Differences between TTL, CMOS, ECL and BiCMOS Logic Families: What You Need to Know

Have you ever wondered how the electronics in your everyday devices, like your TV remote, microwave oven, or mobile phone, work? They rely on complex circuits powered by different logic families. In this article, we’ll explore the key differences between TTL, CMOS, ECL, and BiCMOS logic families, helping you understand the technology behind them and their real-world applications.

But wait, what are logic families? Well, there are two meanings to it.

• Logic Family (Digital ICs): A group of integrated circuits that use specific voltage levels and power supplies to perform logic operations. Examples include TTL (Transistor-Transistor Logic) and CMOS (Complementary Metal-Oxide-Semiconductor).

• Logic Family (VLSI Systems): The design rules that define how the gates are connected within a larger system, like a microprocessor or memory system. The choice of logic family impacts performance, power efficiency, and noise immunity for the entire system.

In simple words, logic families are specific configurations of logic gates that are designed for specific purposes. These logical families contain basic logical systems interconnected to form a larger family of integrated circuitry.

Characteristics of Logic Families

There are several factors that differentiate logic families. Let’s take a look at the most important ones.

• Speed: The response time between input application and output signal.
• Fan-In: The maximum number of inputs a logic gate can handle.
• Fan-Out: The number of circuits a logic gate can drive.
• Noise Immunity: The circuit’s ability to withstand electrical noise.
• Propagation Delay: The time between input application and the output change.
• Power Dissipation: The amount of power consumed by the system.

Classification of Logic Families

Logic families are mainly classified according to the polarity of the devices used. We have two main classifications – bipolar and unipolar.

Bipolar means two polarities. Bi- two, polar-polarities. Here, the circuits have bipolar elements like diodes, transistors, etc. Other passive elements, like resistors and capacitors, also make up the circuit. We can divide bipolar families further divided into saturated and unsaturated logic families. Under the saturated logic families, the transistors used in ICs are driven into saturation. And vice-versa for the unsaturated logic systems.

Unipolar families have components with only one polarity. The circuits have unipolar components like MOSFETs and passive elements.

Transistor-Transistor Logic Families (TTL)

The transistor-transistor logic families are entirely made with bipolar junction transistors (BJTs) and resistors.

But why are they named ‘transistor-transistor logic’ and not simply ‘transistor logic’?That’s because BJTs are used as the main components in both the logic system of the circuit as well as the amplification part of the circuit.

Commercially, the TTL logic components begin with ‘74XX’, such as 7404, 74S86, etc.

For logic gates built using TTL components, the input voltage is applied to the emitter of the transistors. Circuits comprising of the TTL families usually consist of multi-emitter transistors, i.e., multiple parallel transistors with distinct emitter inputs and common base and collector terminals.

FIG.3 A BASIC TWO INPUT TTL NAND GATE ORIENTATION

The transistors A and B in Fig.3 have separate inputs at the transistors, but the bases are made common and biased with a resistor, and the collector terminals have also been combined, acting as the base of the next transistor.

In the given 2-input TTL configuration, the application of a logic high voltage at terminals A and B reverse biases the emitter-base junction of the transistor. Thus, a small current flows in the opposite direction. This small current reaches the base terminal of the output, turning on the transistor. Upon switching on, the transistor pulls down its output to a logic low. Conversely, if either of the inputs at terminals A and B is zero, no current reaches the base of the second transistor. Thus the output becomes VCC.

Apart from using normal transistors, Schottky transistors are also preferred for TTL logic systems. These transistors portray the Schottky effect and thus have higher switching speed and lesser power consumption.

TTL is now considered a little outdated, but is used as switching device is relays and driving lamps, and in printers and video display terminals.

Characteristics of TTL Logic Families

• The output of a TTL device can serve as an input to a maximum of 10 gates, i.e., the fan-out is 10.
• A logic low voltage for a TTL is defined between 0V-0.2V.
• A logic high voltage for a TTL is at 5V.
• The noise margin is at around 4V.
• The propagation delay is about 9ns.
• A typical TTL component draws a power of about 11mW.

