Introduction to Logic Families

Integrated circuits (ICs) are the building blocks of modern electronics. They contain miniaturized electronic components like transistors, diodes, and resistors fabricated on a semiconductor substrate. Digital logic ICs implement Boolean logic functions using logic gates as their fundamental building blocks.

Logic gates are electronic switches that output specific binary values based on their binary inputs. The main logic gates are:

• AND gate: Outputs 1 only if all inputs are 1, else outputs 0
• OR gate: Outputs 1 if any input is 1, else outputs 0
• NOT gate (inverter): Outputs the opposite of the input (0 becomes 1 and vice versa)

These basic gates can be combined to create more complex logic functions like NAND, NOR, XOR, multiplexers, flip-flops, etc. The specific circuit design and fabrication technology used to implement logic gates and functions is known as a logic family.

The two most common logic families are:

1. Transistor-Transistor Logic (TTL): Uses bipolar junction transistors (BJTs) as the main switching element. Runs on a 5V power supply.

2. Complementary Metal-Oxide-Semiconductor (CMOS): Uses complementary pairs of p-type and n-type metal-oxide-semiconductor field-effect transistors (MOSFETs). Can run on lower voltages like 3.3V.

This article will compare and contrast the TTL and CMOS logic families, covering their history, how they work, their key characteristics, advantages and disadvantages, and common applications. Understanding the differences between logic families is important for circuit designers when selecting components and designing systems.

History and Development

TTL Origins

TTL was invented in 1961 by James L. Buie of TRW, improving on earlier transistor-diode logic and resistor-transistor logic designs. TTL became the dominant logic family used in computers and other digital systems throughout the 1960s and 70s.

Some key milestones in TTL development:

• 1964 – Texas Instruments introduces the 5400 series, the first commercial TTL ICs
• 1971 – The Schottky TTL subfamily is introduced, using Schottky diodes to improve switching speed
• 1976 – Low-power Schottky TTL devices introduced
• 1980s – Advanced Schottky TTL, Fairchild Advanced Schottky TTL (FAST), and Advanced Low-power Schottky (ALS) subfamilies developed with improved speed and lower power consumption

While TTL was the dominant logic family for many years, by the 1990s it was largely replaced by CMOS in new designs due to CMOS’s lower power consumption and ability to scale to smaller fabrication nodes.

CMOS Development

The basic principle of complementary MOS transistor logic gates was first described by Frank Wanlass at Fairchild Semiconductor in 1963. Early CMOS ICs were used in aerospace and military applications in the late 1960s due to their low power consumption and high noise immunity.

Important developments in CMOS technology:

• 1968 – RCA introduces the 4000 series, the first commercial CMOS logic family
• 1970s – High-voltage CMOS for automotive and industrial applications
• 1980s – CMOS becomes the dominant logic family in VLSI chips like microprocessors
• 1990s-2000s – Development of smaller fabrication processes from 600 nm to 14 nm and beyond, enabling higher transistor density and performance

CMOS has become the dominant logic family used today due to its low power consumption, high noise immunity, and ability to scale to very high transistor densities using nanometer fabrication processes. Modern microprocessors contain billions of CMOS transistors.

How TTL Works

A TTL gate uses bipolar junction transistors (BJTs) as the switching element and has multiple inputs and one output. The transistors are arranged as a “totem pole” output stage, with two BJTs stacked between the power supply and ground:

The logic state is determined by the voltages at the inputs:

• Logic 0: 0 to 0.8 V
• Logic 1: 2.2 to 5 V

When all inputs are high (logic 1), the upper transistor is off and the lower transistor is on, pulling the output low (logic 0). If any input is low (logic 0), the lower transistor is off and the output is pulled high through the collector resistor.

TTL inputs source current from the previous stage’s output. Standard TTL outputs can source up to 16 mA and sink up to 24 mA. This asymmetry means a TTL output can safely drive more inputs high than low.

TTL gates also have a voltage bias network and clamping diodes on the inputs to ensure correct logic levels and protect the BJTs from damage.

CMOS Operation

A CMOS gate consists of two complementary types of MOSFETs:

• N-type (NMOS): Turns on when the gate voltage is high
• P-type (PMOS): Turns on when the gate voltage is low

The FETs are arranged in a “pull-up/pull-down” network between the power supply and ground. Here is the schematic for a CMOS NAND gate:

When the inputs are high, the NMOS FETs are on and the PMOS FETs are off, creating a low-resistance path between the output and ground (logic 0). When the inputs are low, the opposite is true and the output is pulled up to the positive supply voltage (logic 1).

In a CMOS gate, the output is always connected to either the positive supply or ground through a low resistance path. This is known as “rail-to-rail” operation. The complementary arrangement also means virtually no current flows except during switching, resulting in very low static power dissipation.

The logic threshold of a CMOS input is approximately half the supply voltage. Inputs have very high impedance and draw virtually no current.

Key Characteristics Comparison

Power Supply and Dissipation

One of the biggest differences between TTL and CMOS is their power supply requirements. TTL requires a regulated 5 V supply, while CMOS can operate on a wider range from 3 V to 18 V or more. This allows CMOS to be used in low-voltage battery-powered applications.

