown above
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M1 M2
M4
M5
Vid/2-Vid/2
Single-Ended
M3
(a) (b)
(a)
(b)
(a)+(b)
wp_par wz_par
w
iscvid
isc
isc(s)
vid(s)=gm
2+
µgm
2
¶1
1 + s/wp par
=
µgm
2
¶2+ s/wp par
1+ s/wp par= gm
1+ s/(2wp par)
1 + s/wp par
M.H. Perrott
Summary of Single-Ended Versus Fully Differential
Advantages of fully differential topologies- Improved CMRR and PSRR across a wide frequency range- Twice the effective signal swing- Removal of pole/zero pair due to current mirror
Potentially improved phase margin Disadvantages of fully differential topologies
- Power and complexity
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C1
R1Vin
Vref
Vout
Single-EndedC1a
R1aVin+ Vout+
R1b
Vin- Vout-
C1b
Fully Differential
Most opamp topologies can be modified to supporteither single-ended or fully differential signaling
M.H. Perrott
Telescopic Opamp (Fully Differential Version)
Popular for high frequency applications- Single stage design- Limitation: input and output swing quite limited
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M4
Vin+M3
M1 M2Iref
Vbias1
Vbias2
Vbias3
M4
M5 M6
M7 M8
M9M10
Vin-
CLCL
Vout+Vout-
Controlledby CMFB
M.H. Perrott
Open Loop Response of Telescopic Opamp
Determine K, wdom, wo, wp
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M4
M3
M1 M2Iref
Vbias1
Vbias2
Vbias3
M4
M5 M6
M7 M8
M9M10
Vid/2
CLCL
Vout+Vout-
Controlledby CMFB
(Vout+-Vout-)/Vid
w (rad/s)wdom
20log
20log(K)
wp
0dB
w0-Vid/2
Why is this topology good for high bandwidth applications?
M.H. Perrott
Open Loop Response of Telescopic Opamp
Notice that parasitic pole is much higher than for two stage opamp, yielding higher potential unity gain BW
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M4
M3
M1 M2Iref
Vbias1
Vbias2
Vbias3
M4
M5 M6
M7 M8
M9M10
Vid/2
CLCL
Vout+Vout-
Controlledby CMFB
(Vout+-Vout-)/Vid
w (rad/s)wdom
20log
20log(K)
wp
0dB
w0-Vid/2
where Rout = ((gm4ro4)ro2)||((gm6ro6)ro8)
wdom = 1/(RoutCL)K = gm2Rout
wo =gm2CL
wp ≈gm4
Cgs4 + Cs4,d2
M.H. Perrott
Telescopic Opamp (Single-Ended Version)
Issue: parasitic pole lower than fully differential version
- Singled-ended version not as useful for wide bandwidth16
M4
Vin+M3
M1 M2Iref
Vbias1
Vbias2
M4
M5 M6
M7 M8
M9M10
Vin-
CL
Vout
wp2 ≈gm7
Cgs7 + Cgs8 + Cd3,d5< wp1 ≈
gm4Cgs4 + Cs4,d2
M.H. Perrott
Folded Cascode Opamp
Modified version of telescopic opamp- Significantly improved input/output swing- High BW (better than two stage, worse than telescopic)- Single stage of gain (lower than telescopic)
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Vbias2
Vbias3
Vbias4
M7 M8
M9 M10
CLCL
Vout+Vout-
Controlledby CMFB
M1 M2Iref
M12M11
Vin-Vin+
Vbias1
M5 M6
M3 M4
Ibias1/2 Ibias1/2
Ibias2 Ibias2
Ibias2-Ibias1/2Ibias2-Ibias1/2
Must set Ibias2 > Ibias1/2
M.H. Perrott
Open Loop Response of Folded Cascode Opamp
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Vbias2
Vbias3
Vbias4
M7 M8
M9 M10
CLCL
Vout+Vout-
Controlledby CMFB
M1 M2Iref
M12M11
Vid/2
Vbias1
M5 M6
M3 M4
-Vid/2
(Vout+-Vout-)/Vid
w (rad/s)wdom
20log20log(K)
wp
0dB
w0
where Rout = ((gm6ro6)ro4)||((gm8ro8)ro10)
wdom = 1/(RoutCL)K = gm2Rout
wo =gm2CL
wp ≈gm8
Cgs8 + Cd2,d10,s8
Ro10 is lower than fortelescopic due to higher
drain current in M10
More capacitiveloading than
for telescopic
M.H. Perrott
Two Stage with Cascoded Output Stage
Higher DC gain than with two stage or folded cascode- Two gain stages with boosted gain on the output stage
Same output swing as folded cascode- Lower than for basic two stage
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Vbias2
M8
M9
CL
Vout
Vbias1
M7
M6
IrefM1 M2
M3
M10
CcRc
M4
M5
Vin+Vin-
M.H. Perrott
Open Loop Response Calculations
20
Vbias2
M8
M9
CL
Vout
Vbias1
M7
M6
IrefM1 M2
M3
M10
CcRc
M4
M5
Vid/2
Vout/Vid
wwdom
20log20log(K)
wp
0dB
w0
-Vid/2
where Rout = ((gm7ro7)ro6)||((gm8ro8)ro9)
wdom = 1/((ro2||ro4)CM)K = gm2(ro2||ro4)gm6Routwo =
gm2Cc
wp ≈gm6CL
CM ≈ (gm6Rout)Cc
M.H. Perrott
Two Stage with Cascoded Input Stage
Compared to two stage with cascoded output- Similar DC gain- Improved output swing- Reduced input swing
21
M7
CL
Vout
M6
Iref
M1 M2
M8
CcRc
M5
Vin+Vin-
Vbias1
M11 M12
M9 M10
M3 M4
3Ibias
M13
Ibias
M.H. Perrott
Open Loop Response Calculations
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M7
CL
Vout
M6
Iref
M1 M2
M8
CcRc
M5
-Vid/2
Vbias1
M11 M12
M9 M10
M3 M4
3Ibias
M13
Ibias
Vout/Vid
wwdom
20log20log(K)
wp
0dB
w0
Vid/2
where Rout1 = ((gm12ro12)ro2)||((gm10ro10)ro4)wdom = 1/(Rout1CM)
K = gm2Rout1gm6(ro6||ro7)wo =
gm2Cc
wp ≈gm6CL
CM ≈ (gm6(ro6||ro7))Cc
M.H. Perrott 23
Summary
Opamp topologies can be configured to process fully differential signals- Provides improved immunity to noise from common-mode
perturbations such as power supply noise- Increases effective signal swing by a factor of two- Carries additional complexity for CMFB and increased
power consumption Integrated opamps are often custom designed for a
given application- Each application places different demands on DC gain,
bandwidth, signal swing, etc.- Opamp topologies considered today include telescopic,
folded cascode, and modified two stage Each carries different tradeoffs on the above specifications