Implementation of A 1000 Watts Class H Amplifier For Public Address Applications

 

UNIVERSITY OF NAIROBI

FACULTY OF ENGINEERING DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING

COURSE CODE: FEE 560 COURSE TITLE: ENGINEERING PROJECT

PROJECT NUMBER: PRJ061 PROJECT TITLE: IMPLEMENTATION OF A 1000 WATTS CLASS H AMPLIFIER FOR PUBLIC ADDRESS APPLICATIONS Presented By: KIILU MATTHEW NDOLO Admission Number: F17 / 2101 / 2004 Supervisor: Mr. C. OMBURA Examiner: Dr. WILFRED N. MWEMA

This project report is submitted impartial fulfillment of the requirement  for the award of o f the degree of Bachelor of Science in Electrical Ele ctrical And  Electronics Engineering of The University Univ ersity of Nairobi

th

Date of submission: 20  May 2009

 

DECLARATION

I, Kiilu Matthew Ndolo do hereby declare that this project in its form and content is my original work and to the best of my knowledge has not been published as a patented paper or submitted for any degree award in any other University or Institution of higher learning.

Signed: _________________________ _____________________________ ____

Date: __________________ ______________________ ____

Kiilu Matthew Ndolo F17 / 2101 / 2004

CERTIFICATION

This project has been submitted with the approval of the supervisor supervisor

Signed: _________________________ _____________________________ ____ Mr. C. Ombura (Project supervisor)

 

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Date: __________________ ______________________ ____

 

DEDICATION

I dedicate this project to my father, Eng. Bernard K. Kyengo, whose mentorship, constant encouragement, material support and steadfast belief in my abilities have gone a long way in making me the person I am.

And to God Almighty, without whom, none of this would have been possible.

 

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ACKNOWLEDGEMENTS

I wish to express my heartfelt gratitude to my supervisor Mr. C. Ombura for his patience, support and much needed guidance without which the successful completion and realization of this project would not have become b ecome a reality.

Special thanks also go out to the lecturers of the department of Electrical and Information Engineering (University of Nairobi) for the knowledge and skills they have imparted on me in my five year pursuit of a Bachelors of Science degree in Electrical and Information Engineering.

To friends and family for continued love and support in times when things got tough.

Above all I thank God Almighty for blessings received and for taking t aking me this far.

 

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ABSTRACT

The large dynamic range of signals demands high peak power. Classical class AB amplifiers with high peak power, however, have very poor efficiency at moderate signal levels. At the same time, demands for power efficient amplifiers have become the norm, leading to an increasing conflict between manageable power dissipation and market demands for high output power. To meet these demands power efficient audio amplifiers are essential.

The main objectives of this project were to design and implement a single channel of a 1000 Watts Class H amplifier that t hat would be suitable for public address applications. An audio amplifier with this kind of power output requires high voltages and currents which are quite dangerous and requires extreme care in its design and implementation. This report goes on to introduce the various amplifier classes available for high power po wer audio amplification and dwells mainly on the high efficiency Class H Amplifiers. It carries a detailed overview on their design, operation principles, implementation and protection. Special attention is given to its efficiency, which is the reason why the Class H Amplifier is at the cutting edge of o f audio amplifier technology. For most of this article the following assumptions have been made. 1.  The power amplifier considered has power supply rails that do not sag with load.

2.  Speaker loads are treated as purely resistive.

 

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TABLE OF CONTENTS

Page Number

DECLARATION

I

DEDICATION

II

ACKNOWLEDGEMENTS

III

ABSTRACT

IV

CHAPTER ONE - Introduction To The Project

1.0  Project Background

2

1.1  Statement Of The Problem

2

1.2  Justification Of The Project

2

1.4 Objectives Of The Project 1.5 Scope And Limitations Of The Project

3 3

CHAPTER TWO - Theory And Background

2.0 Power Amplifiers

4

2.1 Power Amplifier Classes

4

2.1.1 Class A

5

2.1.2 Class B

5

2.1.3 Class AB

5

2.2 Multi-Rail Amplifiers

6

2.2.1 Class G

6

2.2.2 Class H

7

2.3 How The Class H Amplifier Circuit Works:

8

Class-H Principle 2.4 Problems Associated With The Class H Design:

9

2.4.1 Power Supply

9

2.4.2 Output Topology

9

2.5 Amplifier Amplifier Limits

 

10

V

 

2.5.1 Clipping

10

2.5.2 Why Clipping Is Bad For an Amplifier

11

2.5.3 Why Clipping Is Bad For Speakers

12

3.0 CHAPTER THREE - Amplifier Efficiency and Justification of the class H design

3.1 Power Dissipation:

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3.2 Dissipation Dissipatio n model for a Class H vs. Class AB amplifier: amplifie r:

13

3.2.1 The class AB amplifier:

13

3.2.2 The class H amplifier:

15

3.3 Comparison of the Efficiencies of Class AB Vs Class H Amplifier: Amplifi er:

19

3.3.1 Efficiency of the class AB amplifier

19

3.3.2 Efficiency of the class H amplifier

21

4.0 CHAPTER FOUR - Project implementation 4.1 Power requiremen requirements ts analysis

27

4.2 Design of the driver and output stages

30

4.3 Power dissipation of the output devices

33

4.4 Balanced Balanced input stage

34

4.5 Preamplifier Preamplifier

36

4.6 Amplifier Amplifier Protection circuits

37

4.6.1 Thermal protection

37

4.7 System System testing

40

4.8 Results and analysis

41

4.9 Discussion Discussion and comparison with theory

45

CHAPTER FIVE - Conclusions and recommendations

 

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INTRODUCTION For years, engineers have been improving the efficiency of audio power amplifiers, and their efforts can be traced back to the first vacuum tube amplifiers. New technology has brought down the size and price of high power amplifiers while improving performance and efficiency. The main circuit configuration most commonly used for audio amplifiers is class AB which is not exceptionally power efficient with a wide dynamic range of signals as is the case with audio signals. This has led to the emergence of a relatively new class of audio amplifiers that is taking the market by storm. This is the high efficiency class H.

This report is intended to give an overall perspective of the design, testing and implementation approach used to design and implement a 1000 Watts high power class H audio amplifier that would be sited for public address applications. It begins by introducing the various classes of amplifiers that form a foundation on which the class H amplifier is built. Justification of the project is done through theoretical analysis carried out to show how this amplifier topology reduces heat dissipati d issipation on in its output semiconductors, thus increasing in its efficiency.

Problems inherent in this design are briefly discussed along with measures taken to counter them. Since the losses in any amplifier occur mainly as heat dissipated as the unit works, the issue of amplifier protection against thermal runaway is also covered, with designs created to counter the problem. The currents and voltages described in this report are high and pose a great danger in the case of any faults. Some test procedures such as driving the amplifier to its limits in order to observe clipping were thus not covered.

