Valve Amplifiers (eBook)

Morgan Jones (Autor)

eBook Download: PDF | EPUB

2011 | 4. Auflage
700 Seiten
Elsevier Science (Verlag)
978-0-08-096641-0 (ISBN)

Infos

Reviews of previous editions:

Jam-packed with theory, circuit analysis, and DIY basics, it will walk you through all stages of design so that you can create your own wonders. Jones is an ex-BBC engineer with a cool writing style and you?ll find it a no-pain education. Hi-Fi News and Record Review

Valve Amplifiers is an extremely well written book, containing a wealth of information that all audio designers and builders will find useful. Glass Audio

Valve Amplifiers is a market leader for one simple reason: in this specialist area it is recognized as the most complete guide to valve and vacuum tube amplifier design, modification, analysis, construction and maintenance. It is truly the all you need to know guide, and enables audio and circuit designers to succeed with their valve amplifier designs and projects.

This book enables readers to understand, create, reconfigure and personalize high-end, audiophile quality amplifiers. Following a step-by-step approach to design, with little maths and lots of know-how, it starts with a brief review of electronic fundamentals relevant to valve amplifiers, simple stages, compound stages, linking stages together, and finally, complete designs.

The new material included in this Fourth Edition ensures this book will stay at the top of any audio designer?s or enthusiast?s reference list.

 What?s new:

Chapter 1: Charge amplifiers

Chapter 2: Additional circuits, semiconductor constant current sources expanded

Chapter 3: Entire new section on noise

Chapter 4: Lots of new measurements to explode or explain audio folklore

Chapter 5: Astonishingly quiet, but cheap and simple HT supply

Chapter 6: New power amplifier

Chapter 7: New hybrid balanced RIAA stage, attenuator law faking

VA3's focus was on distortion, but in VA4, focus is pushed towards background noise reduction. If that wasn?t enough, there?s more explanation, more measurements, more references, and plenty of new one-liners, any one of which might save hours of trouble.

* The practical guide to analysis, modification, design, construction and maintenance of valve amplifiers
* The fully up-to-date approach to valve electronics
* Essential reading for audio designers and music and electronics enthusiasts alike

---

Valve Amplifiers has been recognized as the most comprehensive guide to valve amplifier design, analysis, modification and maintenance. It provides a detailed presentation of the rudiments of electronics and valve design for engineers and non-experts. The source also covers design principles and construction techniques to help end users build their own tool from scratch designs that work. The author's approach walks the reader through each step of designing and constructing, starting with an overview of the essential working principles of valve amplifiers, the simple and complex stages, the process of linking the stages, and completing the design. The book is comprised of seven chapters all of which include a DIY guide discussion of practical aspects. The text starts with familiarization of the fundamentals of electronics, which are essential for designing and building valve amplifiers. Particular attention has been paid to providing solutions for questions that are commonly asked and faced by beginners in valve designing and construction. Valve Amplifiers is a masterful hands-on guide for both experts and novices who work with tube audio equipment, and for electronic hobbyists, audio engineers, and audiophiles. The practical guide to analysis, modification, design, construction and maintenance of valve amplifiers The fully up-to-date approach to valve electronics Essential reading for audio designers and music and electronics enthusiasts alike

