Logic Families

Contents

Define and classify Logic Families

In digital electronics, a logic family is a group of electronic circuits that share a common logic and power supply voltage levels, operating characteristics, and other performance specifications. Logic families are classified based on the type of logic gates and the semiconductor technology used in their construction.

The main types of logic families are:

  1. Transistor-Transistor Logic (TTL): TTL is a popular logic family that uses bipolar transistors and diodes to implement logic gates. TTL circuits operate at a 5-volt power supply voltage and have high noise immunity and low power consumption.
  2. Complementary Metal-Oxide-Semiconductor (CMOS): CMOS is a logic family that uses MOSFETs (metal-oxide-semiconductor field-effect transistors) to implement logic gates. CMOS circuits can operate at lower power supply voltages (typically 3 to 5 volts), and offer high noise immunity, low power consumption, and high speed.
  3. Emitter-Coupled Logic (ECL): ECL is a logic family that uses bipolar transistors to implement logic gates. ECL circuits operate at negative power supply voltages (-5.2 volts) and offer high speed and high fan-out capability.
  4. Gallium Arsenide Logic (GAL): GAL is a logic family that uses gallium arsenide semiconductor technology to implement logic gates. GAL circuits offer high-speed operation and low power consumption, and are commonly used in high-performance applications.
  5. Programmable Logic Array (PLA): PLA is a type of programmable logic device (PLD) that uses a programmable AND array and a fixed OR array to implement logic functions. PLAs can be programmed to implement any logic function and are commonly used in digital signal processing applications.

Other types of logic families include Emitter-Coupled Logic with Silicon-On-Insulator (ECLinSOI), Advanced Schottky TTL (AS-TTL), and Low-Voltage Differential Signalling (LVDS).

In summary, logic families are groups of electronic circuits that share a common logic and power supply voltage levels, operating characteristics, and other performance specifications. The main types of logic families are TTL, CMOS, ECL, GAL, and PLA, which differ in the semiconductor technology used in their construction and their performance characteristics.

Define and Differentiate between types of Integrated Circuits

Integrated circuits (ICs) are small electronic devices that contain a large number of interconnected electronic components on a single chip of semiconductor material. There are several types of ICs, each with their own unique characteristics and applications. The main types of ICs are:

  1. Digital ICs: Digital ICs are designed to process digital signals and perform digital operations. They are used in a wide range of applications, including computers, smartphones, and other digital devices. Digital ICs can be further classified into two types: combinational and sequential.
  2. Analog ICs: Analog ICs are designed to process analog signals and perform analog operations. They are used in a wide range of applications, including audio and video processing, power management, and measurement and control systems.
  3. Mixed-signal ICs: Mixed-signal ICs combine both digital and analog circuitry on a single chip. They are used in applications that require both digital and analog signal processing, such as in communication systems, sensors, and data acquisition systems.
  4. Memory ICs: Memory ICs are designed to store digital information. They can be further classified into two types: Read-Only Memory (ROM) and Random-Access Memory (RAM). ROM is used to store permanent data, while RAM is used to store temporary data.
  5. Microprocessors: Microprocessors are ICs that contain a central processing unit (CPU) and associated control logic. They are used in a wide range of applications, including computers, smartphones, and other digital devices.
  6. Application-specific ICs (ASICs): ASICs are ICs that are designed for specific applications. They are commonly used in industries such as telecommunications, automotive, and aerospace.
  7. Programmable ICs: Programmable ICs are ICs that can be programmed by the user to perform a specific function. They can be further classified into two types: Field-Programmable Gate Arrays (FPGAs) and Complex Programmable Logic Devices (CPLDs).

In summary, integrated circuits (ICs) are small electronic devices that contain a large number of interconnected electronic components on a single chip of semiconductor material. The main types of ICs include digital, analog, mixed-signal, memory, microprocessors, ASICs, and programmable ICs, each with their own unique characteristics and applications.

Describe switching action of PN-Junction, BJT, and MOS

The switching action of PN-junction, BJT, and MOS devices is the process by which these devices can be turned on or off to control the flow of current.

PN-Junction:

A PN-junction is formed when a p-type semiconductor material and an n-type semiconductor material are brought into contact. When a voltage is applied across the junction, electrons from the n-type material move toward the p-type material, while holes from the p-type material move toward the n-type material. At the junction, the electrons and holes combine, creating a depletion region where there are no free charge carriers. When the voltage applied across the junction is reversed, the depletion region widens, and the junction becomes highly resistive, preventing current flow.

