A LOW-DISTORTION CLASS-AB AUDIO AMPLIFIER WITH HIGH

POWER EFFICIENCY

BY

CHAITANYA MOHAN, B.Tech

A thesis submitted to the Graduate School

in partial fulfillment of the requirements

for the degree

Master of Sciences, Engineering

Specialization in: Electrical Engineering

New Mexico State University

Las Cruces, New Mexico

March 2011

“A Low-Distortion Class-AB Audio Amplifier with High Power Efficiency,” a the-

sis prepared by Chaitanya Mohan in partial fulfillment of the requirements for the

degree, Master of Sciences has been approved and accepted by the following:

Linda LaceyDean of the Graduate School

Dr. Paul M. FurthChair of the Examining Committee

Date

Committee in charge:

Dr. Paul M. Furth

Dr. Jaime Ramirez-Angulo

Dr. Jeffrey Beasley

ii

DEDICATION

Dedicated to my father Chandolu Rama Mohan Rao, mother Chandolu

Hemalatha, sister Srujana Mohan Rao.

iii

ACKNOWLEDGMENTS

First I would like to thank my parents Chandolu Rama Mohan Rao and

Chandolu Hemalatha, sister Srujana Mohan Rao and brother-in-law DeepakNadh

Tammana for supporting me at every level of my life. They are the reason behind

my success at every corner in the journey of life. Srujana has been more of a

friend, guide and advisor than a sister.

Dr. Paul M. Furth, the coach of VLSI V6 team is man behind the success

of this thesis. I can proudly say, the knowledge I acquired from him in Electronics

is more than what I have earned in my entire bachelors. The approach towards

every problem and level of analyzing things before hand is what I would like to

get from him.

I would also like to thank Dr. Jaime Ramirez-Angulo for imparting knowl-

edge on analog concepts.

A special note of thanks to my childhood friend Hareesh Gottipati (Nani),

Vidhul Dev and Arka who are more than just friends. I still remember the fights

we had on every other day on almost every topic. The topics included more of

politics, movies, places and almost every current situation, but the discussion

never involved studies.

Swetha Peri is one other person in my life who is more than a friend. She

had the patience to hear everything and take any situation casually with a calm

iv

mind. I would like to thank Swetha for being such a great friend and who always

supported my every decision.

A thanks is just not enough for Harish Valapala. I cannot forget the help

that I got from him, every time when I was supposed to meet the deadline.

I would also like to thank Alex from math department for giving an op-

portunity to work as a math tutor in the final semester. He has been very humble

during my defense and allowed me to work based on my availability, which was

very helpful

Finally I would like to thank all my friends and roommates: Sravan (Buggi),

Varun (Jaffa), Lalith (Makku bro), Venu, Madhusudhan Nagireddy (Madhu),

Suresh (Debri), Nikhilesh (Hadavidi) and especially the V6 group Punith (the

buss), Rajesh, Ramesh, Venkat and Harish.

v

VITA

December 17, 1986 Born in Hyderabad, India.

Education

2004 - 2008 B.Tech. Electronics and Communication Engineering,Jawaharlal Nehru Technological University, India

2009 - 2010 Teaching Assistant,New Mexico State University,USA

Since 2008 M.S in Electrical Engineering,New Mexico State University, USA

Awards and Achievments

2008 - 2011 In-State Tuition, NMSU,USA.

March - 2011 Third place in Graduate Research and Arts Symposium,NMSU, USA.

Field of Study

Major Field: Electrical Engineering (Analog Microelectronics/VLSI Design)

vi

ABSTRACT

A LOW-DISTORTION CLASS-AB AUDIO AMPLIFIER WITH HIGH

POWER EFFICIENCY

BY

CHAITANYA MOHAN, B.Tech

Master of Sciences, Engineering

Specialization in Electrical Engineering

New Mexico State University

Las Cruces, New Mexico, 2011

Dr. Paul M. Furth, Chair

Place: Thomas & Brown Room-207

Date: 03/17/2011 Time: 2:00 PM

A low-distortion three-stage Class-AB audio amplifier is designed to drive

a 16-Ω headphone speaker. High power efficiency in the design was achieved by

using fully-differential internal stages with local common-mode feedback networks

and replica biasing of the output stage. The threshold voltage of NMOS transistors

were made comparable to PMOS transistors by biasing the p-substrate in order

to achieve high linearity. The stability of the amplifier is achieved using multiple

compensation techniques. The audio amplifier is designed to drive widely varying

capacitive loads from 10 pF to 5 nF. The peak power delivered to the load is

vii

93.8mW. The quiescent power of the amplifier is 1.43mW. The output signal

swing is 2.45Vpp for ±1.5V supply. The THD of the amplifier is measured as -

79dB. The design has been implemented in a 0.5µm CMOS process and occupies

0.35 mm2 of area.

viii

TABLE OF CONTENTS

LIST OF TABLES xii

LIST OF FIGURES xiii

1 INTRODUCTION 1

2 BASE FOR AUDIO AMPLIFIERS 4

2.1 Audio Amplifier Specifications . . . . . . . . . . . . . . . . . . . . 4

2.1.1 Headphone Speaker Load . . . . . . . . . . . . . . . . . . . 5

2.1.2 Total Harmonic Distortion in an Amplifier . . . . . . . . . 5

2.1.3 Power Efficiency of an Amplifier . . . . . . . . . . . . . . . 6

2.2 Output Stage Classification . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 Class-A Amplifiers . . . . . . . . . . . . . . . . . . . . . . 7

2.2.2 Class-D Amplifier . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.3 Class-AB Amplifier . . . . . . . . . . . . . . . . . . . . . . 9

2.3 Multi-Stage Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3.1 Pseudo Class-AB Amplifier . . . . . . . . . . . . . . . . . 11

2.3.2 True Class-AB Amplifier . . . . . . . . . . . . . . . . . . . 13

2.4 Common-Mode Feedback Network . . . . . . . . . . . . . . . . . . 14

2.5 Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.5.1 Miller Compensation . . . . . . . . . . . . . . . . . . . . . 16

ix

2.5.2 Reverse-Nested Miller Compensation . . . . . . . . . . . . 17

2.6 Three-Stage Class-AB Amplifier from [1] . . . . . . . . . . . . . . 18

2.6.1 Design from [1] . . . . . . . . . . . . . . . . . . . . . . . . 18

2.6.2 Experimental Results from [1] . . . . . . . . . . . . . . . . 19

2.7 Replica Biasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3 DESIGN OF THE THREE-STAGE CLASS-AB AUDIO AMPLI-FIER 23

3.1 Architecture and Key Aspects of the Audio Amplifier . . . . . . . 23

3.2 Transistor Level Three-Stage Design . . . . . . . . . . . . . . . . 25

3.3 Bias circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4 Input-Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.5 Second-Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.5.1 PMOS differential amplifier . . . . . . . . . . . . . . . . . 31

3.5.2 NMOS differential amplifier . . . . . . . . . . . . . . . . . 32

3.6 Output-Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.7 Compensation used in the Design . . . . . . . . . . . . . . . . . . 35

3.8 Small-Signal Models . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.9 Pole-Zero Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4 SIMULATION RESULTS 43

4.1 DC analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.2 AC analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.3 Transient analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.4 THD analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5 HARDWARE TESTING 54

x

5.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.3 DC Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.4 Transient Measurements . . . . . . . . . . . . . . . . . . . . . . . 57

5.5 THD Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 58

6 DISCUSSION AND CONCLUSION 69

APPENDICES 74

A. HARDWARE TEST PROCEDURE 75

B. POLE/ZERO ANALYSIS USING MAPLE 85

C. MATLAB CODE TO PLOT WAVEFORMS 88

REFERENCES 95

xi

LIST OF TABLES

2.1 Comparison of measured results . . . . . . . . . . . . . . . . . . . 20

3.1 Transistor Dimensions . . . . . . . . . . . . . . . . . . . . . . . . 26

3.2 Poles and Zeros . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.1 Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2 AC Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . 46

4.3 Transient Simulation Results . . . . . . . . . . . . . . . . . . . . . 52

5.1 Hardware Measurements . . . . . . . . . . . . . . . . . . . . . . . 68

6.1 Summary of Hardware Test Results . . . . . . . . . . . . . . . . . 69

6.2 Comparison of results with state-of-the-art ([1]) . . . . . . . . . . 70

6.3 Simulation vs Hardware (LIQ) . . . . . . . . . . . . . . . . . . . . 71

6.4 Simulation vs Hardware (LTHD) . . . . . . . . . . . . . . . . . . 72

6.5 Simulation vs Hardware (MIQ) . . . . . . . . . . . . . . . . . . . 72

6.6 Simulation vs Hardware (HCL) . . . . . . . . . . . . . . . . . . . 73

xii

LIST OF FIGURES

2.1 Schematic of a Class-A amplifier . . . . . . . . . . . . . . . . . . . 7

2.2 Basic design of a Class-D amplifier . . . . . . . . . . . . . . . . . 9

2.3 Schematic of a Class-AB amplifier . . . . . . . . . . . . . . . . . . 10

2.4 Schematic of a three-stage pseudo class-AB amplifier . . . . . . . 12

2.5 Schematic of a three-stage true class-AB amplifier . . . . . . . . . 13

2.6 Schematic of a fully-differential amplifier with common-mode feed-back network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.7 Architecture of Miller compensation for two-stage amplifier . . . . 16

2.8 Architecture of Reverse-Nested Miller compensation for three-stageamplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.9 Architecture of the three-stage class-AB amplifier of [1] . . . . . . 19

2.10 (a) Schematic of two-stage pseudo class-AB amplifier (b) Replicabias circuit to control quiescent at the output stage . . . . . . . . 21