Inhalt

Front Cover 1
Valve Amplifiers 4
Copyright Page 5
Contents 6
Preface 10
Dedication 12
Acknowledgements 14
1. Circuit Analysis 16
Mathematical Symbols 16
Electrons and Definitions 17
Batteries and Lamps 19
Ohm’s Law 20
Power 21
Kirchhoff’s Laws 22
Resistors in Series and Parallel 24
Potential Dividers 29
Equivalent Circuits 29
The Thévenin Equivalent Circuit 30
The Norton Equivalent Circuit 33
Units and Multipliers 34
The Decibel 35
Alternating Current (AC) 36
The Sine Wave 36
The Transformer 39
Capacitors, Inductors and Reactance 40
Filters 42
Time Constants 45
Resonance 46
RMS and Power 48
The Square Wave 49
Square Waves and Transients 50
Random Noise 55
Active Devices 56
Conventional Current Flow and Electron Flow 56
Silicon Diodes 57
Voltage References 58
Bipolar Junction Transistors (BJTs) 60
The Common Emitter Amplifier 62
Considering DC Conditions 64
Input and Output Resistances 64
The Emitter Follower 66
The Darlington Pair 67
General Observations on BJTs 67
Feedback 68
The Feedback Equation 68
Practical Limitations of the Feedback Equation 69
Feedback Terminology and Input and Output Impedances 70
The Operational Amplifier 71
The Inverter and Virtual Earth Adder 72
The Non-Inverting Amplifier and Voltage Follower 73
The Integrator 75
The Charge Amplifier 75
DC Offsets 77
References 78
Recommended Further Reading 78
2. Basic Building Blocks 80
The Common Cathode Triode Amplifier 80
Limitations on Choice of the Operating Point 83
Conditions at the Operating Point 85
Dynamic, or AC, Parameters 88
Cathode Bias 91
The Effect on AC Conditions of an Unbypassed Cathode Bias Resistor 93
The Cathode Decoupling Capacitor 94
Choice of Value of Grid-Leak Resistor 96
Choice of Value of Output Coupling Capacitor 98
Miller Capacitance 98
Reducing Output Resistance of the Previous Stage 100
Guided-Grid, or Beam, Triodes 100
The Tetrode 101
The Beam Tetrode and the Pentode 102
The Significance of the Pentode Curves 104
Using the EF86 Small-Signal Pentode 106
The Cascode 109
The Charge Amplifier 117
The Cathode Follower 118
Sources and Sinks: Definitions 122
The Common Cathode Amplifier as a Constant Current Sink (CCS) 124
Pentode Constant Current Sinks 126
The Cathode Follower with Active Load 128
The White Cathode Follower 129
Analysis of the Self-Contained White Cathode Follower 129
The White Cathode Follower as an Output Stage 132
The & #956
The Importance of the AC Loadline 137
Upper Valve Choice in the & #956
Limitations of the & #956
The Shunt-Regulated Push–Pull Amplifier (SRPP) 140
The & #946
The Cathode-Coupled Amplifier 145
The Differential Pair 148
Gain of the Differential Pair 150
Output Resistance of the Differential Pair 150
AC Balance of the Differential Pair and Signal at the Cathode Junction 151
Common-Mode Rejection Ratio (CMRR) 151
Power Supply Rejection Ratio (PSRR) 153
Semiconductor Constant Current Sinks 154
Using Transistors as Active Loads for Valves 157
Optimising rout by Choice of Transistor Type 160
Field-Effect Transistors (FETs) as Constant Current Sinks 162
Designing Constant Current Sinks Using the DN2540N5 164
References 168
Recommended Further Reading 169
3. Dynamic Range: Distortion and Noise 170
Distortion 170
Defining Distortion 170
Measuring Non-Linear Distortion 171
Distortion Measurement and Interpretation 172
Choosing the Measurement 173
Refining Harmonic Distortion Measurement 174
Weighting of Harmonics 174
Summation and Rectifiers 175
Alternative Rectifiers 177
Noise and THD+N 177
Spectrum Analysers 178
Digital Concepts 178
Sampling 179
Scaling 179
Quantisation 180
Number Systems 180
Precision 180
The Fast Fourier Transform (FFT) 181
The Periodicity Assumption 182
Windowing 182
How the Author’s Distortion Measurements Were Made 183