TTL also has much higher static power dissipation than CMOS. Even when not switching, TTL gates draw a significant “quiescent” current, on the order of 1 to 10 mA per gate. CMOS gates draw very little current except during switching transients, typically a few μA. However, CMOS dynamic power increases with switching frequency, so at very high speeds it can exceed TTL.

Noise Immunity

Noise immunity refers to a logic gate’s ability to tolerate spurious voltage spikes on its inputs without changing state. It is defined as the difference between the maximum low input voltage (V_IL) and minimum high input voltage (V_IH).

TTL gates have a typical noise immunity of 400 mV. CMOS gates have much higher noise immunity, often over 1 V, due to their high input impedance and symmetric input thresholds.

Speed

The maximum switching speed of a logic gate is limited by its propagation delay t_pd, the time it takes for an output to change state after an input changes.

TTL gates are generally faster than 4000 series CMOS, with t_pd on the order of 10 ns vs 50 ns or more for CMOS. However, more recent CMOS subfamilies like 74HC and 74AC series have propagation delays of a few ns, comparable to or better than TTL.

Fanout and Drive

Fanout refers to the number of gate inputs that one gate output can reliably drive. It is determined by the ratio of output drive to input loading.

TTL has asymmetric drive capability, able to sink more current than it can source. A standard TTL output can drive 10 TTL inputs low or 20 inputs high. Low-power TTL has less output drive.

CMOS has much higher input impedance than TTL, so one CMOS output can typically drive 50 or more CMOS inputs. However, CMOS outputs have limited current sourcing/sinking ability on the order of a few mA.

Interfacing TTL and CMOS

While TTL outputs can directly drive CMOS inputs due to CMOS’s high input impedance, CMOS outputs may not be able to drive TTL inputs reliably because of their limited output drive. Open-collector or open-drain buffers may be needed.

TTL and CMOS circuits can be damaged by voltages outside their recommended supply range. 5 V TTL outputs can overstress low-voltage CMOS inputs, so level-translation buffers should be used.

Advantages and Disadvantages

TTL Pros and Cons

Advantages of TTL:
– Faster than 4000 series CMOS
– Good output drive and fanout
– Low cost and wide availability
– Established reliability and second-sourcing

Disadvantages of TTL:
– Requires a 5 V regulated supply
– High static power consumption
– Lower noise immunity than CMOS
– Limited to bipolar fab processes

CMOS Benefits and Drawbacks

Advantages of CMOS:
– Very low static power dissipation
– High noise immunity
– Wide power supply range
– Rail-to-rail output swing
– High input impedance
– Simpler circuit design
– Scales to very small feature sizes

Disadvantages of CMOS:
– Slower than TTL (4000 series)
– Limited output drive capability
– Susceptible to electrostatic discharge (ESD) damage
– More expensive than TTL for simple gates

Common Applications

TTL and CMOS are used in a wide variety of digital systems:

• Computers and peripherals
• Industrial control systems
• Test and measurement equipment
• Consumer electronics
• Telecommunications
• Military and aerospace systems

Some specific applications best suited for each logic family:

TTL:
– Backplane drivers and bus transceivers
– Line drivers and receivers
– Display drivers
– Memory address decoders
– Analog-to-digital converters

CMOS:
– Microprocessors and microcontrollers
– Memory chips
– Application-specific integrated circuits (ASICs)
– Field-programmable gate arrays (FPGAs)
– Wearable and portable devices
– Automotive electronics

Frequently Asked Questions

Q: Can TTL and CMOS ICs be used in the same circuit?

A: Yes, but care must be taken to ensure the output of one type can properly drive the input of the other without causing damage or unreliable operation. Level-translation buffers may be needed between TTL and low-voltage CMOS.

Q: Which is better, TTL or CMOS?

A: It depends on the specific application and design requirements such as power consumption, speed, noise immunity, and cost. In general, CMOS is preferred for new designs due to its lower power usage and scalability.

Q: Are TTL ICs still used today?

A: TTL has largely been replaced by CMOS in new designs. However, many legacy systems still use TTL chips and they are still widely available. Some specialty applications may still use TTL for its specific properties.

Q: Can CMOS outputs drive loads like LEDs or relays?

A: Directly driving high-current loads like relays or multiple LEDs is not recommended for either TTL or CMOS outputs. Buffering with a transistor or dedicated driver IC should be used to protect the logic gate outputs.

Q: What is the difference between 74HC and 74HCT CMOS series?

A: 74HC is a high-speed CMOS series with CMOS-compatible input thresholds (typically 50% of V_CC). 74HCT has the same output characteristics as 74HC but with TTL-compatible input thresholds. It can be directly driven by both CMOS and TTL outputs.

Conclusion

TTL and CMOS are two major logic families used to implement digital integrated circuits. While they perform the same basic logic functions, they differ in their circuit design, power requirements, speed, and interface characteristics.

TTL was the dominant logic family for many years, but has largely been supplanted by CMOS in new designs due to CMOS’s lower static power consumption, higher noise immunity, and ability to scale to smaller fabrication processes.

Understanding the properties and trade-offs of different logic families is important for engineers when designing digital systems. Mixing logic families requires careful consideration of the interface between them to ensure reliable operation.

As VLSI technology continues to advance, the distinction between logic families has blurred, with CMOS being used almost exclusively except in special applications. However, the fundamental principles of logic gate operation remain the same. By learning these foundations, designers can effectively harness logic ICs to create increasingly complex and powerful digital systems.