On the overall, it emerged that the modest gains in efficiency warranted for the complexity of this amplifier design.

 

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CHAPTER ONE Introduction To The Project

2.0 Project Background For years, manufacturers have been improving the efficiency of audio power amplifiers. Power amplifiers are categorized b y circuitry into class A, B, AB, C, D, E, F, G  and H. The class of an amplifier refers to the method in which the components within operate and determine the level of distortion, efficiency, heat dissipation, etc.

The main circuit configuration most commonly used for audio amplifiers is class AB. However, it is not exceptionally power efficient with a wide dynamic range of signals as is the case with music.

The class H amplifier is a composite amplifier equipped to electronically switch between multiple power supplies in response to signal amplitude while using the class AB type as a power-output stage. It has higher efficiency across a wide amplitude

range than the class AB amplifier and is especially suitable for with a wide dynamic range.

2.1 Statement Of The Problem To design and implement a 1000 watts class H amplifier for public address applications.

2.2 Justification Of The Project Many audio amplifiers use the classical Class AB amplifier in the t he output stage. These have a high efficiency when operating at full power. However since audio playback rarely reaches the peak power output of an amplifier, this class of amplifiers suffers from great heat dissipation which makes it very inefficient. A class H topology reduces dissipation across the output devices connected to multiple bipolar supply rails and allows the amplifier to operate with optimized class AB efficiency regardless of output power level.  

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1.3 Objectives Of The Project a.  To study and demystify the operation principles of various classes of amplifier topology b.  To design, simulate and implement a single channel of class H amplifier that is suitable for public address applications

1.6 Scope Scope And Limitations Of The Project The project scope was limited mainly to studying the operation principles, design and implementation of the Class H amplifier. Computer simulations were carried out in Multisim Version 10.0.1 and MATLAB 7.0 to analyze the operation of the design. Verification of results obtained would require the use of the same or o r similar software.

 

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CHAPTER TWO Theory And Background 2.0 Power Amplifiers

A power amplifier takes a line-level signal and reproduces it in a form that will drive a loudspeaker. It converts a low-voltage, high-impedance waveform into a highvoltage, low-impedance waveform. Its main purpose is to reproduce a low-power signal at high power. An ideal amplifier does nothing to the input signal other than make it stronger. However, real world amplifiers are not perfect. There are undesired characteristics that appear in the output signal such as noise and distortion. In the very best amplifiers these undesired characteristics characteristics are quite small but never zero. An audio amplifier  is a normal electronic voltage amplifier optimized for the amplification of low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain. While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts. In practical amplifiers, the actual maximum output voltage to the speakers is slightly less than the value of voltage present on the rails. The voltage and the speaker impedance determines how much power (wattage) will be delivered to the speaker

2.1 Power Amplifier Classes Power amplifier output stage circuits are classified based upon the conduction angle or angle of flow, θ, of the input signal through the amplifying device, i.e. the portion of the input signal cycle during which the amplifying device conducts. The angle of flow is closely related to the amplifier power efficiency and leads to the following classes which are discussed briefly:

 

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2.1.1 Class A Class A amplifying devices operate over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input and are the usual means of implementing small-signal amplifiers. However, they are not very efficient because the device is always conducting, and power is drawn from the power supply even if there is no input at all. For this amplifier class, a theoretical maximum efficiency of 50% is obtainable with inductive output coupling and only 25% with capacitive coupling. Thus if high output powers are needed from a Class A circuit, the dissipated power and the accompanying heat will become quite significant.

2.1.2 Class B Class B amplifiers have a conduction angle of 180o and only amplify half of the input wave cycle. As such they create a large amount of distortion, but their efficiency is greatly improved and is much better than Class A since the amplifying element is switched swi tched off o ff altogether half of the time, and so cannot cannot dissipate power. Class B has a maximum theoretical efficiency efficienc y of 78.5% (i.e., π /4), but can suffer from the drawback of crossover distortion if the handoff from one active element to the other is not perfect. An improvement is to bias the devices so they are not completely off when they're not in use. This approach is called Class AB operation.

2.1.3 Class AB In Class AB operation, each device operates the same way as in Class B over half the waveform, but also conducts a small amount on the other half. As a result, the dead zone is reduced resulting in a minimized crossover distortion when the waveforms from the two devices are combined. Class AB sacrifices some efficiency over class B in favor of o f linearity, and will always have an efficiency that is below 78.5%. Class B or AB push–pull circuits are tthe he most common design type found in aud io power amplifiers.

 

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2.2 Multi-Rail Amplifiers These have more than one rail voltage or pairs of rail voltages in the bipolar supply case. The two main classes involved here are the Class-G and Class-H amplifiers. Multiple rail amplifiers typically use only two voltage rails, which amounts to more than 80 percent theoretical efficiency at maximum power. There are many particulars to this type of design, but the main idea is to minimize the voltage across the output transistors and approach 100 percent efficiency at the output stage.

2.2.1 Class G Class G amplifiers are a more efficient version of class AB amplifiers, which use rail switching to decrease power consumption and increase efficiency. This highefficiency technique uses cascaded Class AB output stages, each connected to a different power supply voltage, with the magnitude of the input i nput signal determining the transistors to be used. Its operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required.

Figure 2.1: Switching Switching Rails of a class G ampl amplifier ifier

It can be:   Series type -



with two output devices connected in series for push and pull,

with either switching on and off to make the higher rail accessible and the other acting as the output device. d evice. This arrangement is not common, but is used

 

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to improve the safe operating area for the output transistors by limiting the voltage across each transistor pair and spreading the wasted power across more transis tr ansistors. tors. Parallel type - with two parallel p arallel push-pull stages connected connected to the two rails.

2.2.2 Class H Class H is similar to class G, except in this case, the rail voltage is modulated by the input signal to create an infinite number of supply rails which are only a few volts larger than the output signal at any given time. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices thus the nickname “rail-tracker” or “tracking” power amplifier. Switched mode power supplies can be used to create the tracking rails. r ails.

The class H design keeps the voltage across the transistors small and the output transistors cool thus significant efficiency gains can be achieved but with the drawback of a more complicated supply design and reduced thermal heat dissipation performance.

Figure 2.2: Rail modulation of a class H amplifier

 

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2.3 How The Class H Amplifier Circuit Works: Class-H Principle The class-H amplifier consists of a BTL (Bipolar Transistor Logic) class-B amplifier and a circuit which lifts the internal supply voltage. Since an audio amplifier only operates in the extended voltage area for short periods, the average dissipated power is only slightly higher than that for an amplifier without the supply voltage lifting circuit, although the peak output power is substantially increased. However, a circuit for re-charging the capacitors is needed.

Figure 2.3: Illustration of the class H principle.