3.1 Architecture of the proposed three-stage class-AB amplifier . . . . 24

3.2 Schematic of the three-stage class-AB audio amplifier . . . . . . . 27

3.3 Schematic of the bias circuit . . . . . . . . . . . . . . . . . . . . . 28

3.4 Schematic of the first-stage . . . . . . . . . . . . . . . . . . . . . . 30

3.5 Schematic of the second-stage PMOS differential amplifier . . . . 31

3.6 Schematic of the second-stage NMOS differential amplifier . . . . 32

3.7 Schematic of the output-stage . . . . . . . . . . . . . . . . . . . . 34

xiii

3.8 (a) Left-half of the input-stage (b) small-signal model for left half. 36

3.9 (a) Right-half of the input-stage (b) small-signal model for right-half. 37

3.10 (a) PMOS differential amplifier (b) small-signal model PMOS dif-ferential amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.11 (a) NMOS differential amplifier (b) small-signal model NMOS dif-ferential amplifier. . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.12 (a) Schematic of output-stage (b) small-signal model for output-stage 40

3.13 Small-signal model of the designed three-stage class-AB audio am-plifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1 Schematic of the DC test-bench . . . . . . . . . . . . . . . . . . . 44

4.2 DC-analysis output . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.3 Schematic of the AC test-bench . . . . . . . . . . . . . . . . . . . 46

4.4 AC output of LIQ circuit . . . . . . . . . . . . . . . . . . . . . . . 47

4.5 AC output of LTHD circuit . . . . . . . . . . . . . . . . . . . . . 47

4.6 AC output of MIQ design . . . . . . . . . . . . . . . . . . . . . . 48

4.7 AC output of HCL circuit . . . . . . . . . . . . . . . . . . . . . . 49

4.8 Schematic of the Transient test-bench . . . . . . . . . . . . . . . . 49

4.9 Transient output of LIQ circuit . . . . . . . . . . . . . . . . . . . 50

4.10 Transient output of LTHD circuit . . . . . . . . . . . . . . . . . . 50

4.11 Transient output of MIQ circuit . . . . . . . . . . . . . . . . . . . 51

4.12 Transient output of HCL circuit . . . . . . . . . . . . . . . . . . . 52

4.13 Schematic for THD measurement . . . . . . . . . . . . . . . . . . 53

4.14 Transient output for measuring THD . . . . . . . . . . . . . . . . 53

5.1 Layout of LIQ amplifier . . . . . . . . . . . . . . . . . . . . . . . 55

5.2 Layout of LTHD amplifier . . . . . . . . . . . . . . . . . . . . . . 56

5.3 Layout of MIQ amplifier . . . . . . . . . . . . . . . . . . . . . . . 57

xiv

5.4 Layout of HCL amplifier . . . . . . . . . . . . . . . . . . . . . . . 58

5.5 Layout of the frame with two LIQ and MIQ. . . . . . . . . . . . . 59

5.6 Layout of the frame with two LTHD and HCL. . . . . . . . . . . . 60

5.7 Micrograph of the chip. . . . . . . . . . . . . . . . . . . . . . . . . 61

5.8 Transient response of LIQ design (an offset of 900mV is addedintentionally for visibility) . . . . . . . . . . . . . . . . . . . . . . 62

5.9 Transient response of LTHD design (an offset of 900mV is addedintentionally for visibility) . . . . . . . . . . . . . . . . . . . . . . 62

5.10 Transient response of MIQ design (an offset of 900mV is addedintentionally for visibility) . . . . . . . . . . . . . . . . . . . . . . 63

5.11 Transient response of HCL design (an offset of 900mV is addedintentionally for visibility) . . . . . . . . . . . . . . . . . . . . . . 63

5.12 THD measurement for LIQ design . . . . . . . . . . . . . . . . . . 64

5.13 THD measurement for LTHD design . . . . . . . . . . . . . . . . 65

5.14 THD measurement for MIQ design . . . . . . . . . . . . . . . . . 66

5.15 THD measurement for HCL design . . . . . . . . . . . . . . . . . 67

xv

Chapter 1

INTRODUCTION

The size of portable devices are decreasing with advances in technology; simi-

larly battery size is also decreasing [2],[3],[4]. The portable devices available in

the present day market such as laptops, cellphone, iPods and other music play-

ers require audio amplifiers that are capable of driving small resistive loads and

wide range of capacitive loads (headphone speakers). Audio amplifiers require

high current at the output stage to drive low resistive loads [5]. The main fea-

tures of audio amplifiers are low power dissipation, high output power and low

distortion[1],[2],[5],[6],[7]. The ideal choice for audio amplifiers are class-AB and

class-D amplifiers [6]. Though class-D amplifiers have high efficiency, low power

dissipation and low distortion [5], class-AB amplifiers are preferred for designing

audio amplifiers because they have better power supply rejection ratio (PSRR)

than class-D amplifiers [2],[6]. Moreover, class-D amplifiers are subject to electro-

magnetic interference [1],[5],[7].

A three-stage pseudo class-AB amplifier from [3] experiences a large qui-

escent current when the output stage current increases. An adaptive biasing

technique is used to transform the pseudo class-AB amplifier to a true class-AB

amplifier [3],[4]. However, the gain experienced by the load through PMOS output

transistor is different from the gain experienced by the NMOS output transistor.

This results in asymmetry at the output, which in turn causes severe distortion.

Although, the bias current of the amplifier is low, the distortion is large.

1

In order to obtain symmetry at the output, the load must experience the

same gain through both the NMOS and PMOS output transistors. Biasing the

output transistors at a low quiescent current is achieved using replica bias. The

replica biasing circuit is used to generate the required bias voltages at the gates

of the output transistor [8]. A local common-mode feedback network is used for

symmetrical gain and to generate a desired common-mode output voltage. Thus

it simultaneously improves the power efficiency and reduces distortion.

As the number of stages in an amplifier increases, the stability starts to

degrade [3],[4],[9],[10],[11]. Thus compensation networks are used to improve the

stability of a multi-stage amplifier. Some of the commonly used compensations

are Miller compensation with nulling resistor, nested Miller compensation and

reverse-nested Miller compensation for multi-stage amplifiers. Miller compensa-

tion with nulling resistor proposed in [10] is used to create a RHP zero to split

poles. Reverse-nested Miller compensation is more desirable for multi-stage am-

plifiers than nested Miller compensation as it improves the bandwidth [3],[9],[10].

Based on the class-AB amplifier in [3],[4], this thesis reports on the design of

a new three-stage class-AB amplifier. The class-AB amplifier has fully-differential

internal stages. A common-mode feedback network is used to provide the symmet-

rical gain and to generate a common-mode voltage at the output. Low quiescent

current at the output-stage is obtained using the replica bias circuit. Substrate

biasing technique is used to attain linearity at the output.

Chapter 2 describes specifications for designing an audio amplifier, the

types of output stages that can be used in the design of an audio amplifier, the

purpose of using using multi-stage amplifiers, the stability issues of multi-stage

amplifiers, and the compensation networks that are used to improve the stability,

bandwidth and transient response of the multi-stage amplifiers. A summary of

2

architecture and experimental results of three-stage class-AB amplifier from [1]

is described. The results of [1] are used as basis for designing a new three-stage

class-AB amplifier with improved figure of merit (FOM).

Chapter 3 explains the architecture of each stage of a three-stage class-AB

amplifier designed in this thesis. The working of replica bias circuit to generate low

quiescent currents at the output stage is discussed. The compensation networks

used for stabilizing the amplifier and the small signal models that explain how the

variation in compensation capacitor values improve the stability is also explained.

Chapter 4 discusses the results that determine the functionality of the

amplifier. The test-benches for DC, AC and transient analysis are explained. A

comparison of results for four designs with variation in compensation capacitor

values is summarized. A test-bench for measuring the total harmonic distortion

(THD) is explained.

Chapter 5 explains the hardware implementation of the three-stage class-

AB audio amplifier. The layout of the design is discussed and the test-setup of

the design for determining the quiescent current is explained in the DC testing.

The hardware testing results obtained are compared with simulation results.

Chapter 6 discusses about the figure of merit (FOM) of the designed am-

plifier. A summary of results obtained in [1] are compared with the hardware

testing results obtained in chapter 5.

APPENDICES contains the test procedure for testing the circuit in real-

time, the Maple work that determines the poles and zeros in an amplifier based

on the small-signal model and the code used for plotting the waveforms.

3

Chapter 2

BASE FOR AUDIO AMPLIFIERS

This chapter give an introduction to specifications of audio amplifiers, types of

amplifier output-stage, multi-stage amplifiers and compensation networks.

2.1 Audio Amplifier Specifications

Amplifiers are used in every electronic device. Though general purpose

op-amps can be used to drive a variety of loads but driving small resistive loads

is a tough task. The modern portable devices such as laptops, cellphones, Ipods

and other music players require audio amplifiers for driving headphone speakers.

The resistance of headphones is small. It can vary from 32-Ω to a much smaller

value depending on the supply voltage.

The device dimensions and supply voltages are decreasing with advances

in technology. In order to provide nearly constant output power, corresponding to

the human perception of loudness, reduced voltages necessitate reduced headphone

speakers. The output power POUT is given as

POUT =V 2RMSRL

(2.1)

Thus for small device dimensions and supply voltages, the load resistance must

be made small to maintain a constant output power. The important aspects in

an audio amplifier are load, total harmonic distortion and power efficiency.

4

2.1.1 Headphone Speaker Load

The output resistive load for an audio amplifier in portable devices is very

small. As the supply voltage in these devices is very small, the resistance of the

headphone must be made small to provide constant output power as shown in

(2.1). If the resistance of the headphone speaker is made large, then for small

supply voltages the output power of the amplifier decreases. Thus the loudness

is reduced. Modern day headphone speakers have resistance as small as 8-Ω.

The quiescent current is small for small supply voltages. The distortion in an

amplifier is inversely proportional to the quiescent current. As the quiescent

current decreases, the distortion in an amplifier increases.