Advantages of TTL Logic Families

• TTL has a strong drive capability.
• It is least susceptible to electrical damage.
• Requires only one supply voltage (otherwise for CMOS)
• Lesser immune to noise when compared to ECL, but more than CMOS.
• Fastest saturation, when compared to other logic families
• Low output impedance for all states

Disadvantages of TTL Logic Families

• TTL dissipates a lot of power, thus not making it suitable for battery-powered devices.
• Not recommended in VLSI chips as it requires more space and isolation
• Expensive compared to MOSFETs

Common TTL Logic ICs

• 74 family
• 74LS (Low-power Schottky) family
• 74F (Fast) family
• 74AS (Advanced Schottky) family
• 74ALS (Advanced Low-power Schottky) family

Emitter – Coupled Logic Families (ECL)

ECL families use overdriven BJT differential amplifiers with single-ended inputs. The emitters of the transistors are connected all together, hence the name ‘emitter-coupled’ logic families.

The transistors in this system never saturate; their logic high and logic low levels are chosen close to one another, thus eliminating the possibility of saturation in the transistors. The transistors are thus operated in the active region or the cut-off region.

These transistors use the property of current switching. Thus this family is also known as Current Steering Logic (CSL) or Current Mode Logic Families (CML). It makes use of a transistor-based differential amplifier to amplify and combine digital signals.

We can see here that the transistors are differential amplifiers, with their emitters coupled. Cryptographic applications and high-speed requirements use ECL Logic.

Characteristics of ECL Logic Families

• The output of an ECL system has a very low impedance, thus having a fan-out of about 25.
• The propagation time is about 1ns, making it the fastest logic family.
• A logic low voltage for ECL is about -1.7V to -1.75V.
• A logic high voltage for ECL is about -0.8V.
• It is the fastest amongst all the logic families.
• It has an average propagation delay time of about 1ns-4ns.

Advantages of ECL Logic Families

• Has a fan-out better than the TTL Logic family
• Offers the highest speed in operation
• ECL systems produce complementary outputs (OR-NOR, AND-NAND).
• Parameters do not vary much with temperature

Disadvantages of ECL Logic Families

• Worst noise immunity compared to TTL and CMOS
• Highest power consumption, when compared with TTL and CMOS
• VLSI design difficult as ECL circuits require resistors, thus increasing system size
• Capacitive loading reduces the fan-out capacity

Complimentary MOSFETs Logic Families (CMOS)

The CMOS family uses MOSFETs in the integrated circuits. Both NMOS and PMOS complement each other and are used symmetrically in each configuration of the circuit, hence the name ‘complimentary.’

Since both PMOS and NMOS are present, an application of input voltage will turn on any one type of the transistor at a time. Thus, at any time, there is no direct link between the power supply and the ground.

Two main attributes of CMOS devices are high immunity to noise and low static power consumption. CMOS consists of only enhancement type of MOSFETs.

 FIG.5 A BASIC TWO INPUT CMOS NAND GATE ORIENTATION

For NMOS transistors, if the input is a 1, the MOS turns on; otherwise, it is off. On the other hand, for the PMOS, if the input is 0, the MOSFET switches on; otherwise, the transistor is off. Thus, if either of the input is 1, the output voltage is equal to VDD.

The FETs are placed such that if either of the NMOS or PMOS transistors are connected in series, the other type is connected in parallel. Thus, only one type of MOS turns on at a time.

We use CMOS chips in RF applications, satellite communication, Bluetooth, and mobile networks. We also see their use in radar applications and Wi-Fi technology. To be honest, CMOS is the backbone of the semiconductor industry. Its applications are countless.

Characteristics of CMOS Logic Families

• CMOS supports a very large fan-out, more than 50 transistors.
• It has excellent noise immunity amongst all families.
• A logic low voltage for CMOS is about
• A logic high voltage for ECL is somewhere between 4.5V to 5V.
• The propagation delay is the worst when compared with TTL and ECL families at about 200ns.

Advantages of CMOS Logic Families

• Has the highest fan-out, when compared with TTL and ECL
• Works well over a wide range of temperature
• Noise immunity is better than TTL and ECL

Disadvantages of CMOS Logic Families

• Average propagation delay is the least in comparison with TTL and ECL

Common TTL Logic ICs

• 4000 family
• 74C family
• 74HC family
• 74HCT family
• 74AC family
• 74ACT family
• 74AHC family

BiCMOS Logic Families (Bipolar CMOS)

BiCMOS, or Bipolar CMOS, combines the best properties of two distinct technologies in the integrated circuit- Bipolar junction transistors and the CMOS logic family. We see the application of BiCMOS in Pentium, Pentium Pro microprocessors.