Figure 2.3 shows the configuration of the class-H amplifier. Transistors T1and T2 are the output power transistors of the class-B BTL amplifier, and R1 is the load resistor. Controlled current source T7 charges C1 to: VC1max = E 1 – VD1 – VCEsatT7

which is about equal to the supply voltages El. When voltage V1 rises and T1 approaches the saturation area, this is detected by the lift control circuit. Lift transistor T5 then conducts and the charged capacitor is switched between the collector of T1 and supply voltage El. V1 can increase to about twice the supply voltage. Precautions must be taken in the lift/recharge control circuit circuit to prevent T5 and T7 from conducting conducting simultaneously. simultaneously.

 

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  Because the quiescent level of V1 is half the supply voltage, V1 can increase to twice the supply voltage, but can only decrease by half the supply voltage. This results in a voltage swing which is symmetrical with respect to ground at the amplifier output.

2.4 Problems Associated With The Class H Design: The primary advantage to this type of arrangement is that higher voltages can be delivered to the load with a higher efficiency than could be delivered using a conventional convention al class AB design. It does, do es, however, have several disadvantages.

2.4.1 Power Supply Transistor technology limits supply voltage. To get around this in class H amplifiers, the output transistors are placed in series and multiple supplies are used. For a transformer to operate efficiently, primary and secondary windings must be as physically close as possible. This is a problem when there are four secondaries on a core common to one primary. No supply will be used more than 1/2 of any audio cycle.  

2.4.2 Output Topology When a transistor is turned on fully and is conducting at its maximum level, it is said to be saturated. In a class H topology, before the second transistor can turn on, the first transistor must reach maximum conduction and go into saturation. Saturation is bad because transistors near, entering, or leaving saturation act non-linearly. In other words, distortion occurs as the audio level goes above and below the saturation point of the lower transistor. This leads to distortion in the output ou tput wavef waveform. orm.

 

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Figure 2.4: Distortion found in a Class H amplifier output signal

The amount of distortion is fixed. Raising and lowering audio level will affect the signal to noise ratio any time the output exceeds 1/4 power. Because signal to noise is actually measured by finding the noise floor with no signal then comparing it to rated output, the additional noise is not accounted for in class “H” specifications!

2.5 Amplifier Limits  2.5.1 Clipping   Any real life amplifier is an imperfect realization of an ideal amplifier. One important limitation of a real life amplifier is that the output it can generate is ultimately limited by the power available from the power supply. An amplifier will saturate and clip the output if the input signal becomes too large for the amplifier to reproduce or if operational limits for a device are exceeded.

 

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100 75 50 25 0

-25 -50 -75 -100

Figure 2.5 : Plot of output ou tput signal with an input signal of 3 volts vo lts and a gain of 50

Figure 2.5 shows that the output signal is no longer a clean sine wave because the peaks of the waveform have been chopped off or "clipped". If the signal illustrated in the figure above is applied to a speaker, the sound would take on a "harsh" or "raspy" "raspy" sound. The T he sound quality will deteriorate further the more the signal is clipped. In extreme cases, a sine wave will approach the shape of a square wave when clipped. Clipping introduces a large number of  harmonic components to a signal and it also increases the RMS level of the signal, something that can lead to blown speakers.

2.5.2 Why Clipping Is Bad For an Amplifier Effects of clipping on the amplifier really depend dep end a lot on how well the amplifier is designed. On the overall, some minor or occasional clipping is no big deal, d eal, but excessive clipping can stress things out. Operating an amplifier in extreme clipping might be more likely to stress out or damage the power supply as compared to the output semiconductors. When extreme clipping occurs, the output o utput transistors may

 

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actually have an easier time because when extreme clipping occurs they t hey are operating more like a switch, either being fully ON or fully OFF. This means that the output transistors have have to d deal eal with less heat dissipation. By being fully ON, they are sending the vast majority of the power supply's power to the speakers. Relatively less heat will be dissipated in the transistor in cases of extreme clipping. clipp ing. However, the power supply has to give out a lot more power than it was likely designed to do, and if the electronic components used with it were marginally m arginally designed or rated, they could fail. Amplifiers used for public address systems should be b e designed to be very rugged in this regard.

2.5.3 Why Clipping Is Bad For Speakers The risk of damage to the speakers depends on o n the characteristic of the music, to what degree clipping is occurring and how conservatively the speakers are rated. There are two major reasons why speakers driven by an amplifier operating in clipping can be bad:  

Signal compression occurs, and

 

More energy is generated in the high frequency range due to increased

•

•

harmonic distortion. This can lead to premature failure of the speakers. 

 

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3.0 CHAPTER THREE Amplifier Efficiency and Justification of the class H design 3.1 Power Po wer Dissipati Dissipation: on: The dissipation of an amplifier in relation to the output power Po and the efficiency η   is given by:

The main dissipation in linear amplifiers is caused by the output current that has to flow from the supply voltage to the output voltage. The voltage drop times the output current gives the dissipated power.

Given a certain amplitude distribution, there is an optimum which strongly depends on the load. Since most loudspeakers are more or less reactive, the current is out of phase with the voltage. This could lead to large currents when the full supply voltage is across the output transistors and thus influences the power dissipation.

3.2 Dissipation model for a Class H vs. Class AB amplifier: 3.2.1 The class AB amplifier: For a single amplifier with a symmetrical power supply +/- VS, the dissipation calculation is the same for positive or negative signals. Therefore, the absolute value of the output voltage V  o, ab s is taken and the t he loudspeaker current I  LS is calculated as:

And

The dissipated energy for the sample is a result of the voltage drop across the output transistors. Therefore:

Where:  f  s  is the sampling frequency and

 

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V S  is half the total supply supply voltage.

In a practical amplifier, the output voltage range does not fully extend to the supply voltage V S, but only to the clipping point, V  clip . With PQ  the quiescent power dissipation, the average power dissipation during one audio fragment of length T  frag   is:

Figure 3.1: Simulation of the operation and dissipation waveform of a class cla ss AB amplifier  

Figure 3.2: Simulation of the operation and dissipation waveform of a class cla ss AB amplifier when amplifying an audio a udio waveform

As seen in the figure above, the class AB amplifier wastes a lot of energy as heat when amplifying audio waveforms.

