Advanced Opamp Topologies (Part II) - VppSimvppsim.com/CircuitLectures/Lecture20.pdf · M.H....
Transcript of Advanced Opamp Topologies (Part II) - VppSimvppsim.com/CircuitLectures/Lecture20.pdf · M.H....
M.H. Perrott
Analysis and Design of Analog Integrated CircuitsLecture 20
Advanced Opamp Topologies (Part II)
Michael H. PerrottApril 15, 2012
Copyright © 2012 by Michael H. PerrottAll rights reserved.
M.H. Perrott
Outline of Lecture
Gain boosting technique Nested Miller technique Replica bias technique Improved slew rate opamp example
2
M.H. Perrott
Recall the 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)
3
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
Can we further boost the DC gain?
M.H. Perrott
Gain Boosting of Current Sources
We can achieve increased output impedance of a current source with an amplifier- The amplifier essentially increases gm1 by factor K
Key issue: what is a convenient implementation of the above circuit? 4
Vref
Rref
Iout
DC Gain = K Rout
M1
Rref
-gmb1vsvgs
vs
ro1gm1vgs
Vref
DC Gain = K
Iout
Rout
Rout = (Kgm1ro1)Rref
M.H. Perrott
A Simple Gain Boosting Amplifier
Common source amplifier utilized
Issue: current source requires significant headroom due to the fact that Vds2 = Vgs4
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Iout
Rout
M1
M3M2
Iref
M4
Ibias
Iout
Rout
M1
M3M2
Iref
Vref
DC Gain = K
K = gm4ro4, Rref = ro2
⇒ Rout = (gm4ro4) (gm1ro1) ro2 ≈ (gmro)2ro2
M.H. Perrott
Folded Cascode Gain Boosting Amplifier
Folded cascode yields
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Iout
Rout
M1
M3M2
IrefVbias2
Vbias3
Vbias4
M7
M8
M6
M5
M4Vbias1
Vbias5
K = gm4 (((gm6ro6)ro5)||((gm7ro7)ro8))⇒ Rout ≈ (gmro)3ro2
Is there a convenient way to set Vbias5?
- Improved headroom and higher gain!
M.H. Perrott
Differential Version of Gain Boosting Amplifier
Leverage fully differential nature of current sources within the opamp- PMOS gain devices are now part of a differential pair- Need CMFB to set common-mode gate voltages of M1
and M27
IoutRout
M1
M4M3
Vbias2
Vbias3
Vbias4
M11
M13
M9
M7
M5Vbias1
Vbias2
Vbias3
Vbias4
M12
M14
M10
M8
M6Vbias1
Iout
M2
Rout
Vbias0
Ibias
M.H. Perrott
Symbolic View of Folded Cascode Gain Boosting Amp
We can apply this to the overall folded cascodeopamp
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IoutRout
M1
M4M3
Iout Rout
Vbias0
M.H. Perrott
Folded Cascode with Gain Boosting
Gain boosting provides substantial increase of DC gain while maintaining good input and output swing- Gain is on the order of (gmro)4
Issue – very complex!9
Vbias4
M7 M8
M9 M10
CLCL
Vout+Vout-
Controlledby CMFB
M1 M2Iref
M12M11
Vin+Vin-
Vbias1
M5 M6
M3 M4
M.H. Perrott
Recall Pole Splitting for Two Stage Compensation
Moves the dominant pole of the second stage to higher frequencies such that it becomes a parasitic pole
Places the first stage pole as the dominant pole- Leverages the gain of the second stage to achieve
capacitor multiplication using the Miller effect
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-gmgm
Vout/Vid
w (rad/s)
20log
wp2 wp1wp1 wp2
Can we extend the pole splitting techniqueto more than 2 gain stages?
