HIGHEFFICIENCYAUDIOPOWERAMPLIFIERS

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HIGHEFFICIENCYAUDIOPOWERAMPLIFIERS  design and practical use

Phd. thesis, by Ronan van der Zee  1999

Contents
1 Introduction .............................................................................1
1.1 Motivation .................................................................................................1
1.2 Problem definition.....................................................................................2
1.2.1 Audio Signals ...........................................................................2
1.2.2 Amplifier Power.......................................................................2
1.2.3 Efficiency or dissipation?.........................................................3
1.2.4 Distortion .................................................................................4
1.2.5 Integration on chip ...................................................................6
1.2.6 Other specifications..................................................................6
1.3 Efficiency of the class AB amplifier .........................................................7
1.4 Scope and Outline ...................................................................................10
2 Measuring and predicting amplifier dissipation................13
2.1 Introduction .............................................................................................13
2.2 How to measure?.....................................................................................13
2.3 Characteristics of audio signals...............................................................14
2.3.1 The test set..............................................................................14
2.3.2 Amplitude distribution ...........................................................15
2.3.3 Frequency distribution............................................................17
2.4 The IEC-268 test signal...........................................................................18
2.4.1 Characteristics ........................................................................19
2.4.2 Completeness .........................................................................20
2.4.3 Accuracy.................................................................................21
2.5 A simple periodic test signal ...................................................................23
2.6 An IEC variant.........................................................................................25
2.7 Conclusions .............................................................................................26
3 Linear and switching amplifiers..........................................27
3.1 Introduction .............................................................................................27
3.2 Linear amplifiers .....................................................................................27
3.2.1 Class G ...................................................................................27
3.2.2 Class H ...................................................................................29
3.2.3 ‘Cool power’ ..........................................................................30
3.2.4 ‘Front/Rear’............................................................................31
3.2.5 Generalisation and modelling ................................................ 32
3.2.6 Limitations of linear amplifiers ............................................. 35
3.3 Switching amplifiers ............................................................................... 36
3.3.1 The class D principle ............................................................. 36
3.3.2 Output stage ........................................................................... 37
3.3.3 Output stage dissipation......................................................... 40
3.3.4 Output filter............................................................................ 41
3.3.5 Modulators and feedback....................................................... 43
3.3.6 Limitations of switching amplifiers ....................................... 49
3.4 Conclusions............................................................................................. 49
4 Combinations of linear and switching amplifiers ............. 51
4.1 Introduction............................................................................................. 51
4.2 Amplifiers in series ................................................................................. 51
4.2.1 Amplifiers with a tracking power supply............................... 51
4.2.2 Chip area considerations........................................................ 54
4.2.3 Bridge topology...................................................................... 57
4.3 Amplifiers in parallel .............................................................................. 58
4.3.1 Problems with amplifiers in parallel...................................... 58
4.3.2 Current dumping .................................................................... 59
4.3.3 Reduction of AB’s output current.......................................... 60
4.4 Conclusions............................................................................................. 61
5 Realisation of a class AB/D bridge amplifier..................... 63
5.1 Introduction............................................................................................. 63
5.2 Circuit principle ...................................................................................... 63
5.3 Circuit analysis and design considerations ............................................. 64
5.3.1 Timing of bridge switching.................................................... 64
5.3.2 Dissipation ............................................................................. 67
5.3.3 Common mode rejection........................................................ 68
5.4 Implementation of a prototype ................................................................ 70
5.4.1 Realisation ............................................................................. 70
5.4.2 Measurements ........................................................................ 71
5.4.3 Discussion.............................................................................. 74
5.5 Conclusions............................................................................................. 74
6 Realisation of a class AB/D parallel amplifier................... 75
6.1 Introduction............................................................................................. 75
6.2 Circuit principle ...................................................................................... 75
6.3 Circuit analysis and design considerations ............................................. 77
6.3.1 Current ripple and coil ...........................................................77
6.3.2 Dissipation .............................................................................79
6.3.3 AB’s output impedance..........................................................80
6.4 Implementation of a prototype with a low switching frequency ............82
6.4.1 Realisation..............................................................................82
6.4.2 Measurement results...............................................................83
6.4.3 Discussion ..............................................................................86
6.5 Implementation of a prototype with a high switching frequency ............87
6.5.1 Realisation..............................................................................87
6.5.2 Measurements ........................................................................89
6.5.3 Discussion ..............................................................................92
6.6 Future possibilities ..................................................................................93
6.6.1 A higher order coupling network ...........................................93
6.6.2 A clocked version...................................................................95
6.6.3 A parallel amplifier in bridge .................................................96
6.6.4 A balanced current output stage .............................................97
6.7 Conclusions .............................................................................................98
7 Conclusions ............................................................................99
7.1 Introduction .............................................................................................99
7.2 This work in relation to other work.........................................................99
7.3 Future developments .............................................................................100
References ...............................................................................103
Summary.................................................................................109
Samenvatting ..........................................................................111
Selected symbols and abbreviations.....................................113
Nawoord..................................................................................115

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1.1 Motivation
During the history of audio registration and reproduction, which started more than
a century ago, there has been a steady improvement in quality. The first record
player, which was in fact a rotating drum, used only the mechanical excitation of
the needle to produce sound. The movements of the needle were transferred to a
diaphragm in a horn, thus forming a true ‘audio amplifier’. Later in time, the
movements of the needle were first transformed into electrical signals. These signals
were amplified by means of vacuum tubes and fed to a loudspeaker. With the
introduction of the transistor, vacuum tubes were replaced by transistors, and later
by integrated circuits. These developments led to audio amplifiers with less
weight, using less power and sounding better. (Regarding this last aspect it is quite
unfortunate that many people are misled by the term ‘warm feeling of tube amplifiers’,
thinking it refers to sound quality rather than to dissipation).
At the same time, the quality of storage media improved. The Phonograph was
succeeded by the Gramophone. Analogue magnetic recording developed from
steel wire to tape. Noise reduction techniques increased the dynamic range. Over
the past years, audio in the consumer domain has essentially become digital.
Audio is stored on media like CD (Compact Disc), DAT (Digital Audio Tape),
DCC (Digital Compact Cassette), MD (Mini Disk), or DVD (Digital Versatile
Disc). Formats include PCM (Pulse Code Modulation) up to 96kHz 24 bit, DSD
(Direct Stream Digital), multichannel sound up to 6 channels, and psycho acoustic
codecs like Dolby AC-3 (Audio Coding 3), MPEG (Motion Picture Expert Group)
layer 1-3 and MPEG-4 AAC (Advanced Audio Coding).
With the introduction of DAB (Digital Audio Broadcasting) and HDTV (High
Definition TeleVision), the audio amplifier is one of the few remaining analogue
components in the audio chain. This not only means that it has to fulfil high re
quirements on standard specifications points like distortion, slew-rate, power supply
rejection, etc. The large dynamic range of digital signals, for instance, demands
high peak power. Classical class AB amplifiers with high peak power,
however, have very poor efficiency at moderate signal levels. Also, good bass reproduction
is getting more and more important, requiring much power of the amplifier.
At the same time, dimensions have become smaller. Mini sets, car radios
and PC multimedia equipment have only little space available, leading to an increasing
conflict between manageable power dissipation and market demands for
high output power and many output channels. Also, more and more equipment becomes
portable. In these cases, a low power consumption is necessary to lengthen
battery life. To meet these demands, highly integrated, power efficient audio amplifiers
are essential.

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