Introduction to logic family

In this article, We will be going through the Logic Family. we will start our article with an introduction to the logic families, then we will go through their classification and go through their characteristics. Also, we will go through the comparison between CMOS, TTL, and ECL logic families. At last, we will conclude our article with the advantages, disadvantages, applications and some FAQs.

What is a Logic Family?

Logic families are different types of technologies being used to build different logic gates. Logic gates are digital circuits that perform basic logic operations like AND, OR, NOT, NAND, and NOR. In other words, it is a group of compatible ICs with the same logic levels and supply voltages fabricated for performing various logical functions. Here, when we say that ICs have the same logic level, we are referring to two types of logic levels that exist -

• In positive logic, 0 is formed by a low voltage level, and a high voltage level forms 1. It means the ON state refers to high voltage as input or output while OFF means low voltage as input or output.
• In negative logic, 0 is formed by a high voltage level, and 1 is formed by a low voltage level. Here, the situation is reversed to that of positive logic. ON means a low voltage input or output while OFF means high voltage as input or output.

Logical functions are the logical gate operations. The most common logical functions are - AND, OR, NOT, NAND, NOR, XOR.

Logic families are fabricated using various semiconductor technologies utilizing diodes and transistors as switching components. Diodes are simple switching elements having two states, ON as when in forward bias and OFF as when in reverse bias.

In the same way, transistors are elements having three terminals- collector, base, and emitter and they utilize base voltage to switch their states by allowing them to flow current from collector to emitter.

Various factors are responsible for choosing which logic families can be used for the given specific scenarios which include switching speed, fan-out capabilities, power consumption, etc.

Classification of Logic Families

Logic families can be broadly categorized as per the following diagram

Unipolar Logic Families

Unipolar means having a single type of charge carrier. This logic family uses transistors that have either electrons or holes(not both) as charge carriers. The most commonly used technology is Metal-Oxide-Semiconductor(MOS). A common example is Complementary MOS (CMOS) logic.

NMOS (N-channel Metal Oxide Semiconductor)

NMOS transistors conduct when a positive charge is applied to the gate terminal. A nChannel Metal-oxide-semiconductor(MOS) transistor is one in which n-type dopants are used in the gate region(channels). A positive voltage on the gate turns the device ON.NMOS is built on a p-type substrate with an n-type source and drain diffused on it. In NMOS, electrons are the majority carriers. When a high voltage is applied to the gate, the conduction starts. A negative supply stops the conduction and thus switches OFF the device. These are generally considered faster to pMOS as the charge carriers here are electrons which are twice as fast as holes.

PMOS (P-channel Metal Oxide Semiconductor Battery-Powered)

PMOS transistors conduct when a negative charge is applied to the gate terminal. A p-channel Metal-oxide-semiconductor(MOS) transistor is one in which p-type dopants are used in the gate region(channels). A negative voltage on the gate turns the device ON.

p-channel MOSFETs consist of a p-type source and drain diffused on a N-type substrate. Holes are the majority of charge carriers here. That means the high voltage supplied to the gate turns the device OFF. Similarly, a low voltage starts the conduction and in turn, switches ON the device. They are more immune to noise fluctuations compared to NMOS devices.

CMOS(Complementary MOS)

In CMOS technology, both n-type and p-type transistors are used to design logical functions. The same signal which turns ON one transistor is used to turn OFF the other transistor. These characteristics of CMOS make it compatible for designing logic devices using only simple switching elements, without the need for a pull-up resistor.

Characteristics of CMOS

Characteristics of CMOS are listed below :

• Low power consumption - CMOS circuit consumes very low power, making them ideal for battery-powered devices.
• Low cost - The CMOS fabrication process is relatively simpler compared to other semiconductor technologies.
• High reliability and noise immunity - They are considered to have a high noise margin and thus are good for circuits that require high tolerance to noise.
• Limited Voltage Swing - They have a low voltage range of operation, making them less suitable for high voltage operations.
• Process Variation - The CMOS fabrication process is highly dependent on process conditions, leading to variations that can affect the performance and reliability of the final product.
• Vulnerability to electrostatic discharge - CMOS is greatly affected by electrostatic discharge leading to permanent device damage on exposure.

Bipolar Logic Families

In bipolar devices, the conduction happens due to both charge carriers - electrons and holes. Bipolar logic families use semiconductor diodes and bipolar junction transistors as the basic building blocks of logic circuits. The simplest bipolar logic elements use diodes and resistors to perform logic operations; this is called diode logic. Most TTL logic gates use diode logic internally and boost their output drive capability using transistor circuits. Some TTL gates use parallel configurations of transistors to perform logic functions. ECL gates use transistors as current switches to achieve very high speed.