 

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3.2.2 The class H amplifier: The class H amplifier benefits from the elimination of wasted heat in the case of class-AB by splitting the power rails into 2 or more TIERS, and switching to the appropriate Rail according to the Voltage Swing at output. This vastly improves output stage heat dissipation by providing low voltage when the signal level is low so that the output transistor has less VCE  across it to t o do the job with less wastage. Taking the absolute value of the output voltage voltage V  o, ab s 

Loudspeaker current is calculated as

Where R LS is the loud speaker current. Modeling the class H amplifier with two power supply rails, V S,high S,high and V S,low S,low, small signals use V S,low S,low  . A signal is small if V   o   is smaller than a certain threshold voltage V  switch , which will lie slightly below V S,low S,low. The dissipated energy per sample due to

the voltage drop across the output transistors is given as:

When V  o is less than the threshold voltage V  switch And

When V  o is greater than the threshold voltage  V  switch  

The class H amplifier also dissipates due to the fact that it has to charge and lift the electrolytic capacitor. With  PQ  the quiescent power dissipation and  Elift the voltage required to charge and lift the capacitor ,  the average power dissipation during one audio fragment of length T  frag  is:

 

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Figure 3.3: Simulation of the operation and dissipation waveform of a class H amplifier  

Figure 3.4: Simulation of the operation and dissipation waveform of a class H amplifier when amplifying an audio waveform  waveform 

As seen from the diagram above, the class H amplifier leads to significant savings since the output transistors always "see" the rail voltage, and have to dissipate the difference between the rail voltage and the voltage across the speakers multiplied by the current, which equals watts of heat. Class H is therefore efficient and not much heat from the amplifier is generated. Its drawback is that there is finite time to switch between one rail voltage and the others, so in the mid volume level, there can be distortion when the rails are moving back and forth.

 

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The figures below show plots comparing the theoretical dissipation of the 1000 Watts class H amplifier topology to that of a classical class AB amplifier with a similar power output.

Figure 3.5: Comparison Comparison of the dissipation d issipation of a class AB and a nd a class H amplifiers. [ Harry Perros Computer Science Department NC State University  ]

Figure 3.6: comparison of the output stage power loss for class AB and class H amplifiers

 

[Katsuhiko Higashiyama - Matsar shita Electric  Industrial co rporation  ] 

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300

200

100

20

40

60

80

100

Figure 3.7: average power dissipation as a function of RMS output voltage. Curve A: class AB amplifier amplifier Curve B: class H amplifier amplifier Curve C: Gaussian Curve D: Exponential 

[

 

Philips Semiconductors, Nijmegen, The Netherlands ] 

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1500

( Class H )

(Class AB)

1000

500

0

0

Figure 3.8: average power consumption for class H vs. class AB amplifiers 

[Yamaha Power Amplifier White Paper August 2008] 3.3 Comparison of the Efficiencies Efficiencies of Class AB Vs Class H Amplifier: The electrical efficiency of an amplifier is defined as the ratio of the power developed in the load to the power drawn from the DC supply. Using simple linear analysis the efficiency of amplifier output stages can be determined.

3.3.1 Efficiency of the class c lass AB amplifier  Class AB technology is the foundation of audio amplifier design. A simple Class-AB output stage is shown in

 

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3.9. When the pair of complementary output devices is operated in the linear region there will be current flowing through them whilst there is a voltage across them. This will give rise to power po wer dissipation, dissipation, and hence reduce efficiency. The devices also need a quiescent bias to reduce crossover distortion as one device takes over from the other.

Figure 3.9: A simple Class AB output stage

The efficiency of a single ended class AB amplifier driving a resistive load can be derived by comparing the power taken from the supply and that developed in the load. Ignoring power loss due to quiescent bias, and assuming a sine wave output, the efficiency over any number of periods is equal to the efficiency over a quarter of a period. The efficiency is defined d efined as: η =

 Po / Power input  Pi.  ,Pi.  Power output, Po

Where

Where R L is load resistance. Over a quarter of a period: p eriod: And

Which yields η =

 

¼p

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= 78.5%.  A practical amplifier can not drive its load to the power supply lines. The voltage difference between the power supply and the maximum output voltage lowers the efficiency. Apart from that, a normal audio amplifier signal is not a sine wave, but music or speech. The average output power of audio signals is at most half the maximum sine wave power. Since an audio signal waveform is complicated and not standardized a calculation of the efficiency in the same way as above is not possible.

3.3.2 Efficiency of the class c lass H amplifier  A class H topology reduces dissipation across the output devices connected to multiple bipolar supply rails and allows the amplifier to operate with optimized class AB efficiency regardless of output power level. Below is a brief analysis showing the theoretical efficiency of a class H amplifier topology.

Demonstration of the theoretical efficiency of a Class H amplifier Case 1: With supply switch at  at  Vs sin α 

 

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Figure 3.10:Class H with supply switch at Vs sin α 

Assuming a maximum output and an ideal BJT, Instantaneous Instantane ous load power is: ; ;

Mean load power is

Instantaneous supply power before switch is ; ;

Instantaneous supply power after switch is ; ;

Mean supply power is

 

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To find the maximum of the function;

Determine the first derivative and equate to zero

This equation is satisfied for;

Where only

 

is a point of maxima

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Case 2: With supply switch at  at  Vs.

Figure 3.11 : Class H with supply switch at Vs sin √2 / 2

Assuming a maximum output and an ideal BJT,

Instantaneous Instantane ous load power ; the load voltage ; the load current

Mean load power is

 

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Instantaneous supply power before switch is ; ;

Instantaneous supply power after switch is ; ;

Mean supply power is

Now since

And . Then

Or

This shows that 85.9 % of power consumption from the supply is converted to power in the load in the case of a purely pure ly resistive load. This implies that the power dissipated by the output stage semiconductors is less than in the case of o f the class AB amplifier.

 

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Figure 3.12 : comparison of the efficiency of class AB and class H amplifiers

[Katsuhiko Higashiyama -  Matsarshi ta Electric Industrial c orporation   ] 

Class H eliminates the draw of excess power that would be converted into heat in a typical amplifier design, instead running at a cool temperature during all stages of operation. Even when sitting idle, the power draw is minimal compared to conventional class AB amplifiers. However, as the signal level increases, the system functions in the same way as a Class AB system, and its advantage in efficiency is lost. Class-H reduces the current consumption from as much as 30% in comparison to same wattage amp in Class-AB, while, making the heat sinking requirements far much less than those of a class-AB amplifier. The class H amplifier is thus more efficient across a wider dynamic amplitude range than the class AB amplifier. However, at full power, class H amplifier is slightly less efficient than class AB, but since majority of the time the amplifier is not at full power, the efficiency advantages of the class H amplifier still carry the day through the reduce amount of dissipated heat.

 

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4.0 CHAPTER FOUR Project implementation

Based on the preceding analysis and theory, the implementation of the 1000 watts class H amplifier was done, keeping in mind to provide some flexibility, but also being very careful to make sure that transistor safe operating area (SOA) is not exceeded. There is also a maximum voltage for any semiconductor, and devices must be selected to ensure they are used within their ratings. This project describes an amplifier, power supply and tests procedures that are all inherently dangerous since the DC currents and a nd voltages involved are quite high. high.

