This text has presented some basic methods for reducing or eliminating the effects of random process variations. Such robust layout structures include unit elements, common centroid layout, finger structures, etc. Based on the small signal equivalent diagram of the MOS transistor, the design and analysis techniques of the current mirror and the differential pair have been illustrated. The layout principles to minimise the effects of process variations have been discussed.
Figure 7.1: Two-stage Miller-compensated operational amplifier.
Figure 7.1 shows an example of a common, analog circuit, a simple, frequently used operational amplifier circuit. Except for one transistor and one capacitor, the circuit consists of a single differential stage and two current mirrors, in which one has two outputs. The circuit can be discussed using the basic principles presented in this text.
Finally, methods for reducing the coupling of noise from the substrate and power supplies have been presented. Hopefully, the demonstration of layout solutions to these common, practical problems will motivate the reader to improve the quality of future layout.
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