Digital Logic Gates

  • A digital logic gate is an electrical circuit that makes logical choices using a combination of digital signals at its inputs.
  • Digital logic gates can have multiple inputs, such as A, B, C, D, and so on, but they usually only have one digital output (Q). Individual logic gates can be coupled or cascaded together to create a logic gate function with any desired number of inputs, combinational and sequential type circuits, or other logic gate functions from conventional gates.

  • TTL (Transistor-Transistor Logic) gates, such as the 7400 series, and CMOS (Complementary Metal-Oxide-Silicon) gates, such as the 4000 series, are the two primary families or kinds of commercially accessible digital logic gates

  • The logic technique used to produce the integrated circuit, or "chip" as it is more widely known, is referred to as TTL or CMOS.

  • In general, TTL logic ICs employ NPN and PNP type Bipolar Junction Transistors for all their input and output circuitry, whereas CMOS logic ICs use complementary MOSFET or JFET type Field Effect Transistors.

  • Simple digital logic gates can be built by connecting diodes, transistors, and resistors to form RTL, Resistor-Transistor logic gates, DTL, Diode-Transistor logic gates, or ECL, Emitter-Coupled logic gates, however these are less frequent currently than the popular CMOS family. 

  • Integrated Circuits, or ICs as they are more popularly known, are classified into families based on the number of transistors or "gates" they include. A simple AND gate, for example, may only have a few individual transistors, but a more complicated CPU may have thousands of individual transistor gates. The number of logic gates or the complexity of the circuits within a single chip are used to classify integrated circuits, with the standard categorization for the number of individual gates being: 

Classification of Integrated Circuits

  • Small Scale Integration (SSI)- A single package can include up to 10 transistors or a few gates such as AND, OR, and NOT gates.

  • Medium Scale Integration (MSI) – contains between 10 and 100 transistors or tens of gates and performs digital functions such as adders, decoders, counters, flip-flops, and multiplexers in a single package.

  • Large Scale Integration (LSI) chips are between 100 and 1,000 transistors or hundreds of gates and execute particular digital activities including I/O chips, memory, arithmetic, and logic.

  • Processors, massive memory arrays, and programmable logic devices are examples of VLSI devices, which include between 1,000 and 10,000 transistors or thousands of gates and perform computational operations.

  • SLSI (Super-Large Scale Integration) is a technology that packs 10,000 to 100,000 transistors into a single device.

  • CPUs, GPUs, video processors, microcontrollers, FPGAs, and complicated PICs all use Ultra-Large Scale Integration (ULSI), which has more than 1 million transistors.

  • While the "ultra large scale" ULSI classification is less common, another degree of integration that indicates the Integrated Circuit's complexity is known as the System-on-Chip or (SOC) for short. Individual components such as the CPU, memory, peripherals, I/O logic, and so on are all manufactured on a single piece of silicon, which represents an entire electronic system on a single chip, literally putting the term "integrated" in integrated circuit.

Moore's Laws

  • Regarding the evolution of semiconductor gate technology, Gordon Moore, co-founder of Intel, predicted in 1965 that "the number of transistors and resistors on a single chip will double every 18 months." There were around 60 unique transistor gates on a single silicon chip or die when Gordon Moore made his famous comment in 1965.

  • In 1971, the Intel 4004 was the world's first microprocessor, with a 4-bit data bus and around 2,300 transistors on a single chip working at 600kHz. Intel Corporation has put 1.2 billion individual transistor gates on its new Quad-core i7-2700K Sandy Bridge 64-bit microprocessor chip, which runs at roughly 4GHz, and the on-chip transistor count is continually expanding as newer processors are released. 

Digital Logic State

  • All digital electronic circuits and microprocessor-based systems are built using the Digital Logic Gate as the basic building block. Basic digital logic gates perform binary number AND, OR, and NOT logic operations.

  • Only two voltage levels or states are allowed in digital logic design, and these states are known as Logic "1" and Logic "0," or HIGH and LOW, or TRUE and FALSE, respectively. In Boolean Algebra and typical truth tables, these two states are represented by the binary numbers "1" and "0," respectively.

  • A simple light switch is a nice example of a digital state. The switch can be in one of two states: "ON" or "OFF," but not both at the same time. Then we

Boolean Algebra

Boolean Logic

Voltage State

Logic '1'



Logic '0'



  • Most digital logic gates and systems use "Positive logic," in which a logic level "0" or "LOW" is represented by a zero voltage, 0v, or ground, and a logic level "1" or "HIGH" is represented by a higher voltage, such as +5 volts, with the switching from one voltage level to the other, from either a logic level "0" to a "1" or a "1" to a "0," is made as

  • There is also a "Negative Logic" system in which the values and rules of a logic "0" and a logic "1" are reversed, but for the sake of this instructional part on digital logic gates, we will only refer to the "Negative Logic."