4.1 Power requirements analysis To get 1000 W into 4 ohms, the current needed is:  2

 I  R = 1000

But R = 4 Ohms  2

Thus  I  R = 1000Watts / 4 Ohms  I   = = √ (250) =15.811 Amperes

This gives the full power current. But VI = I  2 R V = IR

=15.811 Amperes X 4 Ohms

 

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=63.2456 Volts (RMS)

This occurs across the load, but needs to have a bit more, because the supply voltage might collapse under the load, and there will always be some voltage lost across the transistors, emitter resistors, etc. The supply voltage needs to be: V  DC  =  = V  RMS * 1.414 V  DC  =  = 63.2456 * 1.414 = ±89.4292Volts DC

Since losses have not been allowed for yet, there is need to allow around 3-5V for the amplifier, and an additional 10V or so to allow for the supply voltage falling when the amp is loaded. The higher the current, the greater the resistive r esistive (I²R) losses, so 5V was used in this design Thus the peak supply voltage required was: Vsupply = 90 +10 +5 =105 Volts

The high rails were chosen to operate at ±110  Volts in order to allow for a small output voltage swing’s headroom incase the amplifier is driven into clipping, whereas the low voltage rails were chosen to be half that amount i.e. ± 55 Volts. The power supply Transformer was calculated to be rated at: 0.707x = 110 Volts Thus x = 110Volts / 0.707 =155.586 Therefore with a transformer rated at 2 x 80Volts, this gives an unloaded supply of: 160*0.707 = 113.12 Volts

 

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= ±115V DC (230 Volts DC total) So the supply has to be treated with extreme care - it is very ver y dangerous indeed. With a peak voltage of 90 Volts (i.e. the requirement to drive the speaker minus the losses), losses ), the peak p eak current will be: From V = IR

90 Volts = 4 I   Therefore I  =  = 90 Volts / 4 Ohms To give a current of 22.5 Amperes into a 4 Ohm load The RMS current is given as:  I  RMS = √ (1000 / 4)

= √ 250 =15.811 Amperes at full power.

The final supply voltage will be around ±100V, because even with the biggest transformer and filter capacitors, voltage losses occur. The current demand is also prodigious. With a peak voltage of 90V, the peak supply current is 22.5 A into a 4 ohm load. RMS speaker current is just under 15.811 at full power. PCB tracks cannot be used for these current levels, because the extra resistance will cause current balancing problems with the power transistors. All wiring needs to be extremely robust, and must absolutely not allow any possibility of contact or short circuits which might kill the class H amplifier. The supply is also quite capable of vaporizing thin wires and PCB tracks.

 

29

 

While lateral MOSFETs offer some real advantages, they are relatively expensive, and difficult to obtain with voltage ratings above 200V. Vertical MOSFETs (e.g. HEXFETs and the like) are a possibility, but suffer gross non-linearity at very low currents. Therefore, a relatively high quiescent current is needed, and this makes heat removal that much more difficult. Because of the issues discussed above, A bipolar junction Transistor approach was thus settled upon as most appropriate for the output stage. This was primarily dictated by the supply voltage, which exceeds that allowed for any affordable lateral MOSFET. It was even a challenge to get affordable BJTs, but the 2SA1302 and 2SC3281 complementary pair was within ratings, so these were chosen.

4.2 Design of the driver and output stages While most books would normally specify a compound (Sziklai) pair for the output stage, in this case it is a triple stage, and the Sziklai can be prone to oscillation, primarily on the negative side. This is highly undesirable for an amplifier with the power described here. Therefore, to obtain a clean output of a class AB with the high thermal efficiency of the class B, a Deep Darlington configuration was chosen. It was found that by using a three-deep Darlington, the individual devices could be biased separately. The first device in the chain is the pre-driver, and second is the driver. Together these two devices were biased to operate as class AB. In other words, they are biased a little bit on, even without signal. Since they are small devices (compared to the third part of the Darlington) they do not generate a lot of heat, but provide a smooth output to the load even through the zero-crossing region.

 

30

 

Figure 4.1 : Deep Darlington configuration

The last part of the Darlington is the output transistor devices (the 2SA1302 and 2SC3281 complementary pair). These were biased so that they are completely off when no signal is applied, but requires only a small signal s ignal to begin conduction.

The output is designed to switch between two bipolar voltage rails of ±55 Volts and ±110 Volts, according to the amplifier’s output requirements. This reduces the amount of heat dissipation that would have occurred if a traditional class AB output stage had been used. As may be noticed, noticed, the output stage has has many paralleled output transistors. The reason of doing so is to reduce the thermal stress and dissipation incurred by each, thus reducing the threshold for thermal runaway of the output semiconductors. Another reason for paralleling the output transistors is to increase the current to the load for higher power. The output transistor devices i.e. the 2SA1302 and 2SC3281 complementary pair are rated to carry a maximum voltage of 200Volta and were thus suited for the application.

The illustration below shows the implementation of the deep Darlington configuration used in this design. A detailed schematic of the output topology can be found in the servicing diagrams contained in the appendix. app endix.

 

31

 

Output Transistors

Driver Stage

Switching Circuit

+ High Rail

+ Low Rail

Input from preamplifier

Class H amplifierOutput Ground

- Low Rail

-High Rail

Figure 4.2: output stage of the Class H amplifier (unlabelled parts connect to the preamplifier p reamplifier st stage age of the amplifier directly

 

32

 

4.3 Power dissipation of the output devices Next, looking closely at the power dissipation d issipation of the devices, worst case resistive load dissipation occurs when ½ the supply voltage is across both load and transistors. Had this been a Class AB, worst case occurs at a voltage of 55V across the load, and gives a peak (instantaneous) power in both load and output stage of : P = V² / R = 55² / 4

= 756.25W

However, For the Class H amplifier, Worst case dissipation gives: P = V² / R = 27.5² / 4 = 189.0625W

When operating on the low rails. However, dissipation is higher when the amplifier switches to the high rails, but somewhat better than in the case of a class AB. Worst case dissipation when the high rails are loaded occurs at 82.5 Volts.This gives a dissipation of: [(1 – efficiency,η) *maximum available output from the low rails ] + Worst case dissipation at the high rails

Since the high rail has a potential that is twice tw ice hat of the low rail, the additional power dissipated when switching occurs will be similar to the worst case dissipation when only the low rail is loaded. Thus, worst case dissipation for the Class H amplifier will be [(1-η)*Po, low rail] + Pdiss low rail This gives Pdiss, Class H = [(1 – 0.85)*(55 0 .85)*(552 /4)] + (27.5² / 4) = (0.15 * 756.25) + 189.0625

 

33

 

= 113.4375 + 189.0625 = 302.5 Watts

The output stage has eight output transistors and the heat dissipation is evenly spread across them. Thus each transistor dissipates 37.8125  Watts as heat. The peak dissipation into a reactive load with a 45° phase angle is almost double that calculated above, about 604.5 W. The 2SA1302 and 2SC3281 transistors are TO-246 packages, and are specified for 150W dissipation at 25°C. It is worth noting that the driver in this arrangement contributes some of the output, but it only reduces the main transistor's peak dissipation by about 5W. The 2SA1006 and 2SC2336 pre-drivers reduce the loading

on the voltage amplification stage (VAS) and ensure good linearity with acceptably low dissipation of the VAS transistor and its current source. Nevertheless, this heat dissipation is a problem since it might cause thermal runaway in the output transistors. Thus measures had to be taken to ensure hat the output transistors remained within their safe operating area. Since it is not likely to maintain the output transistors at 25°C and there is need to allow for elevated temperature. Cooling is vitally important in this amplifier and substantial heat sinks were required whose temperature increase with dissipated heat is 0.06 °C/ Watt. However, since these were impossible to find, an alternative heat sink design with cooling fins was used along with forced cooling using fans. fan s.