2.1.2 Total Harmonic Distortion in an Amplifier

Total harmonic distortion is very important for amplifiers that drive large

capacitive loads and low resistances such as audio amplifiers. Harmonic distortion

occurs in amplifiers when the AC component of the drain current id is comparable

to DC component of the drain current ID [12]. Every amplifier produces harmonics

for a given fundamental frequency. The level, or amplitude, of these harmonics

is smaller than the amplitude, or level, of its fundamental frequency. The total

harmonic distortion is an important amplifier specification. It is given by

THD =

√V 22RMS + V

23RMS

+ V 24RMS ........+ V2nRMS

V1RMS(2.2)

where V1RMS is the rms voltage of the fundamental frequency and VnRMS is the

rms voltage of the nth harmonic.

The total harmonic distortion is measured in percentage (%). The lower

the value better the sound quality. Audio amplifiers with a THD of less than

0.5% produce an audio signal with noise which is hardly perceived by the human

5

ear. Thus audio amplifiers must be designed with a THD less than 0.5% for high

sound quality. In order to reduce distortion, feedback networks are used.

Apart from distortion, another important feature of an audio amplifier

is power efficiency. The power efficiency of an audio amplifier is made high by

lowering the quiescent current in the amplifier. This is discussed in the next

section.

2.1.3 Power Efficiency of an Amplifier

The battery life of a portable system is very important [1]. There will

always be demand in the market for systems that run for longer time. Thus in

order to have an efficient battery life, the quiescent power of the system should

be minimized. The power efficiency of the amplifier is defined as the ratio of

power delivered to the load to power supplied by the battery [1],[12]. The power

delivered to the load is given in (2.1). The power efficiency (η) of the amplifier is

given by

η% =POUTPS

× 100 (2.3)

where PS is the power supplied by the battery given by

PS = IQ(VDD − VSS) (2.4)

Thus, from (2.3) and (2.4), if PS is reduced, the efficiency of the amplifier will be

improved. In order to reduce PS, the quiescent current (IQ) can be reduced.

As supply voltages are smaller for new technologies, multi-stage amplifiers

are used to drive the load. The quiescent current increases with increase in number

of stages. The quiescent current also depends on the type of amplifier. The

linearity in the operational region and low power dissipation are important aspects

6

for audio amplifiers. Thus two types of amplifiers that provide either of the two

properties or both are discussed in the following sections- Class-A and Class-AB

amplifiers

2.2 Output Stage Classification

The output stage of amplifiers are classified based on the circuit config-

uration and the type of operation [8]. Three output stage classifications of an

amplifier are discussed in the following sections.

2.2.1 Class-A Amplifiers

The high linearity in the operational range of class-A amplifiers make them

ideal for audio applications. Owing to their high power dissipation these ampli-

fiers are replaced with class-AB amplifiers for audio applications. Thus class-A

amplifiers are limited to applications that require only small changes in the output

voltage, as the power consumption can be very low for small output signals. The

schematic of the class-A amplifier is shown in Fig. 2.1.

VB

M2

M1VIN

VOUT

RL

IB

Figure 2.1: Schematic of a Class-A amplifier

7

The gate of the transistor M2 is connected to a DC bias voltage (VB).

This turns the transistor M2 ON and a constant current (IB) is sourced from VDD

to bias transistor M1. Thus it acts as a current source. The input for a class-A

amplifier is at the gate of transistor M1. The gain of the amplifier is Gm1 ·Ro, where

Gm1 is the transconductance of the transistor M1 and Ro is the resistance at the

output node given by Ro = ro1‖ ro2‖RL. The current from transistor M1 increases

when the gate experiences a large signal at the input. The sinking current in

the amplifier is not limited but the sourcing current is limited to IB as shown in

Fig. 2.1. The upper-half cycle at the output observes distortion [8]. Moreover the

efficiency of the class-A amplifier is low. The efficiency of an amplifier is given in

(2.3). For class-A amplifiers the load power is given as

PL =V 2OUT2RL

(2.5)

and the power from battery PS is given as

PS = 2ID(VDD − VSS) (2.6)

The efficiency is maximum when VOUT = ID·RL = (VDD - VSS). The efficiency of

class-A amplifier accounts to 25% [13] using equations (2.3), (2.5), (2.6). Hence

makes it hard to be used in present day market of audio amplifiers.

To overcome the problem of low efficiency, class-AB amplifiers are preferred

over class-A amplifiers. The class-AB amplifiers have low power dissipation and

low distortion. The other classification of output stage amplifier known as class-D

amplifier, also provides low power dissipation and low distortion. This is explained

in the following section.

8

2.2.2 Class-D Amplifier

Class-D amplifiers find applications in audio amplifiers and pulse genera-

tors [8]. The basic design of a class-D amplifier is shown in Fig. 2.2. The output

of a Class-D amplifier is a sequence of pulses. The frequency of the output signal

is higher than input signal frequency. Passive filters are used at the output-stage

to eliminate undesired harmonics.

+

–

+Precision triangular

Wave generator

RL

VoutA1

Figure 2.2: Basic design of a Class-D amplifier

The amplitude of the output pulses is fixed. The conduction of switching

devices occurs during the transition of states. Hence, the power dissipation is

reduced as the transition is only for a short duration. Though class-D amplifiers

have an efficiency up to 95%, the power supply rejection ratio is higher when

compared to class-AB amplifiers, as one of the supply voltage is the output voltage

[8]. Hence, variations in supply voltage create variations at output.

Thus class-AB amplifiers are preferred for audio applications, as they have

higher power supply rejection ratio over class-D amplifiers.

2.2.3 Class-AB Amplifier

The basic schematic of the class-AB amplifier is shown in Fig. 2.3. The

transistor MN is ON for the negative half cycle of the output signal and the

transistor MP is ON for the positive half cycle. Thus, only one transistor sources

9

current for each half cycle while other is turned OFF. This kind of operation

provides a push-pull action. This minimizes the quiescent current of the amplifier.

The Vbat is used to turn ON both MN and MP for the short time when the output

signal is near zero. Thus it acts as a class-A amplifier for very small output signals.

The result is minimized crossover (crossing through zero) distortion.

MN

MP

VIN VOUT

RL

Vbat

Vbat

Figure 2.3: Schematic of a Class-AB amplifier

The efficiency of the class-AB amplifier is determined using (2.3). The load

power is given in (2.5). The power from the battery is the total average power

drawn through the power supplies given by

PS =2 · VOUT · (VDD − VSS)

(π + θ) ·RL+ 2IQ(VDD − VSS) (2.7)

where IQ(VDD - VSS) is the quiescent power (VOUT = 0) per transistor and (π

+ θ) is the conduction angle in radians, (π + θ) ≤ 2π. Thus the maximum

efficiency accounts to 50% ≤ η ≤ 78.5% [13]. The push-pull action, low distortion

and high power efficiency of class-AB amplifiers make them ideal choice for audio

applications.

10

A single stage class-AB amplifier does not provide the enough gain to

drive the load. Thus multi-stage amplifiers are used for driving small loads. As

the number of stages increase, the stability starts to degrade. To stabilize these

multi-stage amplifiers compensation networks are used. The following sections

focus on multi-stage amplifiers and compensation networks.

2.3 Multi-Stage Amplifiers

In the past, a single-stage amplifier was sufficient for providing large gain.

As the device dimensions were large, voltage levels were large to drive resistive

loads. A single-stage amplifier has only one pole, thus single-stage amplifiers are

highly stable. Advances in technology has made the device dimensions smaller.

Low voltages are required to operate these transistors. Hence a single-stage gain

is not sufficient to drive resistive loads.

In order to overcome this problem, two or more stages are cascaded together

to provide a gain that is the product of each gain stage [14],[15],[16].As the number

of stages increase, the stability starts to degrade. A compensation network is

required to provide stability. The compensation network becomes more complex

when the number of stages increases beyond four. A large number of multi-stage

amplifiers are proposed in the literature [9],[10],[11],[13]. A multi-stage pseudo

class-AB amplifier proposed in [17] is used as the basis for audio amplifier designed

in this work.

2.3.1 Pseudo Class-AB Amplifier

The schematic of a multi-stage amplifier is shown in Fig. 2.4. The first-

stage is a folded cascode differential amplifier formed with transistors N1−5 and

P1−4. The gain of the first-stage at node V1 is given by GM3 ·R1, where R1 is the

output resistance at V1. The second-stage is realized using a NMOS common-

source amplifier N6.

11

Vin+

Vb1

Vb3

Vb2

Vin-

Vb2

Vb3

N1 N4 N5 N6

N2 N3

P3

P1

P4

P2

P6

P5

N7

P8

N8

P7

RL CL

Rm3 Rm1Cm3 Cm1Cm2

Ccb

V1 V2

V3

Vout

Figure 2.4: Schematic of a three-stage pseudo class-AB amplifier

The output stage for sinking the current is designed using a NMOS common-

source amplifier N8. The third-stage or the output stage for sourcing the current

is realized using N7 common-source amplifier and P7 and P8 current mirror. Thus

the push-pull action is provided by P8 and N8 transistors.

The gain in the first-stage is a inverting gain. The common-source ampli-

fiers N6 and N8 of second-stage have inverting gain configuration. The gain of the

third stage realized with transistors N7 and P7−8 is a non-inverting gain. As the

two-stages of inverting gain from P8 and N7 common-source amplifiers are cas-

caded to obtain a non-inverting gain. Thus the overall gain from Vin+ to Vout is

non-inverting. This multi-stage pseudo class-AB amplifier is a low voltage design

for driving small loads.

Though it can drive small resistive loads, it cannot be used as audio ampli-

fier. As stated earlier, the low power dissipation and low distortion are the main

features of audio amplifiers. The quiescent current in this amplifier increases with

increase in current at the output stage. Thus the power dissipation of the ampli-

12

fier is large. In order to reduce the high quiescent current in this amplifier a new

technique called adaptive biasing has been proposed in [3],[4].