Designing for Low Distortion 184
Signal Amplitude 184
Cascodes and Distortion 187
Grid Current 188
Distortion due to Grid Current at Contact Potential 188
Distortion due to Grid Current and Volume Controls 189
Operating with Grid Current (Class A2) 190
Distortion Reduction by Parameter Restriction 192
Distortion Reduction by Cancellation 195
Differential Pair Distortion Cancellation 197
Push–Pull Distortion Cancellation 199
The Western Electric Harmonic Equaliser 199
Side-Effects of the Harmonic Equaliser 201
DC Bias Problems 203
Cathode Resistor Bias 203
Grid Bias (Rk=0) 205
Rechargeable Battery Cathode Bias (rk=0) 206
Diode Cathode Bias (rk˜0) 206
Constant Current Sink Bias 210
Individual Valve Choice 211
Which Valves Were Explicitly Designed to be Low Distortion? 211
Carbonising of Envelopes 213
Deflecting Electrons 213
Testing to Find Low-Distortion Valves 214
The Test Circuit 214
Audio Test Level and Frequency 215
Test Results 215
Interpretation 218
A Convention 220
Alternative Medium-µ Valves 220
Weighted-Distortion Results 221
Overall Conclusions 221
Coupling from One Stage to the Next 222
Blocking 223
Transformer Coupling 225
Low Frequency Step Networks 225
Level Shifting and DC Coupling 226
A DC Coupled Class A Electromagnetic Headphone Amplifier 228
Using a Norton Level Shifter 231
Distortion and Negative Feedback 234
Carbon Resistors and Distortion 237
Noise 237
Noise from Resistances 238
Noise from Resistive Volume Controls 238
Noise from Amplifying Devices 239
Grid Current Noise and the Poisson Distribution 241
Electrometers and Grid Current 241
Noise in DC References 245
How the Author’s DC Reference Noise Measurements Were Made 245
Gas Reference Noise Measurements 247
Variation of Gas Reference Noise with Operating Current 247
Semiconductor Reference Noise Measurements and Statistical Summation 247
Variation of Zener Reference Noise with Operating Current 249
Noise of the Composite Zener Compared to a 317 250
Red LED Noise 251
References 251
Recommended Further Reading 252
4. Component Technology 254
Resistors 254
Preferred Values 254
Heat 255
Metal Film Resistors 256
Power (Wirewound) Resistors 259
Ageing Wirewound Resistors 259
Noise and Inductance of Wirewound Resistors 260
Non-Inductive Thick Film Power Resistors 263
General Considerations on Choosing Resistors 263
Tolerance 263
Heat 263
Voltage Rating 264
Power Rating 264
Capacitors 264
The Parallel Plate Capacitor 264
Reducing the Gap Between the Plates and Adding Plates 265
The Dielectric 265
Different Types of Capacitors 266
Air Dielectric, Metal Plate (& #949
Plastic Film, Foil Plate Capacitors (2< &
Metallised Plastic Film Capacitors 271
Metallised Paper Capacitors (1.8< &
Silvered Mica Capacitors (Muscovite Mica, & #949
Ceramic Capacitors 272
Electrolytic Capacitors 273
Aluminium Electrolytic Capacitors (& #949
Tantalum Electrolytic Capacitors (& #949
Variation of Capacitance with Frequency 282
Imaginary Capacitance 282
General Considerations in Choosing Capacitors 284
Voltage Rating 284
Capacitance Value 284
Heat 285
ESR 285
Leakage and ‘d’ 285
Microphony 285
Bypassing 286
Magnetic Components 287
Inductors 288
Air-Cored Inductors 288
Gapped Cores for AC Only 290
Gapped Cores for AC and DC (Power Supply Chokes) 291
Self-Capacitance 292
Transformers 294
Iron Losses 294
DC Magnetisation 298
Copper Losses 299
Electrostatic Screens 299
Magnetostriction 300
Output Transformers, Feedback and Loudspeakers 300
Transformer Models 301
Input Transformer Loading 304
Why Should I Use a Transformer? 306
General Considerations in Choosing Transformers 307
Uses and Abuses of Audio Transformers 308
Guitar Amplifiers and Arcs 308
Other Modes of Destruction 309
