[Adapted from the book review by Jean-Pierre Vanderreydt in Linear Audio Vol 1, April 2010, by kind permission of Linear Audio Publishing]
This book is exclusively about audio power amplifiers design and mainly about class AB power amplifiers except for detailed introduction to class D amplifiers. Bob Cordell, the author, is well known within the audio community. He is a member of the AES, and presented many important papers on power amplifier design and distortion analysis.
The objective of this book is, according to its author: « to address many advanced topics and important design subtleties and to allow at the same time designers relatively new to the field to absorb the material without being overwhelmed ».
Did he achieve his goal? Certainly yes, by more than 90%. It is definitely a design book and not a cookbook or a description more or less exhaustive of existing designs. When digested, this book will give to the reader all the background necessary to design state of the art and flawlessly performing amplifiers. The advanced reader will find topics not addressed in equivalent books and which would otherwise require reading of specialized literature.
The books' division in six parts is judicious and well chosen. It helps to keep an overall view of the field without being lost in details while gradually building up knowledge.
In part one the author covers the basic design principles of a power amplifiers. The voltage amplifying topology analyzed throughout the book is the two stages OTA (Operational Trans-conductance Amplifier). This topology is the most used in power amplifiers, because it is well known and well behaved. This part starts with a description of the basic building blocks used to design the stages of an OTA. Their operation is well described in a qualitative way without using any node equation.
With this background, the author starts a very good design exercise, beginning with the explanation of a basic amplifier and ending in a high performance one. Each refinement step is justified by the improvement in THD1, THD 20 for various loads including zero load. This is a very educative and nice approach which shows the designer the reason why each implemented improvement is beneficial.
The Early effect is correctly treated but some confusion exists among designers and is not completely clarified in the book.
In the next chapter in this first part, the author explains negative feedback (voltage), stability and compensation avoiding heavy math in a very effective but rather dense way.
The next chapter investigates what is certainly the most critical stage in a power amplifier: the output stage. Here the author shines. His treatment of all the design aspects of an output stage is perfect. Understanding and minimizing crossover distortion and output stage stability are keys to the performance of a class AB power amplifier. This treatment will be followed in a later chapter by an even more thorough analysis.
The author revisits, in a second part, the design and topologies of the different stages under the title: Advanced Power Amplifier Design Techniques.
JFETs are presented to be used in the front end without showing a definitive advantage. Then Bob presents in good details various complementary input stages coupled with push pull VAS showing nicely how current sources may not fight each other and how to stabilize the bias current if they are used in a complementary design.
He finalizes his list by presenting a full symmetrical design with a fully differential input amplifier loaded by a self biasing load which operates as common mode feedback. There is no presentation of single stage folded cascode topology. This one is very popular in IC design because it is fast and easy to design with. There is also no discussion of current mode amplifiers known for their speed and very high slew rate. It would have been interesting here to understand if the various increases in complexity presented are justified by an increase in measured performances.
In the next chapter of advanced techniques, the author is doing a very fine and exhaustive job in explaining how to design DC servo's: very thorough!
Next, compensation is revisited. Dominant pole, pole splitting in Miller compensation is well explained as well as the tradeoff between slew rate and GBW. It should be pointed out here how the increase in VAS collector current shifts the non dominant pole to the right and is a design degree of freedom, together with the increase in Miller capacitance, to improve phase margin.
The insertion of a zero in the forward path by the minor loop feedback modification to cancel a parasitic pole is explained. This is a technique used to cancel a pole introduced by capacitive loading at the output of the amplifier, an important feature in audio amplifiers. But there is a risk worthwhile mentioning. The global forward path zero introduced is a pole in the minor loop and could degrade minor loop stability. A zero introduced in the minor loop canceling the introduced pole restores the minor loop stability. This is done by bridging the RC feedback element with a capacitor. This is not a generic compensation technique because it is depending on the specific load and could generate instabilities when the load is removed!
The feedforward right half plane zero (that acts as a negative pole from a phase point of view) of a Miller stage is addressed but this is not critical in a BJT design because the zero is at a very high frequency. The technique used to eliminate it is still useful for another reason. A resistor as mentioned is efficient but other techniques are making the path through the capacitor unilateral so eliminating the forward zero. A way to do so is by connecting the Miller capacitor to a cascoded node in the front end. The advantage of such a solution is that it improves a lot the PSSR of the negative rail which is 0dB in a classical Miller stage. It impedes the forward traveling of negative supply noise through the Miller capacitor.
Next, three advanced compensation techniques improving the open loop bandwidth and therefore the distortion reduction at higher frequencies are well explained
In the next chapter of Advanced Techniques, the output stage design and its crossover distortion is revisited. As mentioned before, this is a very important chapter and is covered in a perfect way. The graph of crossover distortion versus power level is particularly enlightening. The analysis of optimal bias, crossover distortion, driver design and output push pull design as well as stability and sizing is very complete and well written.