2.3.2 True Class-AB Amplifier

To minimize the current through the transistor P7 in Fig. 2.4, two resistors

Rad1 and Rad2 are used, as shown in Fig. 2.5. The diode connected transistor P7

and the resistors Rad1 and Rad2 form the adaptive biasing network [3],[4]. The

value of the resistors Rad1 and Rad2 are made large to minimize the current through

transistor P7 .

Vin+

Vb1

Vb3

Vb2

Vin-

Vb2

Vb3

N1 N4 N5 N6

N2 N3

P3

P1

P4

P2

P6

P5

N7

P8

N8

P7

RL CL

Rm3 Rm1Cm3 Cm1

Cm2

Ccb

V1 V2

V3

Vout

Rad1

Rad2

Adaptive biasing

Figure 2.5: Schematic of a three-stage true class-AB amplifier

The transistor P7 is in cutoff region when the output current is very low.

The load at node V3 is simply Rad2. The phase margin of the amplifier is improved

with the presence of resistor Rad2 and it moves the non-dominant pole to high

frequencies [3],[4].

When the load experiences a large sourcing current through the transistor

P8, the transistor P7 moves to the saturation region. Now, the load experienced

by node V3 is (1

GM7+ Rad1) ‖ Rad2. For large values of resistors, the gain at the

node V3 in Fig. 2.5 is large compared to the gain at node V3 in Fig. 2.4 [3],[4].

13

Thus the power efficiency and gain of the amplifier are improved. Though

the amplifier has good power efficiency but it cannot be used for audio amplifica-

tion, as the distortion in the amplifier is high. The gain experienced by the load

through P8 common-source is a three-stage gain, whereas the gain experienced by

the load through N8 common-source amplifier is a two-stage gain. This creates

non-linearity at the output and leads to distortion. To attain low distortion, load

must experience same gain through the push-pull transistors.

One way to achieve this is by using fully differential internal stages. The

fully differential amplifiers require additional circuitry to attain balanced outputs.

This additional circuitry is a common-mode feedback network. The working of

the common-mode feedback network is explained in the following section.

2.4 Common-Mode Feedback Network

A common-mode feedback network is used for generating a known voltage

at the output of a fully-differential amplifier [12] as shown in Fig. 2.6. The differ-

ence between the inverting terminal and the average of non-inverting terminals of

the common-mode feedback amplifier are amplified with a gain at the output.

If the difference between Vb2 and the average of Vout+ and Vout- is large,

then the voltage VCMFB increases. This increases the VGS of transistors N1 and

N4 and the current through them. Thus the voltage at the output nodes reach

close to Vb2. Hence a known voltage is obtained at the output. The circuit also

works for differential inputs. If one of the input voltage moves above the other

then one of the output goes above Vb2 by a small amount and the other output

moves down the Vb2 by the same amount.

There also exist some concerns while using these feedback networks. The

stability of the amplifier changes and it requires compensation networks to sta-

14

Vb2P1

Vin+Vin-N2 N3

N1 N4

CMFB++ -

P2

Vout+ Vout-

VCMFB

Figure 2.6: Schematic of a fully-differential amplifier with common-mode feedbacknetwork

bilize the differential amplifier, as well as common-mode feedback network. The

following section gives an introduction to compensation.

2.5 Compensation

Stability is an important aspect for amplifiers. The amplifiers with single

stage are highly stable as they have single dominant pole. In multi-stage amplifiers

the number of dominant poles are not limited to one. Thus the stability goes

15

down. Compensation networks are used, to improve the stability of multi-stage

amplifiers. Two types of compensation networks are reported in the following

sections.

2.5.1 Miller Compensation

A Miller compensation uses a resistor and a capacitor in series between

the input and output of an inverting stage [9],[11]. The architecture of the Miller

compensation is shown in Fig. 2.7.

-Av1 -Av2

CM

VIN VOUT

RM

R1

-gm1 -gm2

C1RL

Figure 2.7: Architecture of Miller compensation for two-stage amplifier

If the two poles of the two-stage amplifier are near each other, then a

Miller compensation can be used to split the poles. The dominant pole is moved

towards the low frequencies and the non-dominant pole is moved towards the

high frequencies. The series resistor RM and capacitor CM create a zero. The

dominant pole is given by P−3dB =1

CM (RM+GM2R1RL), the non-dominant pole is

P2 =RM+GM2R1RLCL(R1+RM )RL

, and the zero is 1(RM− 1GM2

)CM. Thus the RHP zero can be

eliminated by selecting RM =1

GM2[9],[11], [13].

16

2.5.2 Reverse-Nested Miller Compensation

A two-stage amplifier is designed by cascading two inverting stages. Sim-

ilarly, a three-stage amplifier is designed by cascading two inverting stages and

a non-inverting stage. In a three-stage amplifier design the first stage is invert-

ing stage that is implemented using a differential amplifier and the next stages

are implemented with common-source amplifiers. If the second stage is made non-

inverting, then nested Miller compensation is used. Likewise, if the second-stage is

implemented with inverting gain configuration and third-stage with non-inverting

gain configuration, then reverse-nested Miller compensation can be used to stabi-

lize the amplifier. The architecture of the reverse-nested Miller compensation is

shown in Fig. 2.8.

-Av2 Av3

CM2

VIN VOUT

RM

-Av1

CM1

R1 R2C1 C2

RL

-gm1 -gm2 gm3

Figure 2.8: Architecture of Reverse-Nested Miller compensation for three-stageamplifier

The dominant pole for this compensation is given by

P−3dB =1

CM1GM2GM3R1R2RL(2.8)

The transfer function adapted from [9] is given by

17

Arnmc =GM1GM2GM3R1R2RL(1− s(

CM2GM2

+CM1

GM2GM3R2)− s2 CM1CM2

GM2GM3)

(1 + s 1CM1GM2GM3R1R2RL

)[1 + s(CM2CLGM3CM1

− CM2GM2

+CM2GM3

) + s2CM1CLGM2GM3

]

(2.9)

The equation 2.9 shows that the amplifier has three poles and two RHP

zeros. The dominant pole is given in equation 2.8. The other two poles are high

frequency poles.

Thus in any amplifier after the required compensation scheme is applied,

the values of compensation capacitors and resistors are determined to move the

non-dominant poles and zeros to high frequencies. This stabilizes the circuit

and improves the bandwidth. A large number of compensation techniques are

proposed to stabilize the multi-stage amplifiers [9],[10],[11],[15],[16],[17]. These

multi-stage amplifiers are used in a wide range of applications.

A three-stage class-AB amplifier is proposed by [1] to drive 16-Ω headphone

speakers. The design and results are summarized in the following section. This

design is taken as a reference to compare the results with our work.

2.6 Three-Stage Class-AB Amplifier from [1]

The architecture of the proposed design is shown in Fig. 2.9. It is a three-

stage class-AB amplifier that drives 16-Ω headphone speakers and a wide range

of capacitive loads.

2.6.1 Design from [1]

The first stage is implemented using a folded-cascode amplifier with an

inverting gain configuration. The second-stage is implemented using common-

source amplifiers with positive gain configuration. A damping factor control stage

is used in the amplifiers that drive large capacitive loads [1],[11]. The damp-

ing factor control stage is used in amplifiers that have large swing to improve

18

Gm2 -Gm3

CC

VIN VOUT

RB

CC2

RL

-Gm1

GmD

RC

CD2CD

VB

CL

Figure 2.9: Architecture of the three-stage class-AB amplifier of [1]

the bandwidth and transient response [11]. The output stage is designed with

common-source amplifiers to provide the required push-pull action. The output

stage of the amplifier is biased at ±1V, and the rest of the amplifier is biased at

±0.6V.

2.6.2 Experimental Results from [1]

The total quiescent current of the amplifier is 730 µA. The THD of the

design is -84.8dB for 1.4VPP , 1 kHz sine-wave output. A figure of merit (FOM)

was defined to compare with other designs. The figure of merit defined by [1] is

the ratio of peak output power to the supply power. A comparison of measured

results is shown in Table 2.1.

Taking the above design as a reference, a new three-stage amplifier is de-

signed. The concept of replica biasing is used in the design to generate the bias

19

Table 2.1: Comparison of measured results

Parameter [18] [19] [20] [1]

Technology -0.35 µmCMOS

65 nmCMOS

130nmCMOS

Capacitance load 0-300 pF 0-300pF 0-12 nF 1 pF - 22 nF

Supply 3.0V 0.8V 2.5V 1.2V/2.0V

THD+N @ max.output

-90dB -69dB -68dB -84dB

Total compensationcapacitance

- - 35pF 14pF

Quiescent power 12.0mW 2.5mW 12.5mW 1.2mW

FOM 8.1 1.3 4.3 33.3

voltages for the output stage. The following section gives a brief introduction to

replica biasing.

2.7 Replica Biasing

A replica bias circuit is used for generating bias voltages for the output

stage [21]. A replica bias circuit can be used in a class-AB amplifier to bias the

common-source amplifiers of the output stage. The bias voltages generated by

the replica bias circuit acts as Vbat as shown in Fig. 2.3. When no input signal

is present, transistors of the output stage are in ON state but in a non-linear

region. The quiescent current in the output stage is set by the current through the

replica bias circuit [8]. The output stage acts as a class-A amplifier, as the output

transistors are either sourcing or sinking current all the time. This eliminates the

dead band region and minimizes the distortion in class-AB amplifiers.

20

The schematic of a two-stage pseudo class-AB amplifier and the replica

bias circuit to control quiescent current adapted from [8] are shown in Fig. 2.10.

