Digital IC Applications Chapter - 1 CMOS Logic - Introduction to Logic Families

LOGIC FAMILIES

The first electrically controlled logic circuits, developed at Bell Laboratories in 1930s, were based on relays.

In the mid-1940s, the first electronic digital computer, the Eniac, used logic circuits based on vacuum tubes. The Eniac had about 18,000 tubes and a similar number of logic gates, not a lot by today’s standards of microprocessor chips with tens of millions of transistors. However, the Eniac occupies a lot of space than a chip could if it fell on you—it was 100 feet long, 10 feet high, 3 feet deep, and consumed 140,000 watts of power!

The inventions of the semiconductor diode and the bipolar junction transistor

allowed the development of smaller, faster, and more capable computers in the late 1950s.

In the 1960s, the invention of the integrated circuit (IC) allowed multiple diodes, transistors, and other components to be fabricated on a single chip, and computers got still better. Also in 1960s, the first integrated-circuit logic families are introduced.

A logic family is a collection of different integrated-circuit chips that have similar input, output, and internal circuit characteristics, but that perform different logic functions. Chips from the same family can be interconnected to perform any desired logic function. On the other hand, chips from differing families may not be compatible; they may use different power-supply voltages or may use different input and output conditions to represent logic values.

The most successful bipolar logic family is transistor-transistor logic (TTL) first introduced in the 1960s, is a family of logic families that are compatible with each other but differ in speed, power consumption, and cost. Digital systems can mix components from several different TTL families, according to design goals and constraints in different parts of the system. These are used in academic labs.

Ten years before the bipolar junction transistor was invented, another type of transistor, called the metal-oxide semiconductor field effect transistor (MOSFET), or simply MOS transistor was introduced. But MOS transistors were difficult to fabricate in the early days, and from 1960s the development of MOS-based logic and memory circuits was practical. Even then, MOS circuits lagged bipolar circuits considerably in speed, and were attractive only in selected applications because of their lower power consumption and higher levels of integration.

From mid-1980s, advances in the design of MOS circuits, i.e., complementary MOS (CMOS) circuits vastly increased their performance and popularity. Eg: Large-scale integrated circuits like microprocessors and memories. Small- to medium- scale applications, TTL was once the logic family of choice but now they use CMOS devices with equivalent functionality but higher speed and lower power consumption.

CMOS logic is most suitable and easy to understand commercial digital logic technology. As a consequence of the industry’s transition from TTL to CMOS over a long period of time, many CMOS families were designed to be somewhat compatible with TTL.