Analogue IC Design has become the essential title covering the current-mode approach to integrated circuit design. The approach has sparked much interest in analogue electronics and is linked to important advances in integratedcircuit technology, such as CMOS VLSI which allows mixed analogue and digital circuits and high-speed GaAs processing.
Inspec keywords: VLSI; current conveyors; current-mode circuits; digital-analogue conversion; switched capacitor filters; analogue-digital conversion; circuit feedback; switched current circuits; analogue integrated circuits; integrated circuit design; current mirrors; continuous time filters; monolithic integrated circuits; neural chips; circuit simulation
Other keywords: dynamic current mirror; continuous-time filter; analogue integrated circuit design; current-mode circuit; current-mode A/D converter; switched-current filter; current-mode analogue amplifier; current conveyor; neural network building block; current feedback; analogue IC design; bipolar current mirror; high frequency CMOS transconductor; current-mode D/A converter; analog interface circuit; analog MOS VLSI; LCR filter simulation; current-copier circuit; switched capacitor monolithic filter; current variable; voltage amplifier; charge variable
Subjects: Analogue circuit design, modelling and testing; Digital circuit design, modelling and testing; Neural net devices; A/D and D/A convertors; Analogue circuits; Amplifiers; Active filters and other active networks; Semiconductor integrated circuits; Time varying and switched networks; Neural nets (circuit implementations); A/D and D/A convertors
State-of-the-art analogue integrated circuit design is receiving a tremendous boost from the development and application of current-mode processing, which is rapidly superceeding traditional approaches based on voltage-mode designs. There are many advantages to be gained from a wider view of analogue signal processing embracing current-mode techniques. This book draws together contributions from the world's most eminent analogue IC designers to provide, for the first time, a comprehensive text devoted to this important and exciting new area of analogue electronics.
This chapter will describe many examples of translinear circuits, which come as close to true current-mode operation as any circuit can. These circuits were based on the remarkable fact that the trans conductance of a BJT is linearly proportional to its collector current, hence the term trans-linear. Further, the word carried with it the connation of “lying somewhere between familiar home territories of the linear circuit and the formidable terrains of the nonlinear”. Indeed, in addition to their well-known application in multipliers and other nonlinear circuits, translinear concepts are found embedded in many contemporary linear integrated circuits. The most familiar example is the current mirror; the classical four-transistor Class-AB output stage of almost any op-amp can be viewed in translinear terms; the current conveyor, used in the recently-rediscovered current-feedback amplifier, is yet another example.
A current conveyor is a four (possibly five) terminal device which when arranged with other electronic elements in specific circuit configurations can perform many useful analog signal processing functions. In many ways the current conveyor simplifies circuit design in much the same manner as the conventional operational amplifier (op-amp).
The contents of this chapter have been arranged into two major parts. The first part concentrates upon techniques which extend the capabilities of the VOA to achieve current-mode performance. Several current-converter circuit designs have been reported which employ the VOA, attempting to provide it with a well-defined current output facility. One of the most successful techniques is VOA supply current sensing, where current-mirrors are used to sense the output current of the VOA via its power supply rails. The authors have developed a number of novel circuit applications based upon VOA supply current sensing and a review of this work will be presented.
The primary goal of this chapter is to highlight the important aspects of transconductor design in the framework of current mode signal processing systems. Emphasis will be placed on large signal performance, with secondary consideration given to frequency response and noise. The chapter begins with an introduction to the mathematical techniques needed to characterize transconductor performance. Using these techniques, the large signal and small signal properties of the differential pair are analyzed. It will be shown that while offering excellent high frequency performance and low noise, its large signal characteristics are nonlinear. Therefore, the dynamic range and efficiency of the differential pair are limited.
Current mirrors find endless uses not only in biasing applications of low to moderate accuracy, where their high output impedance makes them valuable as good approximations to ideal current sources. More complex mirrors provide special capabilities, such as high accuracy over many decades of current, exceptionally high output resistance, very low or high transfer ratios, and so on.
The current mirror is a ubiquitous building block in analog integrated circuits. First applied in bipolar technology, it is now extensively used in CMOS to duplicate, multiply or divide bias currents or signal currents.
Modern communication systems, both microwave and optical, are beginning to rely heavily on the high frequency circuit performance capabilities provided by III-V semiconductor materials, of which the most mature is Gallium Arsenide (GaAs). The applications of GaAs can be divided into digital and analogue.The analogue applications can be further subdivided into microwave circuits, and sampled data circuits based on switched capacitor circuit techniques. Both the high quality switches and the high gain, fast settling operational amplifiers required in these systems can be realised using the GaAs Metal Semiconductor FET, or MESFET.
A filter is a two port that shapes the spectrum of the input signal in order to obtain an output signal with the desired frequency content. Thus, a filter has passbands where the frequency components are transmitted to the output and stop bands where they are rejected.