VCTRL

M4 M5

2IB

VI+VI– M3M2

M1

M6

M7

M8

M9

M11

M10

VB CC

RCCC

RLargeVout

yx

(a)

VCTRL

M5C

IB

VrefM3C

M1C

M6C

MB

CC

RC

MP

MN

IBVBVB

(b)

Figure 2.10: (a) Schematic of two-stage pseudo class-AB amplifier (b) Replicabias circuit to control quiescent at the output stage

The two-stage pseudo class-AB amplifier has a fully-differential first-stage.

The transistors M6 and M7 are common-source amplifiers that provide the push-

pull action. The replica bias circuit is used for controlling the quiescent current

through output-stage transistors M6 and M7. The transistors M5C , M3C , M1C

and M6C are replicas of M5, M3, M1 and M6 respectively. The transistor MB

21

is biased at a voltage VB such that the current through MB is IB. The current

through transistors MP , M5C and M5 are similar, as they have same VSG. Thus,

the voltage at the drain of M5C is similar to the voltage at node ’y’. This causes

the current through M6 to be same as the current through M6C (IB). Hence, a

quiescent current of known value is obtained at the output of a two-stage pseudo

class-AB amplifier.

A feedback network is used along with the replica bias circuit to generate

bias voltages for the output-stage. Three-stage class-AB amplifier designed in our

work use the concept of replica bias to generate bias voltages and control quiescent

current at the output stage. The design of the amplifier is discussed in Chapter. 3.

22

Chapter 3

DESIGN OF THE THREE-STAGE CLASS-AB AUDIO AMPLIFIER

A three-stage class-AB audio amplifier is designed to drive 16-Ω headphone speak-

ers. The audio amplifier has high power efficiency and low distortion, and it is

also capable of driving a wide range of capacitive loads.

This chapter deals with the design of the three-stage class-AB amplifier.

It is organized as follows: The key aspects and architecture of the audio amplifier

is discussed in the first section. It is followed by the design of each stage with

small signal models. The last section of the chapter deals with the stability of the

amplifier that is analyzed with poles and zeros.

3.1 Architecture and Key Aspects of the Audio Amplifier

The architecture of the designed three-stage class-AB audio amplifier is

shown in Fig. 3.1. The design is implemented using fully-differential internal

stages. The first stage is a fully-differential folded cascode amplifier with an

inverting gain configuration. The second-stage is implemented with two two dif-

ferential amplifiers. A non-inverting gain configuration is used in this stage. The

third stage is implemented with PMOS and NMOS common-source amplifiers for

the push-pull action.

The gain experienced by the load at the output through NMOS and PMOS

common-source amplifiers is same. The symmetry in gain is achieved using two

differential amplifiers in the second-stage. In the absence of input signal, a dead

band region is created at the output, as the NMOS and PMOS common-source

23

+

-

-

+

+

-

+

-

Vout

Rc1Cc1

Cc2

Cc2

Rc2

2·Cc3

Cc3

Vin+

Vin–

A1

A2

A2

A3

A3

-

+

+

-

2·Cc3

-

-

Vo1–

Vo1+

Vo2P+

Vo2N+

Cc3

Figure 3.1: Architecture of the proposed three-stage class-AB amplifier

amplifiers are turned OFF. This leads to crossover distortion. To minimize the

distortion, a common-mode feedback network is used in combination with replica

bias in the second-stage to generate bias voltages for the third-stage . This turns

ON the transistors of the third-stage and the dead band region is eliminated.

The linearity in the design is achieved using a technique called substrate

biasing. The threshold voltage of all NMOS transistors are made comparable to

threshold voltage of the PMOS transistor. This is attained by connecting the bulk

of the NMOS transistor to a voltage lower than source voltage. This is explained

in detail in the following sections.

24

3.2 Transistor Level Three-Stage Design

The transistor level schematic of the three-stage class-AB audio amplifier

is shown in Fig. 3.2. The first stage is a fully-differential folded cascode amplifier

realized with transistors M1-M12. The transistors M13-M20 in combination with

resistors R1 and capacitors CS form the common-mode feedback network. The

second-stage is realized using two differential amplifiers. A NMOS differential

amplifier is formed with transistors M40-M47 and the PMOS differential amplifier

is formed with transistors M21-M28. The transistors M37-M39 and M56-M58 are

replica bias circuits. The output-stage is realized with transistors MP and MN .

A Miller compensation is used from the output of third-stage to negative output

terminal of the first-stage. A reverse-nested Miller compensation is used between

input and output of the second-stage. The transistor dimensions are given in

Table 3.2.

In this work, four designs are implemented. The difference in each design

is the dimensions of the transistors M39 and M58, the value of compensation

capacitors and resistors, and the input bias current. A trade-off has been observed

between the total harmonic distortion and quiescent current. This is discussed in

Chapter 4. The design of each stage is explained in the following sections.

3.3 Bias circuit

The schematic of the bias circuit is shown in Fig. 3.3. The bias circuit

internally generates four bias voltages Vb1, Vb2, Vb3 and Vb4. The voltage Vb2

is one VSG below VDD. This is generated by diode connecting the transistor M1A

as shown in Fig. 3.3. Similarly the voltage Vb1 is generated.

A long L (length) diode connected transistor is created by connecting the

gates of transistors M4P1-M4P5 as shown in Fig. 3.3. The voltage Vb3 at the gate

of transistor M3A is 2VDSSAT + VTHP . The current through this transistor is given

25

Table 3.1: Transistor Dimensions

Device Dimensions

M1A, M1B, M1C , M19, M20, M35, M36, M50, M5120µm1.2µm

M11, M12, M27, M28, M48, M5620µm1.2µm , m = 2

M120µm1.2µm , m = 4

M4720µm1.2µm , m = 8

M2C , M17, M18, M33, M3420µm0.9µm

M9, M10, M31, M32, M49, M5720µm0.9µm , m = 2

M220µm0.9µm , m = 4

M4620µm0.9µm , m = 8

M4A, M4B, M4C , M54, M55, M15, M16, M31, M3260µm1.2µm

M4, M5, M13, M29, M37,60µm1.2µm , m = 2

M5, M6, M21, M40, M41, M44, M4560µm1.2µm , m = 4

M23, M2660µm1.2µm , m = 6

M3A, M3B, M3C , M52, M5360µm0.9µm

M7, M8, M14, M30, M3860µm0.9µm , m = 2

M22, M42, M4360µm0.9µm , m = 4

M4P1, M4P2, M4P3 , M4P4, M4P530µm1.2µm

M1N1, M1N2, M1N3 , M1N4, M1N510µm1.2µm

M39150µm0.6µm , m = 4,6

MN150µm0.6µm , m = 40

M58300µm0.6µm , m = 4,6

MP300µm0.6µm , m = 40

26

Vo2

N+

Vo2

P+

Vo1+ R

c2

Cc2C

c2

Vb3

Vbia

s

Vb4

Vb2

Vb3

Vb2

Vb

2

Vb

3

Vb

4

Vb1

M1

CM

1A

M1B

M2

C

M3

C

M4

C

M3B

M4B

M3A

M4A

M4P

1

M4P

2

M4P

3

M4P

4

M4P

5

M1

N5

M1

N1

M1

N2

M1

N3

M1

N4

Vin

+

Vb

4

Vb1

Vb4

Vb3

Vb2

R1

R1

Vin

-

Vb4

Vb2

Vb3

Vr

Vo1+

Vo1-

M1

M2

M11

M9

M12

M10

M19

M17

M20

M18

M3

M4

M7

M5

M8

M6

M14

M13

M15

M16

R2

R2

Vo

1!

Vo1+

Cc3

Cc3

Vb

4

Vb3

Vb4

Vb3

Vb2

Vb

2V

b2

Vb

3

Vo2N!

Vo2N+

Vrn

M35

M33

M36

M34

M28

M26

M27

M25

M24

M23

M22

M21

M30

M29

M38

M37

M32

M31

M39

Vb3

R2/2

R2/2

Vo

1!

Vo1+

2·C

c32·C

c3

Vb

4

Vb1

Vb3

Vb

4

Vb1

Vb

4

Vb1

Vo2P+

Vo2P!

Vrp

M50

M51

M49

M48

M57

M56

M52

M54

M53

M55

M42

M40

M43

M41

M46

M47

M58

M44

M45

Vo2

N+

Vo2

P+

Vo

1!

Cc1

Rc1

Vo

ut

MP

MN

CS

CS

CS

CS

CS

CS

Bia

s ci

rcu

itF

irst

-sta

ge

Sec

on

d-s

tag

eT

hir

d-s

tag

e

Rep

lica

-bia

s

Rep

lica

-bia

s

Vc

Vrc

Vrs

Fig

ure

3.2:

Sch

emat

icof

the

thre

e-st

age

clas

s-A

Bau

dio

amplifier

27

Vb3

Vbias

Vb4

Vb2

Vb3

Vb2 Vb2

Vb3

Vb4

Vb1M1CM1A M1B

M2C

M3C

M4C

M3B

M4B

M3A

M4A

M4P1

M4P2

M4P3

M4P4

M4P5

M1N5

M1N1

M1N2

M1N3

M1N4

Vb3

IB2IB1

Figure 3.3: Schematic of the bias circuit

by

ID =µN · COX

2· W3AL3A

· (VGS − VTHP )2 (3.1)

As the current

IB1 = IB2 (3.2)

The current through the Long ’L’ transistor formed by the transistors M4P1-M4P5

is given by

ID =µN · COX

2· W4PL4P

((2VDSSAT + VTHP )− VTHP )2 (3.3)

This equation can be rewritten in terms of VGS as

ID =µN · COX

2· W4PL4P

· 4(VGS − VTHP )2 (3.4)

28

Thus from (3.1), (3.2) and (3.4) we obtain

W4PL4P

=1

4

W3AL3A

(3.5)

The transistor M4A is biased at the edge of the saturation region, thus pulling

more current from transistor M4A, moves it from saturation to triode region [12].

Hence, the length of the long L transistor is assumed five times the length of M3A

rather than four times. This generates a voltage Vb3 that is VSDSAT away from

Vb2.