There are further classifications of the bipolar logic family in two types

Saturated

In this logic, the bipolar junction transistors(BJTs) used are operated in saturated regions. This means that both the emitter-base and collector-base junctions are forward-biased, allowing maximum current flow through the transistor.

Characteristics of Saturated Logic Families

Characteristics of Saturated Logic Families are listed below :

• Can allow relatively higher current through transistors
• Very fast state switching
• Higher power consumption than non-saturated logic families.
• Better noise immunity
• Examples include Transistor-Transistor Logic (TTL), Diode Transistor Logic (DTL), and Resistor Transistor Logic (RTL). TTL is the most popular category in this classification.

Transistor-Transistor Logic (TTL)

Transistor-transistor logic (TTL) is a digital logic family employing bipolar junction transistors (BJTs) to uphold logic states and facilitate switching operations. Introduced in 1961 by James L. Buie of TRW, TTL remains prevalent in various electronic devices and systems. Renowned for its remarkable performance and adaptability, TTL finds widespread application in logic gates, memory circuits, and microprocessors.

Characteristics of TTL

Characteristics of TTL are listed below :

• Logic Voltage Levels: TTL logic inputs are classified as logical high when they fall between 2V and 5V, and logical low when within the range of 0V to 0.8V.
• Propagation Delay: TTL stands out for having the minimal propagation delay among digital integrated circuits (ICs).
• Power Dissipation: A standard TTL device consumes approximately 10mW of power.
• Noise Margin: TTL boasts a noise margin of about 0.4V
• Fan Out: Typically, TTL exhibits a fan-out capability of 10.
• Supply Voltage: TTL necessitates a supply voltage ranging between 4.75 V and 5.25 V.
• Speed: TTL is renowned for its rapid switching speed.
• Compatibility: TTL devices are compatible with other TTL devices.

Diode Transistor Logic (DTL)

In Diode Transistor Logic, diodes are used for AND and OR operations while transistors are used for logical inversion and amplification. DTL is used to design and fabricate digital circuits that use diodes in the input stage and BJTs at the output stage. DTL is a type of circuit used in current digital electronics for processing electrical signals.

Characteristics of DTL

Characteristics of DTL are listed below :

• Noise margin: DTL circuits have better noise performance than that of RTL due to high noise margin
• Fan-out: DTL circuits typically have High fan-out.
• Logic low level: 0 or 0.2V
• Logic high level: 5V
• Average propagation delay: Average delay is of 9ns which lies between that of RTL and TTL
• Power dissipation: A few milliwatts to about 50 mW

Resistor Transistor Logic (RTL)

Being the pioneering logic family adopted in integrated circuits, RTL (Resistor-Transistor Logic) circuits consist of resistors and transistors, with resistors positioned at the inputs and transistors at the output. NPN transistors serve as switches, while resistors either regulate current or introduce voltage drops. Initially developed with discrete components, it wasn't until 1961 that RTL circuits marked the advent of the first digital logic family fabricated as a monolithic integrated circuit. These integrated circuits found application in significant systems like the Apollo Guidance Computer, which debuted in 1966. The fundamental RTL device is the NAND gate.

Characteristics of RTL

Characteristics of RTL are listed below :

• High packing density: It means more numbers of RTL circuits can be implemented over chipset
• Logic low level: 0.7V
• Logic high level: 3.5V
• Power Dissipation: Power dissipation is high compared to DTL and TTL
• Noise margin: RTL has poor noise margin with noise immunity being around 30% of supply voltage.
• Propagation delay: Delay is high resulting in low speed.

Non-saturated

In non-saturated bipolar logic, the bipolar junction transistors (BJTs) are operated in the active or linear region and not in the saturation region. In other words, the collector-base junction is reverse-biased, limiting the current flow through the transistor.

Characteristics of Non-Saturated Logic Families

Characteristics of non-saturated Logic Families are listed below :

• Comparatively lower current flow than the saturated logic family's transistors
• Slower switching speed
• Lower power consumption
• Examples include Emitter Coupled Logic(ECL) and Schottky TTL.

Emitter Coupled Logic (ECL) family

Emitter-coupled logic (ECL) is a bipolar transistor logic family that is considered to be the fastest logic available. It was invented in 1956 at IBM by Hannon S. Yourke. ECL is also known as current-steering logic (CSL), current-mode logic (CML), or current-switch emitter-follower (CSEF) logic. The key to reducing propagation delay in a bipolar logic family is to prevent a gate’s transistors from saturating, we learned how Schottky diodes prevent saturation in TTL gates.

ECL is used in high-performance applications, such as: Clock-distribution circuits, High-frequency-based applications, Fiber-optic transceiver interfaces, Ethernet, and ATM (Asynchronous Transfer Mode) networks.