4.4 Balanced input stage Balanced audio  is a method of interconnecting audio equipment using impedance-

balanced lines. This type of connection is very important in sound production because it allows for the use of long cables while reducing susceptibility to external noise. The TRS jack was chosen in this case since it is easily available and is found in most mixer inputs and outputs because of its smaller profile. The main reason for using a balanced input stage is that many microphones operate at low voltage levels and some with high output impedance, which makes long microphone cables especially susceptible to electromagnetic interference. The input

 

34

 

stage of this amplifier was therefore a perfect application for a balanced interconnection, which cancels out most of this induced outside noise. With the idea in mind that this power amplifier of a public address system might be located at any distance from the mixing console, it was imperative to use balanced lines for the signal paths from the mixer to this amplifier. This was done by connecting each wire to identical impedances at source and load. This means that much of the electromagnetic interference will induce an equal noise voltage in each wire. Since the preamplifier at the next stage is implemented using a differential amplifier which measures the difference in voltage between the two signal lines, noise that is identical on both bot h wires is rejected.

The wires into the amplifier were also twisted together in order to reduce interference from electromagnetic induction. A twisted pair makes the loop area between the conductors as small as possible, and ensures that a magnetic field that passes equally through adjacent loops will induce equal levels of noise on both lines, which is canceled out by the differential amplifier.

Gain Set Jumper 6

Capacitor removes DC element from the output signal 

7 R20 1kΩ

-VE V in

R24 1kΩ

R21

R22

38

10Ω

5kΩ R16

28

20kΩ

4

R19

33

U1A C6

2

15kΩ

1

39

100nF

3

GND

1

8

IC=200V NE5532IP

5

Output to Preamplifier 

GND R18 20kΩ

32

R15

R17

15kΩ

5kΩ

37

R23 10kΩ

GND

GND

 

+VE V in

Figure 4.3 : Balanced input stage

 

35

 

4.5 Preamplifier The implementation of the preamplifier stage required the use of a differential amplifier. The circuit is completely conventional, using a long tailed pair input stage, directly coupled to the Voltage Amplification Stage. A curren currentt mirror was used for the long tailed pair, as this increases open loop gain but has the disadvantage that it may give rise to stability issues. In a very high power amp, stability is paramount and the amplifier must never oscillate under any normal load condition because the heat created can cause almost instant transistor failure. Owing to this important consideration, carefully matched transistor pairs were chosen i.e. the MPQ6700 matched transistor pair.

The figure below shows the schematics of the preamplifier circuit. To establish the gain, each differential amplifier was treated separately as a long tail pair and basic principles of differential amplifiers used to determine the gain. A detailed derivation of this can be found in the Appendix, along with a short code for simulating its operation in SPICE.

The output of this stage is coupled through a capacitor that eliminates the DC componentt of the input signal while allowing the AC component to pass before going componen to the output stage of the class H amplifier. A detailed schematic diagram of the preamplifier can be found in the servicing diagrams contained in the appendix.

 

36

 

4.6 Amplifier Protection circuits 4.6.1 Thermal protection Environmental hazards facing this amplifier design boil down to heat and moisture, each of which poses its own challenges. Cooling is vitally important in i n prolonging the life of any amplifier. This amplifier will need a very substantial heat sink, and fan cooling is essential. In the case of heat, operation in high ambient temperatures will expose limitations in this system's heat dissipation strategy. Cooling primarily relies on a massive heat sink on which the output transistors are mounted, but high ambient temperatures - caused by weather, limited space or restricted airflow - can reduce the cooling potential of the heat sink. This brings out

the need to use forced cooling. This is supposed to keep the power transformer from overheating and burning up as well as keep the output transistors and other solid state devices from overheating and being destroyed. In general, the cooler the amp runs, the longer it will last. This amplifier was designed to ensure a dedicated air path across the electronics which exceeds peak cooling needs. Only one fan is to be used under most operating conditions, but if temperatures within the unit rise above 45° Celsius, the second fan kicks in increasing airflow through the system in order to minimize thermal runaway of the output semiconductors and the contaminate deposition on sensitive amplifier and control componentry componentry.. The primary fan which is a lower speed fan that kicks in immediately the amplifier is powered. However, the second fan involves quite a bit of complicated circuitry that also serves to indicate the ambient temperature of the amplifier unit. It should kick in when the threshold voltage of 45 degrees Celsius is reached, and it runs at high speed.

Circuit operation: The temperature sensor element LM35DZ detects the ambient temperature of the amplifier and and produces an analogue analogue (voltage) signal signal which is passed passed through the LM3914 bargraph display driver which then drives the LEDs depending on how calibration is set.

 

37

 

For the amplifier, the temperature range is set for the optimum operation range of the transistors an other circuit elements i.e. between 25 degrees centigrade (optimum operation temperature) and 45 degrees centigrade(maximum operation temperature). The outputs corresponding to these temperatures are tapped from the LM3914 bar graph display driver and used as inputs to the AND gate.

When the Upper temperature threshold (limit) is reached i.e. 45 degrees centigrade, the AND gate input tapped from the 45 degrees centigrade input will be HIGH whereas the AND gate input tapped from the 25 degrees centigrade input will also be HIGH.. This means that the Transistor and Relay HIGH Relay will be turned ON. Thus th thee cooling fan system will be switched ON and the ambient amplifier temperature will be

maintained within a safe operating region.

On the other hand, when the lower temperature threshold (limit) is reached i.e. 25 degrees centigrade, the AND gate input tapped from the 25 degrees centigrade input will be HIGH whereas the AND gate input tapped from the 60 degrees centigrade input will be LOW. LOW. This means that the Transistor will re remain main OFF, cutting power to the second cooling fan system since the ambient temperature is at optimum. o ptimum.

This circuit also provides a visual display of the ambient amplifier temperature through the use of LEDs in decade steps. This has the advantage that one the temperature can be monitored by the user of the amplifier amplifier unit.