BJT (Bipolar Junction Transistor):

A BJT is a three-layer device consisting of a p-type semiconductor sandwiched between two n-type semiconductors (NPN configuration) or an n-type semiconductor sandwiched between two p-type semiconductors (PNP configuration). The junction between the layers is called the base-emitter junction, and the other junction is called the base-collector junction. When a small current is applied to the base-emitter junction, the transistor turns on, allowing a larger current to flow from the collector to the emitter. The amount of current flowing through the transistor is controlled by the voltage applied to the base-emitter junction.

MOS (Metal-Oxide-Semiconductor):

MOSFETs are a type of transistor that uses a metal gate separated from the semiconductor material by an insulating oxide layer. The MOSFET has three terminals: the source, the drain, and the gate. When a voltage is applied to the gate, an electric field is created, which attracts or repels electrons in the semiconductor material, allowing or preventing current flow between the source and the drain. In a MOSFET, the gate is isolated from the channel by an oxide layer, which makes the MOSFET highly resistant to current flow in the off state. The MOSFET is a type of transistor that can switch very quickly, making it useful in high-speed digital circuits.

Describe Logic Family Specification

In digital electronics, a logic family is a group of electronic circuits that share the same underlying technology and are designed to perform logical operations. Each logic family has its own set of specifications that determine its operating characteristics and performance. Here is a detailed explanation of some of the key specifications for logic families:

  1. Power Supply Voltage (Vcc): This specification specifies the range of voltages that the logic family requires for proper operation. For example, TTL (Transistor-Transistor Logic) requires a power supply voltage of 5V, while CMOS (Complementary Metal-Oxide-Semiconductor) can operate on a range of voltages from 3V to 15V. The power supply voltage affects the speed, power consumption, and noise immunity of the logic family.
  2. Output Voltage Levels: This specification specifies the voltage levels that the logic family generates for its output signals. For example, TTL logic family generates output levels of 0V and 5V, while the CMOS logic family generates output levels of 0V and Vcc. The output voltage levels affect the noise immunity and compatibility with other circuits.
  3. Input Voltage Levels: This specification specifies the voltage levels that the logic family requires for its input signals. For example, the TTL logic family requires input levels of 0V and 5V, while the CMOS logic family requires input levels between 0V and Vcc. The input voltage levels affect the noise immunity and compatibility with other circuits.
  4. Fan-Out: This specification specifies the maximum number of input gates that a logic family output can drive without degradation of the output signal. For example, the TTL logic family can typically drive 10 inputs, while the CMOS logic family can typically drive 50 to 100 inputs.
  5. Speed: This specification specifies the maximum frequency of the input signal that a logic family can process without errors. Typically, it is measured in nanoseconds or picoseconds. The speed affects the performance of the logic family in high-speed applications.
  6. Power Dissipation: This specification specifies the power consumption of the logic family during operation, usually measured in milliwatts or microwatts. The power dissipation affects the heat generated by the circuit and the battery life in portable devices.
  7. Noise Margin: This specification specifies the amount of noise that a logic family can tolerate on its input signals without causing errors in the output signals. The noise margin affects the noise immunity of the logic family.
  8. Propagation Delay: This specification specifies the time taken by the logic family to produce an output signal after receiving an input signal. The propagation delay affects the timing of the circuit and the maximum frequency it can operate at.

By considering these specifications, designers can choose the most appropriate logic family for their digital electronic circuits based on the requirements of the application. Different logic families have different strengths and weaknesses, and it’s important to select the one that meets the specific needs of the circuit.

Recall the following Transistor Logic: Resistor Transistor Logic(RTL)

Resistor-Transistor Logic (RTL) is a digital logic family that was widely used in the early days of digital electronics. It was invented in the 1950s as one of the earliest digital logic families to be developed. RTL circuits are built using transistors and resistors, with diodes used for logic inversion. The basic building block of an RTL circuit is the transistor switch, which is used to implement logical functions such as AND, OR, and NOT.

The RTL circuit consists of resistors at inputs and transistors at the output side. Transistors are used as the switching device. The emitter of the transistor is connected to the ground. The collector terminals are tied together and given to the supply through the resistor RC. The collector resistor is known as a passive pull-up resistor.

2-input RTL NOR gate

The following figure shows the circuit diagram of the 2-input RTL NOR gate. Q1 and Q2 are the two transistors. A and B are the two inputs, given to the base of two transistors and Y is the output.