Magnetic Screening Cans 309
Magnetic Core Deterioration 309
Thermionic Valves 310
History 310
Emission 311
Electron Velocity 312
Transit Time 313
Individual Elements of the Valve Structure 314
The Cathode 314
Thoriated Tungsten Filament Fragility 317
Direct Versus Indirectly Heated Cathodes 318
The Thermal Problem 318
The Electrostatic Problem 319
The Electromagnetic Problem 319
The Indirectly Heated Cathode Solution 319
Heater/Cathode Insulation 320
Cathode Temperature Considerations 322
Heaters and their Supplies 322
Current Hogging and Heater Power 324
Heater Voltage and Current 326
The Control Grid 329
Grid Current 330
Thermal Runaway due to Grid Current 330
Grid Emission 330
Frame-Grid Valves 331
Variable-µ Grids and Distortion 332
Other Grids 333
The Anode 334
The Vacuum and Ionisation Noise 337
The Getter 338
The Mica Wafers and Envelope Temperature 339
Valve Sockets – Losses and Noise 341
Valve Bases and the Loktal™ Base 341
The Glass Envelope and the Pins 343
PCB Materials 344
References 345
Recommended Further Reading 346
5. Power Supplies 348
The Major Blocks 348
Rectification and Smoothing 349
Choice of Rectifiers/Diodes 349
Rectifiers To Be Avoided (Gas) 355
Rectifiers To Be Avoided (Selenium) 357
Rectifiers To Be Avoided (Copper Oxide) 357
RF Interference/Spikes 358
The Single Reservoir Capacitor Approach 358
Ripple Voltage 359
The Effect of Ripple Voltage on Output Voltage 360
Ripple Current and Conduction Angle 361
Transformer Core Saturation 365
Choosing the Reservoir Capacitor and Transformer 365
Back-to-Back Mains Transformers for HT Supplies 368
Voltage Multipliers 370
The Choke Input Power Supply 372
Minimum Load Current for a Choke Input Supply 373
Current Rating of the Choke 374
Mains Transformer Current Rating for a Choke Input Supply 376
Current Spikes and Snubbers 376
Intermediate Mode: The Region Between Choke Input and Capacitor Input 380
PSUD2 382
Broadband Response of Practical LC Filters 384
Region 1 384
Region 2 386
Region 3 386
Region 4 386
Estimation of Wide-Band LC Response 390
Sectioned RC Filters 391
Regulators 393
The Fundamental Series Regulator 394
The Two-Transistor Series Regulator 396
The Speed-Up Capacitor 397
Compensating for Regulator Output Inductance 399
A Variable Bias Voltage Regulator 399
The 317 IC Voltage Regulator 401
The 317 as an HT Regulator 403
Valve Voltage Regulators 405
Optimised Valve Voltage Regulators 408
Using a Pentode’s g2 as an Input for Hum Cancellation 409
Increasing Output Current Cheaply 409
Regulator Sound 412
Power Supply Output Resistance and Stereo Crosstalk 412
Power Supply Output Resistance and Amplifier Stability 413
The Statistical Regulator 414
Bypassing the Composite Zener 417
Optimising the Statistical Regulator 419
References for Elevated Heater Supplies – the THINGY 420
Common-Mode Interference 423
Heaters and History 423
How Common-Mode Heater Interference Enters the Audio Signal 424
Mains Transformers and Inter-Winding Capacitance 424
Reducing Transformer Inter-Winding Capacitance 425
Post-Transformer Filtering 426
Practical Issues 427
Transformer Regulation 427
HT Capacitors and Voltage Ratings 428
Can Potentials and Undischarged HT Capacitors 429
The Switch-On Surge 430
Mains Fusing 430
Mains Switching 431
A Practical Design 432
HT Regulation 433
HT Rectification and Smoothing (a PSUD2 Exercise) 435
Heater Rectification and Smoothing (a Manual Exercise) 438
Heater Regulation 439
Mains Filtering 440
Adapting the Power Supply to the EC8010 RIAA Stage 441
HT Regulation 443
Reference Voltages 444
HT Rectification and Smoothing (a PSUD2 Exercise) 444
Heater Regulation 446
Heater Rectification and Smoothing (a Manual Exercise) 447
References 448
Recommended Further Reading 449