The combined strengths of BJTs and CMOS makes BiCMOS an exceptional and essential technology introduced in the electronics industry. High speed and low output impedance offered by the BJTs and high input resistance by CMOS are the main qualities combined to give the BiCMOS.

Since it combines two different designs, the fabrication of the BiCMOS chips happens with high precision and accuracy. The chip designing usually begins with a foundation of the CMOS process and continues with bipolar process steps.

Also, the fabrication of CMOS and BJTs require a controlled addition of impurities, thus increasing the complexity of making these chips. All these together make BiCMOS technology really expensive, when compared with the other technologies. In addition to this, the complicated manufacturing of BiCMOS risks the chances of increased leakage current.

The basic circuit configuration is precisely the same as that of a 2-input CMOS NAND gate, but with extra MOSFETs in the circuit.
Qp and Qo are low impedance output drivers.

The circuit input VA is given at PA and NA1, and input VB is given at PB and NB1. The FETs NB3, NA3, and N2 remove the base charge from the BJTs during switching.

Characteristics of BICMOS Logic Family

• BiCMOS offers lower power dissipation than BJTs.
• It offers improved speed when compared with CMOS technology.
• BiCMOS circuits offer high load current sinking and current sourcing

Advantages of BiCMOS Logic Families

• The best circuit to be used where high current sinking and sourcing is involved.
• Have reduced cycle time, when compared to CMOS circuits
• Robust to process changes and temperature changes
• Lower power dissipation than BJTs

Disadvantages of BiCMOS Logic Families

• Greater process complexity
• Manufacturing cost is very high
• Increased time in fabrication

Difference between TTL, ECL, and CMOS

Frequently Asked Questions (FAQs) about Logic Families in Digital Circuits

What is a Logic Family in digital circuits?

A Logic Family refers to a group of digital integrated circuits (ICs) that share common characteristics such as voltage levels, power supply requirements, and the types of logic gates they use. Examples of logic families include TTL (Transistor-Transistor Logic) and CMOS (Complementary Metal-Oxide-Semiconductor).

How do different logic families differ from each other?

Logic families differ primarily in their voltage levels and power supply requirements:

• TTL (Transistor-Transistor Logic) operates at 5V and uses bipolar transistors.
• CMOS (Complementary Metal-Oxide-Semiconductor) uses lower voltage levels (e.g., 3.3V or 5V) and is more power-efficient than TTL, as it only consumes power when switching states.

Other differences include speed, power consumption, and noise immunity, which affect their suitability for specific applications like high-speed processing or low-power devices.

What does a Logic Family mean in VLSI (Very-Large-Scale Integration) design?

In VLSI systems, a logic family refers to the set of design rules and characteristics that govern how logic gates are interconnected in complex circuits, such as processors or memory systems. It dictates how signals flow, how components are integrated, and how much energy is consumed by the system. For example, a processor may use CMOS logic for efficiency, while a high-speed memory might use ECL (Emitter-Coupled Logic) for faster operation.

Why is the choice of Logic Family important in VLSI systems?

The choice of logic family affects several key factors in VLSI design:

• Speed: Some logic families like ECL (Emitter-Coupled Logic) are faster but consume more power, while others like CMOS are slower but more power-efficient.
• Power Efficiency: Choosing a logic family impacts the overall energy consumption, especially in battery-powered devices like smartphones.
• Noise Immunity: Some logic families handle electrical interference better than others, which is crucial in complex systems like processors or communication devices.
• Scalability: The choice can also affect how easily the system can scale as more components are added.

Can I use any Logic Family for any application?

No, the selection of a logic family depends on the requirements of your specific application. For example:

• TTL logic may be chosen for its robustness and speed, but it consumes more power than other families.
• CMOS logic is ideal for low-power applications, making it the preferred choice for mobile devices and portable electronics.
• ECL may be chosen for applications requiring high-speed operation, like high-frequency data processing, at the expense of increased power consumption.

How do I know which Logic Family to choose for my project?

The best logic family for your project depends on the following factors:

• Speed Requirements: If you need high-speed performance, ECL or TTL might be your best choice.
• Power Consumption: For low-power projects (e.g., mobile devices), CMOS is typically the ideal choice.
• Complexity and Scalability: Consider how your system will scale and how many components it will involve.
• Cost: Some logic families are more cost-effective for large-scale production (like CMOS).