When both the inputs A and B are at 0V or logic 0, it is not enough to turn on the gates of both the transistors. So the transistors will not conduct. Due to this, the voltage +VCC will appear at the output Y. Hence the output is logic 1 or logic HIGH at terminal Y.

When any one of the inputs, either A or B is given HIGH voltage or logic 1, then the transistor with HIGH gate input will be turned on. This will make a path for the supply voltage to go to the ground through the resistor RC and transistor. Thus there will be 0 v at the output terminal Y.

When both the inputs are HIGH, it will drive both the transistors to turn on. It will make a path for the supply voltage to flow to the ground through resistor RC and transistor. Therefore, there will be 0 v at the output terminal Y. The below table shows the truth table for NOR gate.

3-input RTL NOR gate

The above discussed 2-input RTL NOR gate is the basis for all the logic circuits built with resistors and transistors. The 3-input Resistor-Transistor Logic NOR gate can also be constructed as shown below. The operation is similar to the 2-input RTL NOR gate.

Limitations

When the transistor is switched on, the power dissipation increases as the current flows through base and collector. Also, the RTL gate has poor noise margin, poor fan-out and the propagation delay is more.

However, RTL circuits have several limitations. One of the main limitations is their slow switching time, which limits their usefulness in high-speed applications. The slow switching time is due to the relatively large size of the transistors used in RTL circuits. Another limitation is the large number of components required to implement complex functions, which leads to high power consumption, cost, and size.

Despite its limitations, RTL remains a useful technology for low-speed and low-power applications where simplicity and low cost are more important than speed and complexity. It is also used in some specialised applications such as radiation-hardened circuits for space applications.

Recall the following Transistor Logic: Direct-Coupled Transistor Logic (DTCL)

Direct-Coupled Transistor Logic (DTCL) is a type of digital logic circuit that uses transistors as its primary building block. DTCL circuits use a direct coupling between the transistors, which means that the output of one transistor is connected directly to the input of another transistor without any intervening components.

DTCL circuits can be used to implement a wide range of digital logic functions, including basic logic gates like AND, OR, and NOT gates, as well as more complex functions like flip-flops and counters.

One of the key advantages of DTCL circuits is that they are relatively simple and inexpensive to design and manufacture, compared to other types of digital logic circuits. However, they are also limited in terms of their speed and complexity, and are generally not used in high-performance applications where speed and precision are critical.

DTCL circuits were widely used in the early days of digital electronics, but have largely been replaced by more advanced technologies like CMOS (Complementary Metal-Oxide-Semiconductor) and TTL (Transistor-Transistor Logic) circuits, which offer better performance and lower power consumption.

Recall the following Transistor Logic: Diode Transistor Logic (DTL)

Diode Transistor Logic (DTL) is a digital logic family that was widely used in the early days of digital electronics. It was invented as an improvement over Resistor-Transistor Logic (RTL) and was popular in the 1960s and 1970s. DTL circuits are built using transistors and diodes, with resistors used for biassing.

The basic building block of DTL is a diode-resistor logic gate, which consists of a diode connected to the base of a transistor. The diode provides the input to the gate, and the transistor provides the output. When the input is high, the diode conducts and the transistor is turned on, resulting in a low output. When the input is low, the diode is reverse-biassed, and the transistor is turned off, resulting in a high output.

To implement other logic gates, multiple diode-resistor logic gates are combined in various configurations. For example, an AND gate can be implemented using two diode-resistor logic gates in series, and an OR gate can be implemented using two diode-resistor logic gates in parallel.

DTL has several advantages, including simplicity of design and low cost. However, it also has several limitations, including limited fan-out, slow speed, and high power consumption. Due to these limitations, DTL has largely been replaced by other transistor logic families, such as TTL and CMOS.

Logic circuit of 2-input DTL NAND gate

The following figure shows the circuit for the 2-input DTL NAND gate. It consists of two diodes and a transistor. The two diodes DA, DB and the resistor R1 form the input side of the logic circuit. The common emitter configuration of transistor Q1 and resistor R2 forms the output side.

How does a 2-input DTL NAND gate operate?

When both the inputs A and B are LOW, the diodes DA and DB become forward biased and so both diodes will conduct in the forward direction. So the current due to the supply voltage +VCC = 5 V will go to the ground through R1 and the two diodes DA and DB

The supply voltage gets dropped in the resistor R1 and it will not be sufficient to turn ON the transistor. So the transistor will be in cut off mode.

Therefore, the output at the terminal Y will have a HIGH value, that is Logic 1. The operation of the gate with the current flow path is shown in the below figure.