6. The Power Amplifier 450
The Output Stage 450
The Single-Ended Class A Output Stage 451
The Significance of High Output Resistance 453
Transformer Imperfections 454
Classes of Amplifiers 456
Class A 456
Class B 456
Class C 456
Class *1 458
Class *2 458
The Push–Pull Output Stage and the Output Transformer 458
Modifying the Connection of the Output Transformer 461
Output Transformer-Less (OTL) Amplifiers 465
The Entire Amplifier 465
The Driver Stage 467
The Phase Splitter 469
The Differential Pair and Its Derivatives 470
The Input Stage 479
Stability 480
Slugging the Dominant Pole 480
Low Frequency Instability, or Motorboating 482
Parasitic Oscillation and Control Grid-Stoppers 483
Parasitic Oscillation of Ultra-Linear Output Stages, and g2 Stoppers 484
Parasitic Oscillation and Anode Stoppers 484
High Frequency Stability and the 0V Chassis Bond 484
Stability Margin 484
Classic Power Amplifiers 485
The Williamson 485
The Mullard 5-20 487
The Quad II 492
New Designs 495
Single-Ended Madness 495
The Scrapbox Challenge Single-Ended Amplifier 495
Choice of Output Valve 496
Choice of Output Class 497
Choosing the DC Operating Point by Considering Output Power and Distortion 497
Specifying the Output Transformer 498
Biassing the Valve 498
The Cathode Bypass Capacitor 499
Finding the Required HT Voltage 500
HT Smoothing 500
HT Rectification 500
The HT Transformer 501
HT Choke Suitability 502
The HT Regulator Option 503
Estimating Amplifier Output Resistance 505
What are the Driver Stage Requirements? 506
Driver Stage Topology 506
Choice of Valve for the Driver Stage 507
Determining the Driver Stage Operating Point 507
Setting Driver Stage Bias 508
Is the Output Resistance and Gain of the Proposed Driver Stage Adequate? 508
But What About Global Feedback? 509
Summing Up 509
Teething Problems 509
Listening Tests 512
Designer’s Observations 512
Conclusions 513
Obtaining more than Single Digit Output Power 515
Sex, Lies and Output Power 515
Loudspeaker Efficiency and Power Compression 516
Active Crossovers and Zobel Networks 516
Parallel Output Valves and Transformer Design 518
Driving Higher Power Output Stages 519
The Crystal Palace Amplifier 520
13E1 Conditions 522
Driver Requirements 525
Finding a Topology that Satisfies the Driver Requirements 525
(1) Minimal Measured Distortion 525
(2) Distortion to be Composed of Low Order Harmonics 525
(3) Push–pull Output with Good Balance 525
(4) Large Undistorted Voltage Swing 526
(5) Sufficient Gain to Enable Global Negative Feedback if Required 526
(6) Low DC Output Resistance to Avoid Problems with DC Grid Current 526
(7) Low AC Output Resistance to Drive Load Capacitance 526
(8) Tolerance of Output Stage Conduction Angle Changes from 360° to 0° 526
(9) Instantaneous Recovery Even After Gross Overload 527
Circuit Topology: Power Supplies and Their Effect on Constant Current Sinks 527
Va(max) and the Positive HT Supply 528
Symmetry and the Negative HT Supply 529
The Second Differential Pair and Output Stage Current 529
Why Not Have Tighter Stabilisation? 530
The First Differential Pair, Its HT Supply, and Linearity 532
Valve Matching 532
The Essential Twiddly Bits 533
The Cascode Constant Current Sink and Stabilisation Against Mains Variation 533
The 334Z Constant Current Sink and Thermal Stability 536
High Frequency Stability 537
HT Regulators 537
Stereo versus Mass 539
Power Supply Design 539
Designer’s Observations 540
Exceeding Vg2 540
GM70 542
Measuring Ik 542
Global Negative Feedback 542
Conclusions 546
The Bulwer-Lytton Scalable Parallel Push–Pull Amplifier 546
Background 546
Designing the Followers to Drive the Output Valves 548
Comparing Cathode and FET Source Followers 548
Output Stage Bias, Balance and Coupling 551
