# Digital Circuit Logic Families

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The modern electronic systems involve digital technology. The digital technology involves logic families and logic gates. You can see all of these advancements easily from your electronic devices such as microprocessors, phones, computers, and industrial processes.

Just as you may be aware already, our electronics work with the help of logic gates built by a lot of transistors. Not only transistors, every electronic component that is able to give “0” and “1” can be used to build this.

This is why we should learn about logic families because there are a lot of logic gate types to be used to build different types of digital processing units.

## What is a Logic Gate?

A logic gate is an electronic component that has one or more inputs and one output. Each logic gate type follows a specific rule to produce a specific output. Their outputs are predictable and certain so we can rely on this component to build a desired logical circuit.

The logic gate types are:

• AND,
• OR,
• NOT,
• NAND,
• NOR, and
• XOR.

## How Does a Logic Gate Work?

Logic gate is an electronic component that has one or more inputs but only one output. This component has a P-N junction like what we find at diodes or transistors, and every semiconductor component. This makes a logic gate a semiconductor element since we can control its conductivity.

Logic gate only operates with two specific voltage levels given to its inputs and generates the two specific voltage levels equal to what we gave to the inputs with respect to the ground. The input and output only have two values, typically they are “+5V ” and “0V”. The positive voltage represents “1” or “True” or “High (H)” while the zero voltage represents “0” or “False” or “Low (L)”.

The applications of this logic component have a lot of variety and are able to complete several tasks as long as it involves logical operation and digital numeric processing.

## What is a Digital Logic Families

Numerous logic families were created as standalone components, each of which contained one or a few fundamental linked logical operations that could be used as “building blocks” to construct systems or as alleged “glue” to connect more intricate integrated circuits.

In very large scale integrated circuits, such as those used in central processors, memory, or other complicated operations, a family of logic implementation techniques is often referred to as a logic family.

To reduce design complexity, some of these logic families employ static methods.

To reduce size, power consumption, and latency, some logic families, such domino logic, use clocked dynamic approaches.

Various solid-state and vacuum tube logic systems were employed for logic circuitry activities prior to the widespread introduction of integrated circuits. But unlike devices with integrated circuits, these lacked standardization and interactivity.

Metal Oxide Semiconductor (MOS) logic is the most popular logic family in contemporary semiconductor devices because of its low power requirements, tiny transistor size, and high transistor density.

## Integrated Circuit and Digital Logic Families

Production methods and circuit configurations used to create digital integrated circuits range widely. Each of these methods is referred to as a particular logic family.

A logic family is a group of various integrated circuit chips that execute distinct logic gate operations like AND, OR, NOT, etc, but have similar input, output, and internal circuit properties.

The theory states that different logic gate functions will have the same electrical properties when created as an integrated circuit using the same method or that is a member of the same logic family (electrically compatible with each other).

These families may differ in terms of speed, cost, power usage, voltage, and current levels.

The use of compatible interface mechanisms for integrated circuits from various logic families is something that digital systems should make sure of. And for that reason, when designing a digital system, we must understand the various logic families and choose the best possible arrangement of integrated circuits.

The following traits must be identical to be utilized to compare performance:

1. Range of supply voltage
2. Quickness of response
3. The loss of power
4. Levels of input and output logic
5. Current ability to submerge
6. Current capacity for sourcing
7. Flexibility
8. Noise resistance
9. Fan-out

## Types of Logic Families

An integrated circuit is built from one of the devices below or combination of both:

• Bipolar devices
• Unipolar devices

Bipolar devices are mainly used for amplification. This device consists of electrons and holes.

Unipolar devices consist of P-type hole carriers  and N-type electron carriers. Unipolar devices are faster at switching speed compared to bipolar.

The logic families are mainly divided into two big groups:

• Unipolar logic families,
• Bipolar logic families,
• Hybrid logic family

## Unipolar Logic Families

The Unipolar logic families are built with Metal Oxide Semiconductor (MOS) like what we find at MOSFET. This logic family has advantages at switching speed and lower power consumption compared to bipolar logic families.

The MOS logic families can be divided further into:

• N-type MOS (NMOS) logic,
• P-type MOS (PMOS) logic,
• Bipolar MOS (BiMOS) logic, and
• Complementary MOS (CMOS) logic.

Metal oxide semiconductor logic, or MOS, is appropriate for systems with a lot of components.

Complementary Metal Oxide Semiconductor (CMOS) Logic: Suitable for low-power systems (VLSI circuits). gradually takes over as the main family of logic.