Similarly, the voltage Vb4 is generated that is VDSSAT away from Vb1. The

bias voltages are replicated to others stages of the amplifier through transistors

M1C-M4C . The current through the branch M1C-M4C is mirrored to the next

stages based on the aspect ratio of the current mirror.

3.4 Input-Stage

The first stage of the amplifier is realized using a fully-differenatial folded

cascode amplifier, as they have wide swing and high gain. The schematic of the

first stage is shown in Fig.3.4. Transistors M1-M12 represent the folded cascode

amplifier. The transistors M13-M20 in combination with resistors R1 and capac-

itors CS form the common-mode feedback network. A common-mode feedback

network is used for generating a known voltage Vr at the output of the folded

cascode amplifier.

The operation of the first-stage is as follows. The voltage Vr is set to 0V,

this allows the current to pass through the transistor M16. The voltage at the

node VX increases. The transistors M20, M11 and M12 form the current mirror.

The current through the transistors M11 and M12 depends on the aspect ratio

29

Vin+

Vb4

Vb1

Vb4

Vb3

Vb2

R1R1

Vin-

Vb4

Vb2

Vb3

Vr

Vo1+

Vo

1-

M1

M2

M11

M9

M12

M10

M19

M17

M20

M18

M3 M4

M7

M5

M8

M6

M14

M13

M15 M16CS CS Vc

VX

Common-mode

Feedback network

Figure 3.4: Schematic of the first-stage

of the mirror. Hence the voltages Vo1+ and Vo1- are pulled towards VSS. The

voltage Vc acts as virtual ground, as it is the center voltage of Vo1+ and Vo1-.

The gain of the first-stage is determined by the gm4,3 resistors R1, as R1�

ro of the transistors. The tail of the folded cascode amplifier and common-mode

feedback network are cascoded to provide better matching between the transistors

and to obtain good precision in matching the currents.

Under equilibrium condition, the voltage Vo1+ and Vo1- are approximately

at 0V. This voltage is used for biasing the second stage. The implementation of

second-stage is explained in the following section.

3.5 Second-Stage

The second stage of the amplifier is implemented using a NMOS differential

amplifier and a PMOS differential amplifier. The NMOS differential amplifier is

used for biasing PMOS common-source amplifier of the third-stage and the PMOS

30

differential amplifier is used for biasing NMOS common-source amplifier of the

third-stage.

3.5.1 PMOS differential amplifier

The schematic of a PMOS differential amplifier is shown in Fig. 3.5

R2 R2

Vo1– Vo1+

Cc3 Cc3

Vb4

Vb3

Vb4

Vb3

Vb2Vb2Vb2

Vb3

Vo

2N–

Vo2

N+

VrnM35

M33

M36

M34

M28

M26

M27

M25

M24M23

M22

M21

M30

M29

M38

M37

M32M31

M39

CS CS Vrc

CMFB

Replica bias

Figure 3.5: Schematic of the second-stage PMOS differential amplifier

Transistors M21-M28 form the PMOS differential amplifier. The common-

mode feedback network (CMFB) is realized with transistors M29-M36 and resistors

R2 and capacitors CS. Transistors M37-M39 form the replica bias circuit. The cur-

rent through this circuit is determined by the current source formed by transistors

M37 and M38. The voltage Vrn is at VGS above VSS. The common-moded feedback

network generates the known voltage Vrn at the output of the PMOS differen-

tial amplifier. The gain of the amplifier is determined by resistors R2 and the

transconductance gm23,24 .

31

Under equilibrium, the common-mode voltage Vrc is equal to V o2N+. This

voltage is used as the input for the NMOS common-source amplifier of the third-

stage. Since, V o2N+ is equal to Vrn, transistors M39 and MN form a virtual

current mirror. Thus, the quiescent current of the output stage is determined by

the current through M39. The ratio of currents depends on the ratio of multiplicity

of transistor dimensions.

3.5.2 NMOS differential amplifier

The NMOS differential amplifier is similarly designed. The schematic of

the NMOS differential amplifier is shown in Fig. 3.6.

Vb3

R2/2 R2/2

Vo1– Vo1+

2·Cc3

Vb4

Vb1

Vb3

Vb4

Vb1

Vb4

Vb1

Vo2

P+

Vo2

P–

Vrp

M50 M51

M49

M48

M57

M56

M52

M54

M53

M55

M42

M40

M43

M41

M46

M47

M58

M44 M45

CS CS

2·Cc3

Vrs

CMFB

Replica bias

Figure 3.6: Schematic of the second-stage NMOS differential amplifier

Transistors M40-M47 form the NMOS differential amplifier. The common-

mode feedback network (CMFB) is realized with transistors M48-M55 and resis-

tors R22

and capacitors CS. Transistors M56-M58 form the replica bias circuit. The

32

current through this circuit is determined by the current-source formed by tran-

sistors M56 and M57. The voltage Vrn is at VSG below VDD. The common-moded

feedback network generates the known voltage Vrp at the output of the NMOS

differential amplifier. The gain of the amplifier is determined by resistors R22

and

the transconductance gm44,45 .

Under equilibrium, the common-mode voltage Vrs is equal to V o2P+. This

voltage is used as the input for the PMOS common-source amplifier of the third-

stage. Since V o2P+ is equal to Vrp, transistors M58 and MP form a virtual

current mirror. Thus, the quiescent current of the output stage is determined by

the current through M58. The ratio of currents depends on the ratio of multiplicity

of transistor dimensions.

The dimensions of the transistors M46 and M47 are doubled to obtain twice

the bias current than the current through M21 and M22 of Fig. 3.5. Thus, the

gain of the NMOS differential pair is 2·gm44,45·R22 , which is equivalent to the PMOS

differential amplifier gain gm23,24·R2. The gain of the PMOS differential amplifier

is made equal to the NMOS differential amplifier to attain symmetry at the output

of the amplifier.

Unlike the first-stage, the second-stage is a single-ended differential ampli-

fier. The output of the NMOS and PMOS differential amplifiers are the inputs

for PMOS and NMOS common-source amplifiers of the third-stage, respectively.

The design of the third-stage is discussed in the next section.

3.6 Output-Stage

The output-stage of the amplifier is implemented with huge PMOS and

NMOS common-source amplifiers. In order to have same current ID through

transistors MP and MN , the dimensions of the PMOS transistor MP is made twice

the size of NMOS transistor MN , as the mobility of the electrons is approximately

33

about 2.5 times the mobility of holes. The schematic of the output-stage is shown

in Fig. 3.7

Vo2N+

Vo2P+

Vo1–

Cc1Rc1

Vout

MP

MN

Figure 3.7: Schematic of the output-stage

The input capacitance CGS associated with transistor MP is twice the

capacitance CGS of the transistor MN , as dimensions of the transistor MP is

twice the size of transistor MN . The pole of the second-stage PMOS differential

amplifier is approximately at 1R2·CGSN

. In order to have the same pole at the output

of NMOS differential amplifier, the resistance R2 is halved and the compensation

capacitance is doubled. The pole of NMOS differential amplifier is approximated

as 1R22·(2CGSN )

.

34

Output swing at the second-stage NMOS differential amplifier must be

equal to the output swing of PMOS differential amplifier to attain linearity at the

output. The swing at the output of second-stage is determined by voltages Vrn

and Vrp shown in Fig. 3.5 and Fig. 3.6. The voltage Vrn is made equal to Vrp

to obtain same swing at the input of third-stage and this is achieved by biasing

the bulks of NMOS transistors at a voltage lower than VSS. This increases the

threshold voltage of NMOS transistors. Hence, more voltage is required at the

gate of transistor M39 to allow the current from transistors M37 and M38 to pass

through. Thus, VGS of transistor M39 increases and this is comparable to Vrp.

The designed amplifier has three stages. Hence, the stability of the am-

plifier cannot be achieved without a compensation network. Miller compensation

and reverse-nested Miller compensation are used to stabilize the three-stage class-

AB amplifier. A brief description of the compensation network used in this design

is given in the following section.

3.7 Compensation used in the Design

The first-stage is an inverting gain configuration and second stage is a non-

inverting gain configuration. The output-stage is implemented with an inverting

gain configuration. A Miller compensation network is applied between the output

of first-stage and the output of the third-stage. Reverse-nested Miller compensa-

tion is used across the second-stage for NMOS and PMOS differential amplifiers

as shown in Fig. 3.1.

A symmetry is maintained while using the compensation networks at the

second-stage. This simplifies the small-signal model of the amplifier. The small-

signal model of each stage is discussed in the next section.

35

3.8 Small-Signal Models

The small-signal model of a symmetric folded-cascode amplifier is repre-

sented with a current source and a resistor parallel to it. The input-stage in this

design is not symmetric, as the compensation capacitor CC1 is connected to Vo1-

and there is no compensation capacitor to Vo1+. Hence, the circuit is divided

into two equivalent circuits. The left-half of the folded-cascode amplifier and its

corresponding small signal model is shown in Fig. 3.8, where gm1 is the transcon-

Vb4

Vb3

Vb2

Vin+

Vb1

Vb4

R1

Vo1

-

CS

M1

M2

M11

M9

M3

M7

M5

(a)

R1

gm1·Vin/2

Vo1-

(b)

Figure 3.8: (a) Left-half of the input-stage (b) small-signal model for left half.

ductance of the transistor M3. The input signal is assumed asV in2

, since Vin =

(Vin+ - Vin-) and Vin+ is only half of the signal Vin. The resistor R1�ro. Thus

R1 is the output resistance at node Vo1-. One end of the resistor R1 is connected

to node Vo1- and the other end is at virtual ground.