Providing Gain 554
Gain Stage CCS and Gain Balance 554
Balanced Inputs on Power Amplifiers 555
The Volume Control and Baffle Step Compensation 556
Audio Circuit Comments 557
Power Supplies 558
Global Negative Feedback 560
References 560
Further reading 561
7. The Pre-Amplifier 562
Input Selection 563
Disparate Levels between Sources 563
Adjacent Contact Capacitance (Crosstalk Between Sources) 564
Contact and Leakage Resistance (Noise) 565
Solutions and Problems Peculiar to Electromechanical Switches (Relays) 565
Volume Control 566
Limitations on the Control’s Value (Disturbing Frequency Response) 567
Logarithmic Law (Perceived Volume Not Changing Smoothly with Rotation) 568
Switched Attenuators (Disturbing Channel Matching) 569
Switched Attenuator Design 570
Spreadsheets and Volume Controls 573
Volume Controls for Digital Active Crossovers 574
Volume Control Values and Their Effect on Noise 577
Grid-Leak Resistors and Volume Controls 578
Balanced Volume Controls 580
Light-Sensitive Resistors as Volume Controls 580
Transformer Volume Controls 582
Balance Control 583
Law Faking 583
Cable Driver 587
Determination of Required Quiescent Current 587
Choice of Follower Valve 589
Practical Considerations 590
Adding Gain 592
Polarity Inversion 593
Tone Control 594
Obtaining a Clean Signal from Analogue Disc 600
Comparison of Analogue Levels between Vinyl and Digital Sources 600
RIAA and Replay Rumble 601
The Mechanical Problem 602
Arm Wiring and Moving Coil Cartridge DC Resistance 603
Hum Loops and Unbalanced Interfaces 604
Balanced Working and Pick-Up Arm Wiring 604
RIAA Stage Design 606
Determination of Requirements 607
Implementing RIAA Equalisation 609
‘All in One Go’ Equalisation 611
Split RIAA Equalisation 612
The Final Choice 614
A Simplified Example RIAA Stage 614
Noise and Input Capacitance of the Input Stage 614
Valve Noise 620
1/f Noise 621
Connecting Devices in Parallel to Reduce noise 621
Valve Noise Summary 622
Noise Advantage due to RIAA Equalisation 622
Stray Capacitances 623
Calculation of Component Values for 75& #956
180& #956
3180& #956
Awkward Values and Tolerances 627
The EC8010 RIAA Stage 629
The Input Stage 629
Optimising the Input Transformer 632
The Second Stage 633
The Output Stage 634
Refining Valve Choice by Heaters 634
Choosing the Implementation of RIAA Equalisation 635
Grid Current Distortion and RIAA Equaliser Series Resistances 635
3180& #956
The 75& #956
The Computer Aided Design (CAD) Solution 637
3180& #956
75& #956
Practical RIAA Considerations 639
RIAA Direct Measurement Problems 639
Production Tolerances and Component Selection 642
RIAA Equalisation Errors due to Valve Tolerances 643
The Balanced Hybrid RIAA Stage 643
No Step-Up Transformers 644
Semiconductors to the Rescue 644
Miller Capacitance 645
DC Stabilisation and Consequent Gain Reduction 646
JFET Noise 646
BJT Noise 647
Choosing between the BJT and JFET: Equalisation, Distortion and HT Power 648
Reconciling the Balanced Decision with Practicalities 649
Implications of the Block Diagram 649
The Unity-Gain Cable Drivers 650
Deciding the HT Voltage 651
Input Stage BJT Miller Capacitance 652
VCE and BJT Linearity 653
Input Resistance and Bias Current 654
Input Stage Noise 655
RIAA Calculations 656
The Source Followers 657
The Constant Current Sinks 658
The HT Supply 658
Total Gain and Channel Balance 660
Summary 660
References 661
Recommended Further Reading 661
Appendix 662
Valve Data 662
Standard Component Values 666
Resistor Colour Code 666
Plastic Capacitor Coding 668
Cable 668
Square Wave Sag and Low Frequency f–3 dB 669
Playing 78s 671
Equalisation 672
CD 674
Sourcing Components: Bargains and Dealing Directly 675
References 677
Index 678

