Logic Gate Families
There are different 'types' of logic devices - made in different ways.
They are classified in "families", of which the most important are TTL and CMOS. These are the ones that examining boards usually specify in their syllabuses.
TTL
The transistor-transistor-logic family was developed to use bipolar transistor switches for logical operations.
It defines the binary values within the following ranges:
logic state |
Threshold voltage/V |
Upper limit voltage/V |
0 |
0 |
0.8 |
1 |
2 |
5 |
Intermediate values will give uncertain output. Therefore circuits are designed to ensure that does not happen.
TTL is the largest family of digital ICs. They are inexpensive, but draw a lot of power and must be supplied with +5 volts. Individual gates may draw 3-4 mA.
Low power Schottky versions of TTL chips draw only 20% of the power, but are more expensive. These chips have LS in the middle of their numbers.
CMOS (Complementary Metal Oxide Semiconductor) made from MOSFETs
The complementary metal oxide semiconductor family has equivalents to most of the TTL chips.
CMOS chips are much lower in power requirements (roughly 1 mA compared to 3-4 mA) and operate with a wide range of supply voltages (typically 3 to 18 volts).
A CMOS chip number will have a C in the middle of it, e.g., the 74C04 is the CMOS equivalent to the TTL 7404.
A major drawback to their use is their extreme sensitivity to static electricity - they must be carefully protected from static discharges when handling them and dealing with circuits in which they are placed.
ECL (Emitter Coupled Logic) for extremely high speeds
NMOS, PMOS for VLSI large scale integrated circuits.
P- and N-channel Metal Oxide Semiconductors (PMOS and NMOS) offer the advantage of higher component density than TTL chips but are sensitive to damage from electrical discharge - like CMOS. There are also not nearly so many TTL equivalents. (But remember that you can use universal gates to construct all others)
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