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This book deals with the design and implementation of low-voltage bipolar Operational Amplifiers (OpAmps). It is the result of the research that was started about ten years ago by Prof. Johan H. Huijsing. In 1987, he was joined by M. Jeroen Fonderie, who then began his four-year Ph.D. study. This search for better ways to implement low-voltage operational amplifiers has been carried out at the Delft University of Technology, the Netherlands, in cooperation with Philips Semiconductors, Signetics, Sunnyvale, CA The material presented in this book will be of particular interest to professional designers of integrated amplifiers and to graduate students in this field. It is assumed that the reader has a basic knowledge of analog electronics and integrated circuit technology. In Chapter 1, the question is answered why a low-voltage design is examined and why the bipolar technology is chosen. Further, the objectives of this work are defined and some design considerations are given. In Chapter 2, input stages are discussed. The input stage of an OpAmp should amplify the differential-mode input voltage, independent of the common-mode input voltage. This commonmode input voltage must be able to have any value that fits within the supply voltage range. Input stages that are able to realize this at supply voltages down to 1.8 V and down to 1 V are discussed. Chapter 3 deals with output stages. The output voltage of the OpAmp must be able to have any value within the supply voltage range. Besides this rail-to-rail output-voltage range, the output stage should also be able to supply the load that is connected to the OpAmp with an output current in the order of 10 rnA The current gain should therefore be as great as possible. The frequency response of the output stage should be very good, because the frequency behavior of the complete OpAmp is directly dependent on that of the output stage. In addition to the input and output stage, the OpAmp comprises several other circuit parts. The intermediate stage, between the input stage and the output stage boosts the overall gain of the OpAmp. The class-AB current control is required to efficiently control the currents in the output stage. The protection circuits prevent destruction of the OpAmp if it is driven beyond its specification limits. And finally the PTAT reference circuit is required to define all the currents that flow through the OpAmp. All these circuit parts are briefly discussed in Chapter 4. The OpAmp consists of several stages in series and each of these stages introduces at least one pole frequency in the transfer function of the
OpAmp. If feedback is applied to the OpAmp, it oscillates if these poles are too close to each other, and, therefore, a frequency-compensation technique is used to split the poles apart. Both this Miller compensation and a new technique, the multi-path-driven Miller compensation, are thoroughly analyzed in Chapter 5. In Chapter 6, several realizations are evaluated and the measurements that have been performed to verify the theory are discussed. A Figure of merit, the bandwidth-topower ratio, is defined in order to be able to compare the performance of the realized OpAmps. In the final Chapter, the major conclusions of this book are recapitulated and some references to further research are given