Similarly, the small-signal model for right-half of the folded cascode am-

plifier is as shown in Fig. 3.9, where gm1 is the transconductance of M4 and Rc2

is the compensation resistor. Under quiescent conditions the current through M3

36

Vb4

Vb3

Vb2

Vin-

Vb1

Vb4

R1Vo1+

Rc2

Vx

CS

M1

M2

M12

M10

M4

M8

M6

(a)

R1gm1·(-Vin/2)

Vo1+Rc2

Vx

(b)

Figure 3.9: (a) Right-half of the input-stage (b) small-signal model for right-half.

is equal to current through M4. Thus the gm’s are equal. The signal Vin- is

represented as −V in2

. The resistor R1 is approximately equivalent to resistor R1

shown in Fig. 3.8(a).

The second-stage is implemented with NMOS differential amplifier and

PMOS differential amplifier. Thus for each amplifier, the small-signal models are

drawn separately. The PMOS differential amplifier has symmetry at compen-

sation, hence only the output side of the differential amplifier is considered for

drawing the small-signal model as shown in Fig. 3.10.

The transconductance of the transistor M24 is gm2. The resistor R2 is

the output resistance at node V o2n+. Cc2 and Cc3 are compensation capacitors.

Capacitor C2 in Fig. 3.10(b) is the input capacitance CGS of the transistor MN

of the output-stage. The voltage Va is (Vo1+ - Vo1-) and the voltage Vo1+ is

represented as V a2

.

37

R2

Vo1+

Cc3

Vb4

Vb3

Vb2

Vo2n+

Cc2

Vx

CS

M28

M26

M24

M22

M21

(a)

Cc3

R2

Vo2n+ Vo1+Cc2

C2gm2·Va/2

Vx

(b)

Figure 3.10: (a) PMOS differential amplifier (b) small-signal model PMOS differ-ential amplifier.

The part of NMOS differential amplifier and it corresponding small-signal

model are shown in Fig. 3.11. Similar to the PMOS differential amplifier, the

compensation in NMOS differential amplifier is symmetric.

The input capacitance C2 of the transistor MP of output-stage is twice

the input capacitance of NMOS transistor MN of the output-stage, as the size of

PMOS transistor MP is twice the NMOS transistor MN . Thus to provide the same

pole frequency at the output of second-stage the common-mode feedback resistor

is made R22

and the compensation capacitor Cc3 is doubled. The transconductance

of the transistor M45 is 2·gm2, as the current through the transistor M45 is twice

the current through transistor M24 in Fig. 3.10. The signal Vo1+ isV a2

. The

current through current source in Fig. 3.11(b) is given by 2·gm2 · V a2

38

R2/2

Vo1+

2Cc3

Vo2p+

Vb3

Vb1

Vb4

Vx

Cc2

M43

M41

M46

M47

M45

(a)

2Cc3

R2/2

gm2·Va

Vo2p+Vo1+Cc2

2·C2

Vx

(b)

Figure 3.11: (a) NMOS differential amplifier (b) small-signal model NMOS differ-ential amplifier.

The third stage is implemented with NMOS and PMOS common-source

amplifiers. These transistors provide the required push-pull action for the class-AB

output-stage. The schematic of output-stage and its corresponding small-signal

model are shown in Fig. 3.12.

The transconductance of transistors MN and MP is gm3, as the quiescent

current through transistors MN and MP is same. Cc1 and Rc1 are the Miller

compensation capacitor and nulling resistor across the outputs of input-stage and

output-stage. Cout is the load capacitance. Rout is the output resistance of the

amplifier and is given by

Rout = RL‖roN‖roP (3.6)

39

Vo2N+

Vo2P+

Vo1–

Cc1Rc1

Vout

MP

MN

(a)

Rout

Vout

Cout

gm3·Von+

gm3·Vop+

Vout-

Rc1Cc1

(b)

Figure 3.12: (a) Schematic of output-stage (b) small-signal model for output-stage

Rout ≈ RL as RL � roN,P

The complete small-signal model of the designed three-stage class-AB au-

dio amplifier is shown in Fig. 3.13. The equations for current at each node are put

in a software Maple to determine the poles and zeros in the amplifier. The equa-

tion of the currents at each node and equations used to determine the pole/zero

frequencies is shown in APPENDIX B.

3.9 Pole-Zero Analysis

The gain of the amplifier is determined from the equations in APPENDIX B

as

Gain = gm1 ·R1 · gm2 ·R2 · gm3 ·Rout (3.7)

The designed audio amplifier has six poles and five zeros. The poles and zeros

obtained are shown in Table 3.2. The fourth pole is at high frequency, hence the

last two high frequency poles are neglected. The third zero is a RHP zero. The

40

R1

gm

1·V

in/2

Vo1-

R1

gm

1·(

-Vin

/2)

Vo1+

Rc2

Vx

Cc3

R2

Vo2

N+

Cc2

C2

gm

2·V

a/2

2·C

c3

R2/2

gm

2·V

a

Vo2

P+

Cc2

2·C

2

Rout

Vout

Cout

gm

3·V

o2

N+

gm

3·V

o2

P+

Rc1

Cc1

Fig

ure

3.13

:Sm

all-

sign

alm

odel

ofth

edes

igned

thre

e-st

age

clas

s-A

Bau

dio

amplifier

41

other two zeros are high frequency zeros, thus neglected. The first zero (ωZ1) is

used, to cancel the second pole (ωP2). Similarly the second zero ωZ2 cancels third

pole ωP3. Thus the system acts as a two-pole system, where the fourth-pole is

considered as second-pole.

Table 3.2: Poles and Zeros

Poles / Zeros

ωP12

gm2·R1·R2(2·Cc2+2·gm3·Rout·Cc1+3·Cc3)

ωP22·Cc2+2·gm3·Rout·Cc1+3·Cc3

Cc1·(2·gm3·Rout·R2·C2+2·R1·Cc2+3·R1·Cc3)

ωP32·gm3·Rout·R2·C2+2·R1·Cc2+3·R1·Cc3

R1·R2·C2·(6·gm3·Rout·Cc3+4·gm3·Rout·Cc2+2·Cc2+3·Cc3)

ωP4gm2·(2·gm3·Rout+1)Cout·Rout·gm2+2·C2

ωZ12

Cc1·R1+2·R2·C2

ωZ21

R2·C2 +2

Cc1·R1

ωZ3 -gm22·Cc3

42

Chapter 4

SIMULATION RESULTS

To test the functionality of the designed three-stage class-AB audio amplifier, the

amplifier is subjected to DC, AC and transient analysis tests. The following sec-

tions discuss the response of the of the amplifier. The obtained waveforms are

plotted using the Matlab code given in APPENDIX C. Four designs have been im-

plemented by varying the input bias current, dimensions of M39 and M58 shown in

Fig. 3.2, compensation capacitor and resistor values and its corresponding results

are plotted. The designs are named after their performance as LIQ (Low quies-

cent current), LTHD (Low THD), MIQ (Moderate quiescent current) and HCL

(High load capacitance). The design parameters of the four designs are given in

Table 4.1.

Table 4.1: Design Parameters

Design IB Rc1 Rc2 Cc2 Cc3 M39 & M58

HCL 8 µA 1 kΩ 1 kΩ 500 fF 500 fF m = ×4

LTHD 9 µA 1 kΩ 2 kΩ 200 fF 300 fF m = ×4

MIQ 9 µA 1 kΩ 2 kΩ 200 fF 300 fF m = ×6

LIQ 8 µA 1 kΩ 2 kΩ 300 fF 300 fF m = ×6

43

The bulk terminal of all the NMOS transistors are connected to -3V for all

the tests performed.

4.1 DC analysis

The DC analysis determines the symmetry and linearity of the amplifier.

The test-bench for DC analysis of the amplifier is shown in Fig 4.1. The input is

a DC voltage varied from -2mV to 2mV. To obtain the differential input voltage,

two voltage controlled voltage sources (VCVS) are used. One VCVS is with 0.5

gain and the other is with -0.5 gain. The input bias current of the amplifier is

8µA.

vdd

vdd

vss

vss

Vin-

Vin+

Vr

Vout

vss_sub

vss

_su

b

biasI

biasV

Vb2

CL

+

+-

+-

Vin

egain= -0.5

egain= 0.5 RL=16 Ω+ +

+

Figure 4.1: Schematic of the DC test-bench

The simulation results obtained for a resistive load of 16-Ω and capacitive

load of 500 pF are shown in Fig 4.2

The maximum current through the NMOS and PMOS transistors of the

output stage is 83.5mA. The maximum swing at the output is observed as ±1.25V.

4.2 AC analysis

The AC analysis determines the stability of the amplifier. The test-bench

for the AC analysis is shown in FIg. 4.3. Input is a 1V AC signal given at positive

input terminal of the amplifier. A large resistor and capacitor are used in the

feedback network to provide open-loop operation.

44

−2 −1.5 −1 −0.5 0 0.5 1 1.5 2−1.5

−1

−0.5

0

0.5

1

1.5

Out

put V

olta

ge (V

)

−2 −1.5 −1 −0.5 0 0.5 1 1.5 2

0

20

40

60

80

100

DC Input (mV)

Out

put−

Stag

e Cu

rrent

(mA)

MN = OFFMP = ON

MN = ONMP = ON

MN = ONMP = OFF

ID, MN ID, MP

Figure 4.2: DC-analysis output

The simulation results of the LIQ design is given in Fig. 4.4. The circuit

has input bias current of 8µA. The open loop gain of the amplifier is 48.4 dB

The input bias current of the LTHD design is made 9µA. The magnitude

and phase plots of the design is given in Fig. 4.5. The open-loop gain of the

amplifier is 53.7 dB.