Textauszug

Chapter 1

Circuit Analysis

Publisher Summary

This chapter illustrates Circuit Analysis. Electrons are charged particles. Charged objects are attracted to other charged particles or objects. Charged objects come in two forms—negative and positive. Unlike charges attract, and like charges repel. Electrons are negative and positrons are positive, but while electrons are stable in the universe, positrons encounter an electron almost immediately after production, resulting in mutual annihilation and a pair of 511 keV gamma rays. An electron is very small, and does not have much of a charge, so one needs a more practical unit for defining charge. That practical unit is the coulomb (C). One could say that 1 C of charge had flowed between one point and another, which would be equivalent to saying that approximately 6,240,000,000,000,000,000 electrons had passed, but much handier. Simply being able to say that a large number of electrons had flowed past a given point is not in it very helpful. One might say that a billion cars have traveled down a particular section of motorway since it was built, but if he/she were planning a journey down that motorway, he or she would want to know the flow of cars per hour through that section.

In order to look at the interesting business of designing and building valve amplifiers, we need some knowledge of electronics funmentals. Unfortunately, fundamentals are not terribly interesting, and to cover them fully would consume the entire book. Ruthless pruning is, therefore, necessary to condense what is needed in one chapter.

It is thus with deep sorrow that the author has had to forsaken complex numbers and vectors, whilst the omission of differential calculus is a particularly poignant loss. All that is left is ordinary algebra, and although there are lots of equations, they are timid, miserable creatures and quite defenceless.

If you are comfortable with basic electronic terms and techniques, then please feel free to go directly to Chapter 2, where valves appear.

Mathematical Symbols

Unavoidably, a number of mathematical symbols are used, some of which you may have forgotten, or perhaps not previously met:

a≡b a is totally equivalent to b

a=b a equals b

a≈b a is approximately equal to b

a∝b a is proportional to b

a≠b a is not equal to b

a>b a is greater than b

a<b a is less than b

a≥b a is greater than, or equal to, b

a≤b a is less than, or equal to, b

As with the = and ≠ symbols, the four preceding symbols can have a slash through them to negate their meaning (a ∋ b, a is not less than b).

√a the number which when multiplied by itself is equal to a (square root)

an a multiplied by itself n times. a4=a×a×a×a (a to the power n)

± plus or minus

∞ infinity

° degree, either of temperature (°C), or of an angle (360° in a circle)

parallel, either parallel lines, or an electrical parallel connection

Δ a small change in the associated value, such as ΔVgk.

Electrons and Definitions

Electrons are charged particles. Charged objects are attracted to other charged particles or objects. A practical demonstration of this is to take a balloon, rub it briskly against a jumper and then place the rubbed face against a wall. Let it go. The balloon remains stuck to the wall. This is because we have charged the balloon, and so there is an attractive force between it and the wall. (Although the wall was initially uncharged, placing the balloon on the wall induced a charge.)