## Bipolar Logic Families

Bipolar logic family mainly consists of bipolar devices such as transistors and diodes (active elements), along with resistors and capacitors (passive elements). We can divide the bipolar logic families further into:

• Saturated bipolar logic family
• Unsaturated bipolar logic family

The saturated bipolar logic family are:

• Transistor –  Transistor Logic (TTL),
• Diode – Transistor Logic (DTL),
• Integrated – Injection Logic (IIL),
• Resistor – Transistor Logic (RTL), and
• Diode Logic (DL).

The saturated bipolar logic family are:

• Emitter – Coupled Logic (ECL), and
• Schottky TTL.

Transistor-Transistor Logic, or TTL, is a standard logic family that has been around the longest.

ECL, or emitter coupled logic, is appropriate for systems that need to operate quickly.

## Hybrid Logic Families

This logic family is the combination of bipolar and unipolar and the example is:

• Bipolar Complementary MOS (BiCMOS) logic.

## Logic Families Characteristics

Just like another electronic components, the logic families also have several characteristic we should aware of, they are:

### Voltage and Current Threshold Level

Ideally we use 5V voltage and 0V to represent logic “1” and “0” respectively. However, in practical use, we rarely get these values accurately. Thus, we need to define the minimum and maximum parameters.

VIL(max ) – low input voltage

The maximum input voltage to be considered as low (0) input. If the input voltage is higher than this, it will not be detected as low (0).

VIH(max ) – high input voltage

The minimum input voltage to be considered as high (1) input. If the input voltage is lower than this, it will not be detected as high (1).

VOL(max ) – low output voltage

The maximum output voltage to be considered as low (0) output. If the output voltage is higher than this, it will not be detected as low (0).

VOH(max ) – high output voltage

The minimum output voltage to be considered as high (1) input. If the output voltage is lower than this, it will not be detected as high (1).

IIL – low input current

The current that flows into the input terminal when low input voltage within range is applied.

IIH – high input current

The current that flows into the input terminal when high input voltage within range is applied.

IOL – low output current

The current that flows from the output terminal when low output voltage within range is generated.

IOH – high output current

The current that flows from the output terminal when high output voltage within range is generated.

The output current that flowing into the

### 2. Fan-in and Fan-out

Fan-in is the number of inputs a gate has. Two-input gate has two fan-in.

Fan-out is the number of inputs an IC family can drive without falling outside the specified output voltage level. A fan-out of 3 indicates that the gate can drive 3 inputs of that IC family with sufficient current.

Fan-out is also known as loading factor. It limits us not to load it with gates more than the fan-out value.

### 3. Noise Margin

Noise in electrical circuits is something we want to eliminate since it distorts our electrical voltage, current, and signal. The noise margin or noise immunity is the toleration of an electronic device to the noise without changing the output signal.

The measurement quantity of noise immunity or noise margin can be seen below.

We found other variables: VNH and VNL and we will learn those two properly.

High Noise Margin Voltage

Low Noise Margin Voltage

Any noise with negative value greater than VNH can disturb the high logic output out of the range.

Any noise with a positive value greater than VNL can disturb the low logic output out of the range.

### 4. Propagation Delay

When we operate a logic gate, the output will not be generated instantly right after we apply our inputs. There is a small delay response to change the state of the output when the inputs are changed.

This time delay is known as the propagation delay.

From the example above, we found other variables: tPLH and tPHL.

The propagation delay when the output transitions from HIGH (1) to LOW (0) is tPLH.

The propagation delay when the output transitions from LOW (0) to HIGH (1) is tPLH.

There are some notes you should remember:

1. Not every tPLH and tPHL have equal value, and
2. Time delay is measured when the point reaches 50% of the targeted point.

In ideal conditions, the time delay should be zero and we want it as short as possible.

The lower the better.

### 5. Power Dissipation

As with other electrical components, the voltage and current applied to them will dissipate partially as heat. This is also true for logic families, this is why you can feel hot temperature when your hand touches or goes near a running microprocessor.

Unlike microprocessors, logic families dissipated power measured in milliwatts.

We should be careful to reduce the power dissipation to protect the IC from overheating.

To minimize this effect, we often find only one terminal dedicated to VCC in an IC.

The drawn power by an IC from the power supply is calculated from the product of the drawn voltage and current from the power supply.

There are two variables we need to remember along with their extreme conditions.

First is the ICCH (current drawn when the outputs of all gates are HIGH)

Second is the ICCL (current drawn when the outputs of all gates are LOW)

In order to calculate the average power dissipation, we can use the equations below

Where PD is power dissipation.

### 6. Operating Temperature

The acceptable operating temperature for industrial and home appliances is between 0oC and 70oC while military purposes ranged from -55oC to 125oC.

You can expect the logic families to work properly as long as its operating temperature is within range.