A High-Gain High-Speed Low-Power Class-AB Operational Amplifier

Hassan Sarbishaei

Tahereh Kahookar Toosi

Ehsan Zhian Tabasy

Reza Lotfi

Integrated Systems Lab, Ferdowsi University of

Mashhad, Iran

hsarbishaei@ieee.org

tktoosi@ieee.org

ezhian@ieee.org

rlotfi@ieee.org

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

Introduction

Demand for portable equipments + Technology scaling

=> Low-power low-voltage circuit design.

Design bottleneck in LP LV circuit design : Fast and accurate opamps.

=> Low-voltage high-gain opamp architecture is of much interest.

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

High-gain Opamps

Three common methods

– Cascoding and gain boosting – Cascading– Positive feedback

Integrated Systems Lab, Ferdowsi University of Mashhad

High-gain Opamps (Cont’d)

Disadvantages

– Cascoding : Not suitable for low-voltage applications.

– Cascading : Frequency compensation complexity.

– Positive Feedback : Nonlinearity issues, Sensitive to process variations.

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

Positive Feedback Amplifier

)1( ololmi

cli

o AAgv

v

i

o

oioomm

ol

v

v

ggggg

Aif

2112

1

[Amourah et al. ISCAS2001]

Integrated Systems Lab, Ferdowsi University of Mashhad

Positive Feedback Amplifier (Cont’d)

Advantages– High DC-gain– No CMFB circuit is required.– High PSRR from negative rail.

Disadvantages– Limited output voltage swing – Process variation dependency

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

Current Driven Bulk Technique

Based on equations above :

ox

i

ox

ox

ox

BFmsth C

Q

C

Q

C

QV 0

0 2

FsiDB qNQ 20

00 thBD VQNI

Disadvantages : Lower output impedance and slew rate. Low frequency pole-zero pair.

[Ardalan et al. ICECS2004 ]

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

Single-Stage Class-AB Structure

Flipped Voltage Follower (FVF)

– Almost constant VGS(MFVF)

– Low output impedance– High sinking capability

[Ramirez-Angulo et al. ISCAS2004 ]

Integrated Systems Lab, Ferdowsi University of Mashhad

Single-Stage Class-AB Structure (Cont’d)

Low-Voltage Single-Stage Class-AB Amplifier using FVF block

Advantage– Small, non slew rate

limited settling time.

[Baswa et al. ISCAS 2004 ]

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

Proposed Opamp

Combination of all three previous techniques

=> High gain, High slew-rate and high swing opamp.

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

Simulation Results

Integrated Systems Lab, Ferdowsi University of Mashhad

Simulation Results (Cont’d)

DC-gain at all process and temperature corners :

Integrated Systems Lab, Ferdowsi University of Mashhad

Outline

Introduction High-gain Opamps Positive Feedback Amplifier Current Driven Bulk Technique Single-Stage Class-AB Structure Proposed Opamp Simulation Results Conclusion

Integrated Systems Lab, Ferdowsi University of Mashhad

Conclusion

A high-gain high-speed low-power class-AB operational amplifier was presented.

Positive feedback gain enhancement technique was used to boost DC gain.

Class-AB technique was employed to get a higher slew rate with lower power.

And finally to overcome the swing limitation, CDB technique was applied to load transistors to decrease threshold voltage.

The proposed opamp can be used as the first stage of a two stage operational amplifier with rail-to-rail output swing.