Charged objects come in two forms: negative and positive. Unlike charges attract, and like charges repel. Electrons are negative and positrons are positive, but whilst electrons are stable in our universe, positrons encounter an electron almost immediately after production, resulting in mutual annihilation and a pair of 511 keV gamma rays.

If we don’t have ready access to positrons, how can we have a positively charged object? Suppose we had an object that was negatively charged, because it had 2,000 electrons clustered on its surface. If we had another, similar, object that only had 1,000 electrons on its surface, then we would say that the first object was more negatively charged than the second, but as we can’t count how many electrons we have, we might just as easily have said that the second object was more positively charged than the first. It’s just a matter of which way you look at it.

To charge our balloon, we had to do some work and use energy. We had to overcome friction when rubbing the balloon against the woollen jumper. In the process, electrons were moved from one surface to the other. Therefore, one object (the balloon) has acquired an excess of electrons and is negatively charged, whilst the other object (woollen jumper) has lost the same number of electrons and is positively charged.

The balloon would, therefore, stick to the jumper. Or would it? Certainly it will be attracted to the jumper, but what happens when we place the two in contact? The balloon does not stick. This is because the fibres of the jumper were able to touch the whole of the charged area on the balloon, and the electrons were so attracted to the jumper that they moved back onto the jumper, thus neutralising the charge.

At this point, we can discard vague talk of balloons and jumpers because we have just observed electron flow.

An electron is very small, and doesn’t have much of a charge, so we need a more practical unit for defining charge. That practical unit is the coulomb (C). We could now say that 1 C of charge had flowed between one point and another, which would be equivalent to saying that approximately 6,240,000,000,000,000,000 electrons had passed, but much handier.

Simply being able to say that a large number of electrons had flowed past a given point is not in itself very helpful. We might say that a billion cars have travelled down a particular section of motorway since it was built, but if you were planning a journey down that motorway, you would want to know the flow of cars per hour through that section.

Similarly in electronics, we are not concerned with the total flow of electrons since the dawn of time, but we do want to know about electron flow at any given instant. Thus, we could define the flow as the number of coulombs of charge that flowed past a point in one second. This is still rather long-winded, and we will abbreviate yet further.

We will call the flow of electrons current, and as the coulomb/second is unwieldy, it will be redefined as a new unit, the ampere (A). Because the ampere is such a useful unit, the definition linking current and charge is usually stated in the following form.

One coulomb is the charge moved by one ampere flowing for one second.

(coulombs)=current(amperes)×time(seconds)

This is still rather unwieldy, so symbols are assigned to the various units: charge has symbol Q, current I and time t.

=It

This is a very useful equation, and we will meet it again when we look at capacitors (which store charge).

Meanwhile, current has been flowing, but why did it flow? If we are going to move electrons from one place to another, we need a force to cause this movement. This force is known as the electro motive force (EMF). Current continues to flow whilst this force is applied, and it flows from a higher potential to a lower potential.

If two points are at the same potential, no current can flow between them. What is important is the potential difference (pd).

A potential difference causes a current to flow between two points. As this is a new property, we need a unit, a symbol and a definition to describe it. We mentioned work being done in charging the balloon, and in its very precise and physical sense, this is how we can define potential difference, but first, we must define work.

One joule of work is done if a force of one newton moves one metre from its point of application.

This very physical interpretation of work can be understood easily once we realise that it means that one joule of work would be done by moving one kilogramme a distance of one metre in one second. Since charge is directly related to the mass of electrons moved, the physical definition of work can be modified to define the force that causes the movement of charge.

Unsurprisingly, because it causes...

Produkt-Details

• Erscheint lt. Verlag: 14.10.2011
• Sprache: englisch
• Themenwelt: Kunst / Musik / Theater ► Musik
• Themenwelt: Sachbuch/Ratgeber
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-08-096641-1 / 0080966411
• ISBN-13: 978-0-08-096641-0 / 9780080966410