M.H. Perrott
Nested Miller Compensation
Advantage: increased DC gain with high input and output swing
Issue: more parasitic poles to deal with- Leads to lower unity gain bandwidth for reasonable
phase margin
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-gmgmgm
Vout/Vid
w (rad/s)
20log
wp1wp2wp1 wp2,wp3wp3
Proving to be a useful technique in advanced CMOS processeswhich offer fast speed (high gm/C) but low intrinsic gain (low gmro)
Eschauzier, JSSC Dec 1992
M.H. Perrott
Nested Miller Example
Intermediate gain stages must be non-inverting in order to achieve stable feedback
Compensation resistors should also be included to eliminate the impact of RHP zeros- Not shown for simplicity
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M7
M6
IrefM1 M2
M3
M8
Vout
CL
Cc
M4
M5
Vin+Vin-M10 M11
M12 M13
M9
Vbias Cc2
M.H. Perrott
Recall the Telescopic Opamp
Key issue is input swing- Can we improve this?
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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
Replica Bias Technique
Allows current source to maintain its output current even for low Vds using dynamic bias of Vgs- Allows extended input common-mode range
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Gulati, JSSC Dec, 1998
M4
Vin+M3
M1 M2
IrefVbias1
Vbias2
Vbias3
M4
M5 M6
M7 M8
M9M10
Vin-
CLCL
Vout+Vout-
Controlledby CMFB
Vin+
M11 M12
Vin-
K
M.H. Perrott
Recall: Slew Rate Issues for Opamps
Output currents of practical opamps have max limits- Impacts maximum rate of charging or discharging load
capacitance, CL- For large step response, this leads to the output lagging behind the ideal response based on linear modeling We refer to this condition as being slew-rate limited
Where slew-rate is of concern, the output stage of the opamp can be designed to help mitigate this issue- Will lead to extra complexity and perhaps other issues
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Vin
Vout
Vdd
Vss
Vin
Vout
ideal
slew-rate limitedCL
M.H. Perrott
Key Observations for Slew Rate Calculations
First stage- Max I1 = Ibias1- Min I1 = -Ibias1
Second stage- Max I2 = Ibias2- Min I2 = Large
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Vid Voutavd1 avd2
I1
Cc
CL
I2
M6
M1 M2
M3
Vout
CL
CcRc
M4
Vid/2-Vid/2
Ibias1 Ibias2
Current Limits
How can we improve opamp slew rate?
M.H. Perrott
Class A and AB Amplifiers/Buffers
Class A- Maximum slew rate in one direction is set by the
nominal bias current Class AB
- Maximum slew rate is not set by the nominal bias current Goal: low nominal bias current
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IbiasM2
M1
Vin
Vout
Vbias
M2
M1
Vin
Vout
Ibias M1
Vout
M2
Ibias
Vbias
Vin
Class A Amplifier Class AB Amplifier/Buffer
M.H. Perrott
Class AB Opamp
Low bias current can be achieved for Vin+ = Vin-- Must properly set Vbias
Much higher current when Vin+ ≠ Vin-
DC gain can be increased through cascoding of output stage 18
M1
M3
Vbias
M2
M4
CL
Vout+Vout-
M8 M12
M6 M10
Ibias
M9 M5
M7M11
CL
Vin- Vin+
Vbias
Ibias
Costello, JSSC Dec 1985
M.H. Perrott
Biasing Network for Class AB Opamp
Bias current set by- Ratio of device sizes of M1-M4 versus M13-M16- Iref current
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M1
M3 M4
CL
Vout+Vout-
M8 M12
M6 M10
Ibias
M9 M5
M7M11
CL
Ibias
M15
M16
Vin+
Iref
M13
M14
Vin-
Iref
M2
M.H. Perrott 20
Summary
Opamps invite a wide variety of techniques to address different application requirements- Cleverness can substantially improve performance and
robustness- Changing of CMOS processes over time leads to new
techniques which were previously unnecessary or unpractical
Four techniques discussed today- Gain boosting- Nested Miller- Replica bias- Class AB stages