4 - CIRCUITS

Summary

Metal-oxide-semiconductor (MOS) transistor circuits lie at the core of most modern digital systems. They are used to construct logic, memories, and the drivers and receivers for communication lines. In this chapter we briefly survey digital MOS circuit design. The intent is to provide enough background on circuits to understand the circuit constraints on the system-level electrical problems discussed in later chapters.

We begin by describing the structure and operation of the metal-oxide-semi-conductor field-effect transistor or MOSFET. The I–V characteristics of the MOSFET are described. We also address lumped circuit models of device parasitics and introduce a number of simpler models that are useful for approximate hand analysis of circuits.

After presenting the basic circuit elements, we move on to describe common circuit building blocks or idioms such as switches, gates, source followers, current mirrors, and source-coupled pairs. These idioms form the basic building blocks or subcircuits from which larger circuits are built. The first step in analyzing or designing a circuit is always to partition the circuit into a number of these idioms. The overall function of the circuit is then easily discerned, and its parameters follow from the parameters of its constituent subcircuits.

Our exploration of circuits starts in Section 4.3.1 with an examination of switch circuits. Many MOS circuits, including pass-transistor networks, Manchester carry chains, and multiplexers can be accurately modeled by considering each MOSFET to be a switch with a series resistance.