Digital Electronics: Principles, Devices and Applications

Logic Families 1515.5 CMOS Logic FamilyThe CMOS (Complementary Metal Oxide Semiconductor) logic family uses both N-type and P-typeMOSFETs (enhancement MOSFETs, to be more precise) to realize different logic functions. Thetwo types of MOSFET are designed to have matching characteristics. That is, they exhibit identicalcharacteristics in switch-OFF and switch-ON conditions. The main advantage of the CMOS logicfamily over bipolar logic families discussed so far lies in its extremely low power dissipation, which isnear-zero in static conditions. In fact, CMOS devices draw power only when they are switching. Thisallows integration of a much larger number of CMOS gates on a chip than would have been possiblewith bipolar or NMOS (to be discussed later) technology. CMOS technology today is the dominantsemiconductor technology used for making microprocessors, memory devices and application-specificintegrated circuits (ASICs). The CMOS logic family, like TTL, has a large number of subfamilies. Theprominent members of CMOS logic were listed in an earlier part of the chapter. The basic differencebetween different CMOS logic subfamilies such as 4000A, 4000B, 4000UB, 74C, 74HC, 74HCT,74AC and 74ACT is in the fabrication process used and not in the design of the circuits employed toimplement the intended logic function. We will firstly look at the circuit implementation of variouslogic functions in CMOS and then follow this up with a brief description of different subfamilies ofCMOS logic.5.5.1 Circuit Implementation of Logic FunctionsIn the following paragraphs, we will briefly describe the internal schematics of basic logic functionswhen implemented in CMOS logic. These include inverter, NAND, NOR, AND, OR, EX-OR, EX-NORand AND-OR-INVERT functions.5.5.1.1 CMOS InverterThe inverter is the most fundamental building block of CMOS logic. It consists of a pair of N-channeland P-channel MOSFETs connected in cascade configuration as shown in Fig. 5.34. The circuitV DDQ 1AY=AQ2Figure 5.34CMOS inverter.