Characteristics of ECL logic family

Characteristics of ECL Logic Families are listed below :

• Power noise: ECL circuits generate relatively little power noise
• Propagation time: The propagation time for ECL can be less than a nanosecond
• Small voltage swing: ECL achieves its high-speed operation by employing a relatively small voltage swing and preventing the transistors from entering the saturation region
• No external inverters: ECL devices operate without the need for any external inverters to simultaneously create the true and complementary output of the desired function at the outputs
• Small voltage swing: ECL has a small swing which generally varies with difference of 0.8V

Schottky TTL

Schottky TTL employs an internal architecture akin to standard TTL, with the notable inclusion of Schottky transistors. These transistors are essentially conventional bipolar transistors augmented with a Schottky diode bridging the base-collector junction. A Schottky diode, characterized by its semiconductor-metal composition, boasts a notably low cut-in voltage of typically 300 millivolts, in contrast to the 600 mV threshold of other prevalent semiconductor diodes. This low cut-in voltage restricts the base-collector voltage to approximately 400 mV, effectively preventing the transistor from entering saturation. Consequently, this limitation mitigates the transition time required for the transistor to shift from saturation to cutoff state.

Characteristics of Schottky TTL logic family

• Low power consumption: They basically operate in non-saturated region so usually have less power consumed compared to normal TTL family.
• Reduced switching time: Schottky diodes have a low forward voltage drop, often between 0.3 and 0.5 volts, which enables quicker switching time. In other words, Schottky TTL is faster.
• Reduced propagation delay time: By preventing saturation of transistors, it reduces the propagation delay.
• Simple Circuit design: It has low complexity compared to ECL family.

Characteristics of a Logic Family

• Operating Speed: This refers to the time taken for the output voltage to change in response to a change in the input voltage. It is desirable for this time to be minimized.
• Fan-in: This denotes the number of inputs connected to a logic gate. For instance, in an AND gate, the fan-in is 2, whereas in a NOT gate, it's 1.
• Fan-out: Fan-out indicates the total number of outputs that a gate can manage without significant alteration in output voltage.
• Noise Immunity: Noise immunity gauges the capacity of a circuit to endure noise or electrical interference without causing a notable deviation in the output.
• Power Dissipation: Power dissipation refers to the power required for operation. When a circuit transitions from one state to another, power is dissipated. Typically, there are two forms of dissipation: static power dissipation, which is the power consumed when the circuit's state remains unchanged, and dynamic power dissipation, which is the power utilized during state transitions.

Comparison of a Logic Family

The following table presents a comprehensive comparison of popular logic families on various parameters

Advantages and Disadvantages of Different Logic Families

The following table classifies the major benefits and limitations of the three most common logic families - Complementary - MOS (CMOS) family, Transistor-Transistor Logic (TTL) family, and Emitter-Coupled-Logic(ECL) family.

Advantages of CMOS

• Extremely low power consumption
• High fan-out (~50)
• Can operate in wider temperature ranges(-55 to 125 degree C)
• No static power dissipation. Power is dissipated only for switching MOSFETs.
• Best Noise Immunity

Disadvantages of CMOS

• Slow speed of operation
• Propagation delay time is around 50ns while this is around 10-12ns in TTL

Advantages of TTL

• Least susceptible to electrical damage
• Noise immunity is better than ECL but less than CMOS
• Compatible with other logic families
• Lesser propagation delay than CMOS
• Better switching speed

Disadvantages of TTL

• Moderate power consumption.
• Prone to temperature variations
• Large power dissipation
• Poor noise immunity

Advantages of ECL

• Fastest speed
• Lesser temperature interference

Disadvantages of ECL

• Power consumption is higher than TTL and CMOS
• Lower operating voltage
• Very low noise immunity

Applications of the Logic Gate

Given below are the Applications of the Logic gate

Applications of CMOS

• Digital ICs: Microprocessors, Microcontrollers, Memory chips
• Embedded systems: Robotics, Automotive electronics
• Signal Processing: Analog-to-digital converters(ADCs), filters, amplifiers
• Medical devices: MRI scanners, Pacemakers

Applications of TTL

• Legacy systems: Industrial plants having legacy systems implanted
• Testing instruments: Oscilloscopes, Logic analyzers, signal generators

Applications of ECL

• High-speed computing: In legacy mainframe and super-computers
• Telecommunications: High-speed switches, routers and communication interfaces
• Military and Aerospace: Radars, Missile guidance system

Conclusion

IC industries are very fast-paced and transformative and thus, we have constant innovations in the field of IC designs and implementation. We have various new, cutting-edge technologies coming up resulting in more and more logic families becoming obsolete. In this article, we have covered the basics of logic families, their characteristics, and their intended purposes.

Various factors on which we measure the capability of a logic family are also discussed including benefits, limitations, and their application.