# LOGIC FAMILIES IN DIGITAL ELECTRONICS – TTL, CMOS, AND ECL

Most electronic systems which are responsible for modern advances are based on digital technology. All digital systems, computers, and microprocessors are assembled from simple circuits called logic circuits. The basic building blocks of logic circuits are logic gates. And logic gates themselves are simple electronic circuits comprising of diodes, transistors, and **resistors,** in other words, logic families in digital electronics include logic gates constructed using passive devices like resistors, transistors, diodes, etc.

## DIGITAL INTEGRATED CIRCUITS

**Digital Integrated circuits** are produced using several different circuit configurations and production technologies. Each such approach is called a specific **logic family**. A **logic family** is a collection of different integrated circuit chips that have similar input, output, and internal circuit characteristics, but they perform different **logic gate** functions such as **AND**, **OR**, **NOT**, etc. The idea is that different logic gate functions, when fabricated in the form of an integrated circuit with the same approach, or which belongs to the same logic family, will have identical electrical characteristics (electrically compatible with each other). These families may vary by speed, power consumption, cost, voltage, and current levels.

In case of integrated circuits belonging to different logic families, digital system should ensure compatibility interfacing techniques. And that is the reason why we must be clear about different logic families and use the best combination of integrated circuits during the design of a digital system.

The characteristics which are bound to be identical and used to compare performance are:

- Supply voltage range
- Speed of response
- Dissipation of power
- Input and output logic levels
- Current sinking capability
- Current sourcing capability
- Flexibility
- Noise immunity
- Fan-out

## INTEGRATED CIRCUIT AND LOGIC FAMILY

Many **logic families** were produced as individual components, each containing one or a few related basic logical functions, which could be used as “**building blocks**” to create systems or as so-called “glue” to interconnect more complex integrated circuits. A logic family may also refer to a set of techniques used to implement logic within very large-scale integrated circuits such as central processors, memories, or other complex functions. Some such logic families use static techniques to minimize design complexity. Other logic families, such as domino logic, use clocked dynamic techniques to minimize size, power consumption, and delay.

Before the widespread use of integrated circuits, various **solid-state** and **vacuum tube** logic systems were used for logic circuitry operations. But these were not as standardized and interactive as the integrated circuit devices. The most common logic family in modern semiconductor devices is **Metal Oxide Semiconductor (MOS) logic**, due to lower power consumption, small-sized transistor, and high transistor density.

## WHAT IS LOGIC GATES?

The electric circuits which perform logical operations are called **Electric Gate**. A **Logic Gate** is an electronic circuit that has two or more inputs but only one output. Logic follows well-defined rules, producing predictable digital output from a certain input. Main Logic gates are **AND, OR, NOT, NAND, NOR, and XOR**. Digital logic gates NAND and NOR are called universal logic gate because we can construct all other logic gates using NAND gate or NOR gate alone.

## HOW THE LOGIC GATES FUNCTION?

In digital electronics, a gate is defined as an electronic circuit having only one output but two or more inputs. These electronic circuits consist of p-n junction diodes, transistors, and resistors. Gates work with only two voltage levels at their input and output terminals with respect to ground or common, ie., the input and output voltages can have only two values. These values are typically chosen to be 0 Volt (zero volt) and some positive dc voltage value, say +5 Volt. For the 0 Volt logic level, it is customary to use the term that has two or more LOW (or the letter L) and the symbol 0 (zero). While referring to the positive voltage level, we use the term HIGH (or the letter H) or the number 1 (one). A signal appears at the output of a gate only for certain combinations of the input signals. This leads to a number of different types of gates. Gates enable us to perform several tasks which involve logical decision-making, hence the nomenclature logic gates.

## LOGIC FAMILIES IN DIGITAL ELECTRONICS

The digital integrated circuits are designed using **bipolar devices** or **Metal Oxide Semiconductor (MOS)** or a combination of both. There are two kinds of semiconductor devices. The logic families in digital electronics fall under the first kind of Bipolar logic family and the other is the **Unipolar logic family**.

### BIPOLAR LOGIC FAMILY

There are two kinds of operations in bipolar integrated circuits: The saturated Bipolar Logic family and the **Non-saturated Bipolar Logic family**.

Saturated Bipolar Logic Families are:

- Diode logic (DL)
- Resistor Transistor Logic (RTL)
- Diode Transistor Logic (DTL)
- Integrated Injection Logic (IIL or I2L)
- Transistor Transistor Logic (TTL)

Non-saturated Bipolar Logic Families are:

- Schottky TTL
- Emitter Coupled Logic (ECL)

### UNIPOLAR LOGIC FAMILY

The unipolar logic family consists of **Metal Oxide Semiconductor (MOS) logic families**. They are:

- P-type MOS (PMOS) Logic
- N-type MOS (NMOS) logic
- Complementary MOS (CMOS) logic
- Bipolar MOS (BiMOS) logic
- Bipolar CMOS (BiCMOS) logic

## CLASSIFICATION OF LOGIC FAMILIES IN DIGITAL ELECTRONICS

Logic families in digital electronics are mainly classified as Bipolar Logic Families and Unipolar Logic Families.

**Bipolar Logic Families:**It mainly uses bipolar devices like diodes, transistors in addition to passive elements like resistors and capacitors. These are sub classified as saturated bipolar logic family and unsaturated bipolar logic family.- Saturated Bipolar Logic Family: In this family the transistors used in ICs are driven into saturation. For example:
- Transistor-Transistor Logic (TTL)
- Resistor-Transistor Logic (RTL)
- Direct Coupled Transistor Logic (DCTL)
- Diode Transistor Logic (DTL)
- High Threshold Logic(HTL)
- Integrated Injection Logic (IIL or I 2 L)

**Unsaturated bipolar logic family:**In this family the transistors used in IC is not driven into saturation. For example:- Schottky TTL
- Emitter Coupled Logic(ECL)

- Saturated Bipolar Logic Family: In this family the transistors used in ICs are driven into saturation. For example:
**Unipolar Logic Families:**It mainly uses Unipolar devices like MOSFETs in addition to passive elements like resistors and capacitors. These logic families have the advantages of high speed and lower power consumption than Bipolar families. These are classified as:- PMOS or P-Channel MOS Logic Family
- NMOS or N-Channel MOS Logic Family
- CMOS Logic Family

## FEATURES OF LOGIC FAMILIES IN DIGITAL ELECTRONICS

- TTL – Transistor-Transistor Logic: Standard logic family; used for the longest time.
- ECL – Emitter Coupled Logic: Suitable for systems requiring high-speed operations.
- MOS – Metal Oxide Semiconductor Logic: Suitable for systems with high component density.
- CMOS – Complementary Metal Oxide Semiconductor Logic: Suitable for systems with low power consumption (VLSI circuits). Gradually becomes the dominant logic family.