This present chapter attempts to generalise on the realisation of monolithic filters, using a common framework based on the transconductor element, which encompasses many switched capacitor and integrated continuous-time filter realisation architectures. We focus particularly on the distinction of whether voltages or currents are used as the simulating variables, or charges in the switched capacitor case.
In this chapter, the technique is fashioned to the needs of filtering although, like switched-capacitors, it may be applied to many other signal-processing functions. The chapter starts with a brief review of the switched-capacitor technique and then introduces the concepts of sampled-current signal-processing and current memory. Circuit modules are developed which are suitable for implementing filters and this is illustrated by the design of a 6th order low-pass filter. The causes of analogue error are outlined and circuit enhancements are given for improving performance and for rendering the modules amenable to design automation techniques.
In present-day information systems, signal processing is increasingly being carried out by digital VLSI integrated circuits. Broad fields of importance are ASICs and RAMs. Interface functions are required between the “real world” and the silicon system.
A 10-bit current-scaling D/A converter has been implemented in a standard 3 μm P-well CMOS process. This converter uses an array of two-collector lateral bipolar transistors to create its weighted current sources. By using lateral bipolar transistors rather than MOS devices, the converter achieves much better linearity at low full scale current levels. The converter is also capable of operating over a wider range of full-scale currents than either MOS transistor based converters or R/2R ladder based converters. In addition, by using split-collector lateral bipolar transistors, the converter achieves high resolution and high linearity in a very small active area. This chapter discusses the application of current mode techniques to the design of a specific set of analog to digital and digital to analog converters.
The term “current copier” was first used by one of us, for whom analog signal processing was the main focus. In this case, the name arose from the system-level requirement for a circuit capable of distributing multiple copies of an input, output, or state variable in current form within a current-mode discrete-time analog filter. Groeneveld et al. use the term “calibration circuit”, indicative of the function performed by the cell in their monolithic current-mode D/A converter. The algorthmic A/D converter reported by Nairn and Salama employs a “current matching” circuit as its essential feature, ideally eliminating dependence on the matching or ratioing of any circuit components. These multiple geneses suggest that the time has come for this circuit, for reasons of both technological feasibility and usefulness of function. Further, the variety of names is a clear indication of the variety of systems to which a cell capable of copying a current can be applied. In this chapter, we review applications of “current copier” circuits, and suggest new ones. Deferring a discussion of scientific nomenclature to one more capable we will use the name “current copiers” in this chapter because (a) it is the name used in the first publication on this subject tιl and (b) we feel it is the most comprehensively descriptive of the circuit operation and use.
This chapter has presented a new integrable current conveyor topology which offers the promise of improved accuracy, wider frequency bandwidth, smoother transient response, and higher output impedance. This expectation has been confirmed by the performance of the PA630/PA630A ICs, which incorporate this topology and are fabricated on a complementary bipolar IC process. Based on these results, the characteristics of applications previously described in the literature will be enhanced, and new uses such as professional audio electronics are practicable. These new current conveyor ICs should stimulate the already growing interest in analogue current-mode signal processing, and assist in making these techniques more accessible to the general electronics community.
The term “current feedback” when applied to voltage amplifiers is a source of considerable confusion and misunderstanding. When applied to operational amplifiers, the term is commonly used to describe a method of feedback relying on an inherently low impedance inverting input. When applied to instrumentation amplifiers, the term has been utilised (by the author, unfortunately) to denote a differential feedback system relying upon the use of a linearised voltage to current converter as the feedback element. Furthermore, the basic techniques described in reference have their origins in instrumentation amplifier design. The term “active feedback” has also been carelessly applied, and has frequently been perpetuated in the description of any feedback system using other than passive elements in a feedback loop. However, the term “active feedback” seems to have been initially used to describe the use of a voltage to current converter as an overall feedback element. The author has to admit that the latter usage is a useful and (potentially) non-ambiguous description for such a type of feedback, and from here on the term “current feedback” will be applied only in the foremost context.
This chapter will present some interesting circuit approaches for the hardware implementation of neural network in analog MOS VLSI with emphasis on current-mode signal processing.
Analogue integrated circuit design has played an important role in the development of integrated circuit technology. As the level of integration increased in integrated circuit technology, digital circuit implementation became more desirable than analogue circuit implementation because of its robustness and simplicity of design. However, an all-digital implementation of complex integrated circuits is only found in certain types of applications such as memories and microprocessors and even in these integrated circuits, various types of analogue circuits are used. At the present time, a typical applications specific integrated circuit (ASIC) might contain 80% digital and 20% analogue circuits. The focus of this book represents techniques for achieving higher performance in analogue circuits in the face of VLSI technologies. Current mode analogue signal processing offers some important speed advantages over voltage signal processing.
