8.1 Introduction

Many high speed analogue circuits have been developed for bipolar technology. These circuits have generally used current as the main processing variable, and some of them are reviewed in Chapters 2, 4, 15 and 16. Work aimed at extending the maximum frequency capability of CMOS technology, described in Chapters 5 and 9, has also tended to employ current processing techniques. In designs for both bipolar and CMOS technology, a number of key current-based components have been found to be particularly valuable. These include the current mirror (Chapters 6 and 7), the high gain (or operational) transconductance amplifier (Chapter 4) and the high precision transconductor, which provides a tunable linear resistive element for integrated circuit filter realisation as described in Chapter 9.

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 [1]. 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.

Examination of the designs developed for high speed GaAs analogue sampled data circuits reveals that they too rely extensively on current-processing and utilise a range of high quality fast current-based...