The MIQ design is tested with a input bias current of 9µA. The simulation

results of the design is given in Fig. 4.6 and open-loop gain of the amplifier is

50.6 dB

45

+

vdd

vdd

vss

vss

1F

1Vac

Vin-

Vin+

Vr

Vout

vss_sub

vss

_su

b

biasI

biasV

Vb2

1GΩ

CLRL=16 Ω

+

++

Figure 4.3: Schematic of the AC test-bench

Table 4.2: AC Simulation Results

Design GainPhaseMargin

GainMargin

Gain Bandwidth

HCL 51.5 dB 72.04 ◦ 15.52 dB 1.23 MHz

LTHD 53.7 dB 70.8 ◦ 11.5 dB 2.03 MHz

MIQ 50.6 dB 70.3 ◦ 14.5 dB 1.56 MHz

LIQ 48.4 dB 70.7 ◦ 16.4 dB 1.24 MHz

The simulation results of the HCL design is given in Fig. 4.7. The circuit

has input bias current of 8µA.The open loop gain of the amplifier is 51.5 dB

The open loop gain, gain margin, phase margin and unity gain frequency of

four designs are tabulated and given in Table 4.2. The gain-bandwidth product of

the LTHD design is 2.03 MHz. MIQ design has the highest phase-margin among

the four designs.

46

100 101 102 103 104 105 106 107 108−100

−50

0

50

Gai

n (d

B)

100 101 102 103 104 105 106 107 108−400

−300

−200

−100

0

100

Frequency (Hz)

Phas

e (d

eg)

Figure 4.4: AC output of LIQ circuit

100 101 102 103 104 105 106 107 108−80

−60

−40

−20

0

20

40

60

Gai

n (d

B)

100 101 102 103 104 105 106 107 108−400

−300

−200

−100

0

100

Frequency (Hz)

Phas

e (d

eg)

Figure 4.5: AC output of LTHD circuit

47

100 101 102 103 104 105 106 107 108−80

−60

−40

−20

0

20

40

60

Gai

n (d

B)

100 101 102 103 104 105 106 107 108−400

−300

−200

−100

0

100

Frequency (Hz)

Phas

e (d

eg)

Figure 4.6: AC output of MIQ design

4.3 Transient analysis

The transient analysis determines the time domain response of a amplifier.

A square wave of ±100mV and 50kHz frequency is given at the negative input

terminal. The rise time and fall time of the input is 10 nS. The amplifier has a

inverting gain of ’4’. The test-bench for transient analysis is shown in Fig. 4.8.

The simulation result for LIQ is shown in Fig. 4.9. The load capacitance

of the amplifier is varied from 10 pF to 1.5 nF.

LTHD design has no ringing at the output up-to 1 nF. The output of the

amplifier for different load capacitances is shown in Fig. 4.10.

48

100 101 102 103 104 105 106 107 108−80

−60

−40

−20

0

20

40

60

Gai

n (d

B)

100 101 102 103 104 105 106 107 108−400

−300

−200

−100

0

100

Frequency (Hz)

Phas

e (d

eg)

Figure 4.7: AC output of HCL circuit

vdd

vdd

vss

vss

Vin-

Vin+

Vr

Vout

vss_sub

vss

_su

b

biasI

biasV

Vb2

40KΩ

CL

10KΩ

Va=100mV

Freq=50KHz

RL=16 Ω

+

+ +

+

Figure 4.8: Schematic of the Transient test-bench

49

5 10 15 20 25

−0.1

0

0.1

Inpu

t (V)

5 10 15 20 25

0

0.5

1

1.5

2

2.5

Time (µS)

Out

put (

V)

CL = 1.5 nF

CL = 500 pF

CL = 10 pF

Figure 4.9: Transient output of LIQ circuit

5 10 15 20 25

−0.1

0

0.1

Inpu

t (V)

5 10 15 20 25

0

0.5

1

1.5

2

2.5

Time (µS)

Out

put (

V)

CL = 1 nF

CL = 500 pF

CL = 10 pF

Figure 4.10: Transient output of LTHD circuit

50

The simulation result for MIQ is shown in Fig. 4.11. The load capacitance

of the amplifier is varied from 10 pF to 1 nF.

5 10 15 20 25

−0.1

0

0.1

Inpu

t (V)

5 10 15 20 25

0

0.5

1

1.5

2

2.5

Time (µS)

Out

put (

V)

CL = 1 nF

CL = 500 pF

CL = 10 pF

Figure 4.11: Transient output of MIQ circuit

The load capacitance for HCL design is varied from 10 pF to 5 nF, and

there was no ringing at the output. The slew-rate of the design is 1.25 V/µS. The

simulation result for HCL design is shown in Fig. 4.12.

4.4 THD analysis

A transient analysis has been performed on the amplifier to determine the

total harmonic distortion. The amplifier is connected in inverting gain configura-

tion with gain 1. The input is a sine wave of 2.45VPP and frequency of 1kHz. The

test-bench for the measuring THD is shown in Fig. 4.13.

The input has a dead time of 10 µS. The quiescent power of the amplifier is

determined at 5 µS, when there is no input signal. The THD of designs is measured

by using the THD-option of the simulator for a sample of the output signal. To

51

5 10 15 20 25

−0.1

0

0.1

Inpu

t (V)

5 10 15 20 25

0

0.5

1

1.5

2

2.5

Time (µS)

Out

put (

V)

CL = 500 pF

CL = 10 pF

CL = 5 nF

Figure 4.12: Transient output of HCL circuit

Table 4.3: Transient Simulation Results

Design CLOAD THDQuiescentPower

PeakPower

FOM

HCL 10 pF - 5 nF -77 dB 1.77 mW 97.6 mW 55.23

LTHD 10 pF - 1 nF -80.89 dB 1.98 mW 97.6 mW 49.27

MIQ 10 pF - 1 nF -78.56 dB 1.66 mW 97.6 mW 59.40

LIQ 10 pF - 1.5 nF -77.29 dB 1.47 mW 97.6 mW 66.25

measure the THD precisely, the time step parameters have been modified. The

step is given as 1µ and Max. step as 1µ.

The output waveform for measuring the THD is shown in Fig. 4.14. The

THD and quiescent power measurements for four designs are shown in Table 4.3.

52

vdd

vdd

vss

vss

Vin-

Vin+

Vr

Vout

vss_sub

vss

_su

b

biasI

biasV

Vb2

40KΩ

CL

40KΩ

Va=1.25V

Freq=1KHz

RL=16 Ω

+

+ +

+

Figure 4.13: Schematic for THD measurement

0 0.5 1 1.5 2 2.5−1.5

−1

−0.5

0

0.5

1

1.5

Inpu

t (V)

0 0.5 1 1.5 2 2.5−1.5

−1

−0.5

0

0.5

1

1.5

Time (mS)

Out

put (

V)

Figure 4.14: Transient output for measuring THD

53

Chapter 5

HARDWARE TESTING

The three-stage class-AB audio amplifier is fabricated in ON-SEMI 0.5µm pro-

cess through MOSIS. The hardware test is the real-time test for determining the

functionality of the amplifier. This chapter describes the layout techniques used

in the design, the experimental setup used for testing and the tests performed to

determine the operation of the audio amplifier.

5.1 Layout

The layout of four designs is implemented using the virtuso environment

of Cadence. A common-centroid technique is used to attain matching between

devices. The current at the output-stage is huge, hence strapping technique is

used for the huge current to flow through the transistor of the output-stage. In

order to avoid noise that can be coupled with bias voltage, the bias voltage Vb1

and Vb4 are shielded with substrate biasing voltage and Vb2 and Vb3 are shielded

with VDD. The layout of the four designs are shown in Fig. 5.1, Fig. 5.2, Fig. 5.3

and Fig. 5.4.

The designs are arranged in the frame of 40 pins. The layout of the frame

with the designs is shown in Fig. 5.5 and Fig. 5.6. The micrograph of the chip is

shown in Fig. 5.7.

5.2 Experimental Setup

The following apparatus are used for measuring the outputs:

54

Figure 5.1: Layout of LIQ amplifier

1. Oscilloscope: Hewlett Packard: 54600B: 100MHz, Digital Storage Oscillo-

scope.

The oscilloscope is used for plotting the input and output waveforms for a

transient response.

2. Function Generator: Agilent: 33120A: 15 MHz Function/Arbitary Wave-

form Generator.

The function generator is used for generating the input signals/waveforms.

3. Digital Multimeter: Agilent: 34401A: 612

Digit Multimeter.

55

Figure 5.2: Layout of LTHD amplifier

The Digital multimeter is used in this work for determining the DC voltages

AC voltages and DC currents.

4. Stanford Research System: SR770 FFT Network Analyzer.

The Stanford research system is used to determine the THD in the design.

The DC, Transient and THD measurements are obtained using the above

apparatus. The test procedure for these measurements is given in APPENDIX A.

5.3 DC Measurements

The amplifier is connected as a voltage follower and both input terminals

of the amplifier are connected to ground. The output voltage is measured using

56

Figure 5.3: Layout of MIQ amplifier

the digital multimeter. Under quiescent conditions the voltage obtained at the

output is offset voltage. The offset voltages of all the designs are discussed in

Chapter 6. The quiescent current is measured with the digital multimeter by

creating an open circuit between the VDD and the VDD pin of the chip.

5.4 Transient Measurements

The test-setup for transient measurements is similar to the test-bench used

in Fig. 4.8. The results obtained for each design for different capacitive loads are

plotted using the Matlab code given in APPENDIX C.

57

Figure 5.4: Layout of HCL amplifier

5.5 THD Measurements

The amplifier is used in inverting gain configuration with a gain of 1. A in-

put signal of 2.45VPP and 1kHz frequency is applied at the negative input terminal

of the amplifier. The output amplitude is measured using a digital multimeter.

The output is connected to the Stanford research system (SRC) to measure the

THD at the output. The SRC gives the amplitude of fundamental frequency and

its harmonics. The amplitude of first 10 harmonics for each design are shown in

Fig. 5.12, Fig. 5.13, Fig. 5.14 and Fig. 5.15.

58

LIQ

MIQ

Figure 5.5: Layout of the frame with two LIQ and MIQ.

The noise floor is at -106dB. The hardware measurements for all the designs

are shown in