Still in the same section, the author is turning to his forte: the design of MOSFET power amplifiers. After having read this chapter, the reader will know in depth everything required to make a state of the art MOSFET amplifier: theory of power MOSFET operation, vertical and lateral MOSFETs, parasitic oscillations and cure, biasing and driving for low crossover distortion, comparison with BJTs, protection, paralleling and matching; another great chapter.
Next a full chapter is devoted to error correction techniques which the author was one of the first to promote and implement. It is very nice to find in a single place such a treatment of this technique including analysis and detailed design with all the refinements. The reviewer particularly liked the explanation of HEC (Hawksford-type Error Correction) with a negative feedback loop enclosing a positive feedback loop that creates the very high forward gain..
This advanced section ends with a chapter on other sources of distortion. This is a potpourri of all the distortion sources not yet mentioned. It is again nice to have a single place were all these mechanisms are listed and detailed. On aspect missing is the distortion input voltage generated by non linear input current on the input impedance and techniques to avoid it like bootstrapping.
Part three is concerned with the implementation and the making of a real product.
A first chapter addresses in depth output stage thermal design. Then, protection mechanisms, power supply design, ground layout and EMI control are thoroughly described. I particularly liked the chapter on clipping control which is an important aspect for sound quality and is very often neglected.
Part four on simulation and measurement is again a good surprise. Not every designer or would-be designer has been trained in Spice simulation. The availability of top class free software like LTspice imposes the use of this tool. The two chapters following are doing a great job in providing the newcomer with a very easy start in using Spice, and the seasoned user with a really great and detailed approach on making accurate models. The chapter on making one's own models is by itself worth the value of the book. A missing aspect is the explanation of the different existing techniques to simulate correctly the loop gain without changing the loading of the circuit (Middlebrook, Tian probe) and to understand their advantages and shortcomings. I also would have liked to see the problem of analyzing multi-loop stability issues being addressed.
In the rest of this section, the author explains very nicely all the different aspects of audio measurements and provides the reader with very handy DIY tools. The focus is of course on distortion measurements in all its aspects.
The next section covers specific topics. The first one is about the feedback controversy and its influence on the sounding of amplifiers. This endless debate is not solved here but the author closes very well many doors and correctly eliminates arguments that are all too often used by pseudo specialists. The chapter on how to make amplifiers without feedback and the difficulties associated with it is great reading.
Next balanced inputs, bridge amplifiers and fully differential amplifiers are explained.
The last chapter of this section is devoted to integrated circuit power amplifiers. The availability of high quality specialized IC's will allow the designer to make amplifiers that rival full discrete ones and this chapter explains very well how to achieve this.
A very intriguing example shows a MOS amplifier biased with the LT1116 and driven by the LM49830. This is a design worthwhile to be tested because, according to the author, the transient thermal distortion of the output stage should be under control.
This is one of the simplest amplifier to realize (but not simple to design) and perhaps one of the best performing ones.
The last section is an introduction to class D amplifier operation. This is a field in itself and information is scattered among textbooks, papers and application notes. Like in the Spice chapters, the author is doing a great job here in summarizing all the aspects of class D operation. Using the same methodology, the explanations remain qualitative without math. This is necessary if you want to get introduced and get a good overview of this complex new field.
The author starts with the basic concepts of a classical PWM modulator. By describing the operation of a buck DC-DC converter, he starts with a good and rather deep overview of the difficulties that the switching of high power will bring. Single ended and bridge configurations are then introduced.
Digging deeper, the issues of input anti-aliasing filter and output reconstruction filters are addressed. Next, the requirement for negative feedback is explained and the difficulties to include the output of a reconstruction filter in the loop are fully reviewed leading to the concept of self oscillating loops.
The author then shows the simplicity and performance of some self oscillating modulators that include the output filters in the overall feedback with all its advantages.
Finally, the author describes the operation of Sigma Delta amplifiers with their associated quantization noise. Noise shaping is well explained but it is a difficult concept to understand without a feedback block diagram showing how the noise transfer function is different from the signal transfer function. This noise is attenuated at low frequencies because it is divided by the loop gain and thus attenuated by the inverse of the low pass filter transfer function.
The last chapter is about measuring class D amplifiers which is obviously different from linear amplifiers.
This is a great book to read and read again. The progressive approach and the advanced details are making this book useful both for the seasoned designer and the electronic student wanting to understand practical analog design. The explanations avoiding equations are great and are giving a good physical background to the operation of the discussed circuits. This will appeal to people wanting a practical approach to electronics up to an advanced level.
If you want to be comfortable in understanding state of the art audio power amplifier designs, if you want to make your own state of the art amplifier, build it and test it, then you should buy this book and read it thoroughly.
[Adapted from the book review by Jean-Pierre Vanderreydt in Linear Audio Vol 1, April 2010, by kind permission of Linear Audio Publishing]
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Top reviews from other countries
Wurgle
All the expected basics in good detail plus it covers balanced designs in some detail ...
Reviewed in the United Kingdom on February 10, 2018