 

38

 

V1 RL1

R5

12V 100R 12V

L1

D10

U2

1

150.0

VOUT

  2

RV1 R1

3

U1

      3

7 6 4 8

0R22

LM35

5

R4 0R22

9 1k

SIG

    +       V

VRO RHI RLO ADJ MODE          V       2

10 9 8 7 6 5 4 3 2 1

                   

LED-RED

LEDRED D8 10 11 D7 12 D6 LED-RED 13 D5 14 D4 15 LED-RED 16 LED-RED D3 17 LED-RED 18 LED-RED D2 1 LED-RED

LM3914

D1

LED-RED

R2 R3 0R22

0R22

C1

RV2

12V

FAN

D9

LED-RED

U3:A 1 3 2

Q1 MPS6531

74S03

V2 1V

1u

C2 10u 1k

RV3 1k

  Figure 4.4 : The cooling circuit.

 

39

 

Many professional power amps and loudspeaker systems provide some of protection, either to protect the speakers from an amp fault, and/or vice versa. Some of these are implemented at a very basic level - for example the use of a poly-switch which is a non-linear resistor, having a low resistance at normal temperatures and a much higher resistance at some designated temperature. Unlike ordinary thermistors whose characteristics are more or less linear, the poly switch has a rapid transition once the limit has been reached. However, the introduction of a non-linear element is going to add some degree of distortion, and because of a finite resistance, will degrade damping. The basic requirement of a speaker protector requires that any potentially dangerous DC flow to the speakers should be interrupted as quickly as possible. There are a few issues that need to be solved to ensure that this will happen fast enough to stop the loudspeaker drivers from being damaged, and this becomes more critical if a biamped (and even more so with triamped) system is being used. Naturally, one can simply rely on fuses. Although these also have finite resistance, it is small and use of fast blow fuses can be quite effective. The rating becomes quite critical, and fast blow types are essential. The problem with this approach is that if the fuse is of a suitable value to provide good protection, it will be subjected to considerable thermal stress since it is operating at close to its limits. Metal fatigue will create the problem of nuisance blowing, where the fuse blows simply because it is 'tired' of the constant flexing caused by b y temperature variati variations. ons.

4.7 System testing A version of the amplifier was sumulated in National Instrument’s Multisim version 10.0.1 (a more modern version of Electronic Workbench) where a test signal was

applied into the input stage and the corresponding co rresponding output recorde recorded. d. The test signals included:  

a. A simple sine wave from a function generator with a frequency of 1 KHz and an amplitude of 100mV. b.  A Voice input captured as a Wave file format through the computers Sound Card.  

40

 

The output from the amplifier’s output stage was captured on a virtual oscilloscope found in Multisim version 10.0.1 and the output traces were plotted as a function of time.

4.8 Results and analysis  When the balanced input stage was fed with a 100 mV sine wave input, the configuration produced an output whose amplitude was 750 mV. Thus its gain was found to be 7.5. A plot of the output waveform shows that the input stage faithfully reproduces the input signal, only at a larger magnitude, thus its linearity ensures that a stronger version version of the original o riginal signal is sent to the preamplifier

The preamplifier stage was fed with the 750 mV sine wave input from the input stage configuration and it amplified the difference, producing an output whose amplitude was 1.475 V. Thus its gain was found to be 1.98. A plot of the output waveforms is shown bellow.

Figure4.5: plot of the sine wave input signal and the corresponding output of the  preamplifier stage.

 

41

 

The result obtained when the signal reached the output stage was similar, only that this time it had a gain of 73.77. this means that the output voltage was a mere 7.37 Volts which implied that the output was drawn from the amplifier’s Low Rails. An input of 100 mV is quite small and was mainly used to test the operation of the basic class AB amplifier output stage before switching occurred.

The output response of the class H output stage was also measured with a small output signal applied to the input stage. The results obtained are shown in the plots below:

Figure4.6: plot of the sine wave input signal and the corresponding output of the Class H amplifier stage.

The output from the amplifier’s output stage was fed to the oscilloscope’s channel B, whose vertical scale was at 20 Volts/ division whereas the output from the balanced input stage was fed to the oscilloscope’s channel A whose vertical scale was at 5 Volts/ division. From the above plot, output from the amplifier is seen to be 44Volts. Calculating overall system gain:

 

42

 

Gain = output / input = 44V / 100mV =440 for an input signal of 100 mVolts.

In this case, The instantaneous amplifier output current at this stage was calculated to be 11 amperes and the RMS current was found to be 7.777  amperes. Thus the instantaneous output power was calculated as 240.93 Watts. Calculating the efficiency at this stage: Since the output was drawn from the low voltage supply rail, the power input will be given by (55 * 0.707)2 / 4 = 378 Watts Thus efficiency = 240.93 / 378 =0.6359 =63.59%

At this point, the class H output stage is operating as an optimized class AB output stage.

However, when subjected to an input signal with a wider dynamic range (Voice input in the form of a wave sound captured through the computer sound card), the result looked as shown below:

 

43

 

Figure 4.7: Output of the Input stage when fed with a voice input

Figure 4.8 : Output of the Class H stage when fed with a voice input

 

44

 

From figure 4.8, an arbitrary point was chosen at random and the magnitude of the input signal was calculated by working in reverse: Gain = output / input 7.5 = 2.7 / input Thus input = 2.7 / 7.5 =360 mV

Which is well within the excitation range obtainable when someone speaks into a microphone.

Thus the overall system gain will be: Gain = output / input =65.544 V / 360mV =182.066

The instantaneous amplifier output current at this stage was calculated to be 16.386 amperes and the RMS current was found to be 11.584  amperes. The instantaneous

output power was calculated as 536.84 Watts.

The output at this stage was being drawn from the High Voltage rails. This is proof that the switching circuit of this amplifier design actually works.

4.9 Discussion and comparison with theory

From the simulations carried out on the system design, the class H amplifier was seen to modulate the voltage supply rails in accordance to the input signal. The analysis carried out on the simulation results showed that the class H amplifier switched between the voltage supply rails according to the power output requirements of its output stage. The efficiencies calculated using these results, however, were slightly

lower than the theoretical efficiencies quoted previously but on overall, much better than those of class AB amplifiers with similar power ratings.

 

45

 

CHAPTER FIVE Conclusions and recommendations

From the analysis and design carried out in this report, a single channel of a class H amplifier was built. Test procedures that push the amplifier to its limits were not carried out because of the dangerous current and voltage levels involved. However, voltage amplification of input signals was successful albeit with a bit of distortion.

The modest gains in efficiency warrant the additional complexity when viewed in perspective of long term energy savings. The benefits were seen as worthwhile and the apparent limitations i.e. marginal dissipation improvement and distortion of little concern. The objectives of the project were thus achieved successfully. 

