Digital Design Using VHDL A Systems Approach

In this chapter we will see how to build logic circuits (gates) using complementary metal– oxide–semiconductor (CMOS) transistors. We start in Section 4.1 by examining how logic functions can be realized using switches. A series combination of switches performs an AND function while a parallel combination of switches performs an OR function. We can build up more complex switch-logic functions by building more complex series–parallel switch networks.

In Section 4.2 we present a very simple switch-level model of an MOS transistor. CMOS transistors come in two flavors: NMOS and PMOS. For purposes of analyzing the function of logic circuits, we consider an NMOS transistor to be a switch that is closed when its gate is a logic “1” and that passes only a logic “0.” A PMOS transistor is complementary – it is a switch that is closed when its gate is a logic “0” and that passes only a logic “1.” To model the delay and power of logic circuits (which we defer to Chapter 5), we add a resistance and a capacitance to our basic switch. This switch-level model is much simpler than the models used for MOS circuit design, but is perfectly adequate to analyze the functionality and performance of digital logic circuits.

Using our switch-level model, we see how to build gate circuits in Section 4.3 by building a pull-down network of NMOS transistors and a complementary pull-up network of PMOS transistors. A NAND gate, for example, is realized with a series pull-down network of NMOS transistors and a parallel pull-up network of PMOS transistors.