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

5

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

6

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

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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

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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

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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

14

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