 

46

 

List of Symbols/Abbreviations Used AC- Alternating Alternating Current VO - Output Ou tput voltage Vs - Supply rail voltage RL - Load resistance Ibias - Class AB quiescent bias current Ø - Load phase angle Zlood Load impedance η - Efficiency E fficiency Po - Output power Pomax - Maximum sinewave sinewave output power Pi - Input power Pinst - Instantaneous power Pavg - Average power Pd, Pdiss - Dissipated power Vi - Input Voltage Vo - Output voltage VS -  -Positive Positive or negative supply voltage (equal) V DD Positive supply voltage VSS - Negative supply voltage Vthr - Threshold voltage PDF - Probability Density Function f - Frequency f ssw w,f switch switch - Switching frequency f sin sin - Frequency of a sinewave Z - Impedance a - Amplitude as fraction of the power supply Ro - Output resistance Ron - On-resistance of a MOS transistor Lo - Output inductance A - Gain t - Time Tsw - Switching time Io - Output current Ithr - Threshold Threshold current E - Energy

 

47

 

LIST OF FIGURES

Figure 2.1: Switching Switching Rails of a class G amplifier amplifier Figure 2.2: Rail modulation of a class H amplifier Figure 3.5: Comparison of the dissipation dissipation of a class AB and a cclass lass H amplifiers.

[ Harry Perros Computer Science Department NC State University ]

Figure 3.6: comparison of the output stage power loss for class AB and class H amplifiers [Katsuhiko Higashiyama - M atsarshita atsarshita Electric Industrial corporation ] Figure 3.7: average power dissipation as a function of RMS ou output tput voltage. Curve A: class AB amplifier ampli fier Curve B: class H amplifier amplif ier Curve C: Gaussian Curve D: Exponential Exponential [

Philips Semiconductors, Nijmegen, The Netherlands ] 

Figure 3.8: average power consumption for class H vs. class AB amplifiers  Figure 3.9: A simple Class AB output stage Figure 3.10:Class H with supply switch at Vs sin α  Figure 3.11 : Class H with supply switch at Vs sin √2 / 2 Figure 3.12 : comparison of the efficiency of class AB and class H amplifiers Figure 4.1 : Deep Darlington configuration Figure 4.2: output stage of the Class H amplifier Figure 4.3 : Balanced input stage Figure 4.4 : The cooling circuit. Figure4.5: plot of the sine wave input signal signal and the corresponding output of the preamplifier stage. Figure4.6: plot of the sine wave input signal signal and the corresponding output of the Class H amplifier stage.

Figure 4.7: Output of the Input stage when fed with a voice input Figure 4.8 : Output of the Class H stage when fed with a voice input

 

48

 

APPENDIX

SPICE File For A Differential Amplifier SPICE FILE 

BJT_DIFFAMP1.CIR BJT_DIFFAMP1.C IR - BJT DIFFERENTIAL AMPLIFIER * * SIGNAL SOURCE VS 1 2 AC 1 SIN(0 100MVPEAK VCM 2 0 SIN(0 0MVPEAK 5KHZ) * * POWER SUPPLIES VCC 11 0 DC +15V VDD 12 0 DC -15V * Q1 3 1 5 Q2N5551 Q2 4 2 5 Q2N5551 RC1 11 3 1000 RC2 11 4 1000

1KHZ)

RE 5 12 8.25K * * .model Q2N2222 NPN(Is=3.108f Xti=3 Eg=1.11 Vaf=131.5 Bf=217.5 Ne=1.541 + Ise=190.7f Ikf=1.296 Xtb=1.5 Br=6.18 Nc=2 Isc=0 Ikr=0 Rc=1 + Cjc=14.57p Vjc=.75 Mjc=.3333 Fc=.5 Cje=26.08p Vje=.75 + Mje=.3333 Tr=51.35n Tf=451p Itf=.1 Vtf=10 Xtf=2 Rb=10) * * * CHECK DISTORTION WITH F OURIER SERIES ANALYSIS .FOUR 10KHZ V(3,4) * * ANALYSIS .TRAN 5US 200US .AC DEC 5 1K 100MEG * * VIEW RESULTS .PRINT TRAN V(3) .PRINT AC V(3) .PROBE .END

Servicing diagrams:  

49

 

This section contains the schematics of the amplifier.

 

50

 

1000 Watts Class H Amplifier Circuit – Input Stage preamplifier

V2 20 V GND GND

V1 36 V GND

D2

D3

1N5353B

1N5353B

1

GND

4

C3 100nF C4

3

470nF C5

R3 511Ω

Q7

5

2N5551

470uF C6 100nF

R1

Q3

Q8

500Ω

2N5551

2N5551 8

47Ω

R9 7 R13

Q2

Q5

MPS6601 9

11 MPS6601

13

D6

100Ω

R4 68.1Ω

20

10 R5 68.1Ω

R12 100Ω

6

18

17

HFA04TB60

R11 27kΩ

12 R6 8.25kΩ

19

R10 R7 8.25kΩ

D5 30

15

C7

2.7kΩ

30pF

R14

14

16

11kΩ

1N4148 21

D1

R15 576Ω

1N4148 22

R19

R18 68.1Ω

R17 68.1Ω

24

23

R16 1kΩ

R20 1kΩ D7

29

Q10

Q9

MPS6651 26

MPS6651

28

R21

100Ω 25

Q12

2

HFA04TB60 35

47Ω

Q11

R22

31

36

500Ω 2N5401

2N5401

C1

Q1

100nF C8

32 R23Ω 511

2N5401

470uF C9

27

470nF D8

D4

1N5353B

1N5353B

C10 GND GND

V3 -36 V

34

100nF

GND V4 -20 V GND

 

33

GND

GND

53

 

References

1. Charles L. Alley and Kenneth W.Atwood , Electronics engineering engineering third edition. ISBN 085750024 2. John Everett (1992). Vsats: Very Small Aperture Terminals. IET. ISBN 0863412009. 3. A.P. Malvino, Electronic Principles (2nd Ed.1979. Ed .1979. ISBN 0-07-039867-4) p.299. 4. Paul Horowitz, Winfield Hill (1989). The Art of Electronics. Cambridge Universi University ty Press. 5. www.National Semiconductor Corporation.com 6. Sachse, Herbert B. Semiconducting Temperature Sensors and their Applications. JomoKenyatta JomoKeny atta Memorial Library (JKML) TK 7871.98 .S23. 7. William J. Tompkins and John G. Webster. Interfacing Sensors Sensors to IBM PC. Jomo Kenyatta Memorial Library (JKML) TK7887.5, 157. 8. www.Crownaudio.com www.Crownaudio.com 9. Wilfred Njoroge Mwema, Analogue Electronics Electronics, third year syllabus.

 

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