Logic Families

Logic families are sets of chips that may implement different logical functions but use the same type of transistors and voltage levels for logical levels and for the power supplies. The logic families vary by speed, power consumption, cost, voltage & current levels.

In digital designs, a circuit arrangement of the circuit elements in a special manner will result in a particular logic family. The electrical characteristics of the IC will be identical. In other words, different parameters like Noise margin, Fan in, Fan out, etc. will be identical.

Different IC’s belonging to the same logic family will be compatible with each other.

Digital IC gates are classified not only by their logic operation but also by the specific logic circuit family to which it belongs. Each logic family has its own basic electronic circuit upon which more complex digital circuits and functions are developed.

Different types of Logic Gate Families

• RTL: Resistor-transistor logic gate family
• DCTL: Direct-coupled transistor logic gate family
• RCTL: Resistor-capacitor transistor logic gate family
• DTL: Diode-transistor logic gate family
• TTL: Transistor -transistor logic gate family
• IIL: Integrated injection logic gate family

Characteristics of logic families

• Fan-in: the number of inputs to a gate.
• Fan-out: the number of standard loads driven by a gate output. Power Dissipation: the amount of power needed by the gate.
• Propagation delay: The signals through a gate take a certain amount of time to propagate from the inputs to the output. This interval of time is defined as the propagation delay of the gate.
• Noise Margin: Noise margin is the maximum noise voltage added to an input signal of a digital circuit that does not cause an undesirable change in the circuit output.

Significance of Logic Families

If all the ICs are of the same logic family then they are compatible with each other. Hence, the intended logic functions are performed and the goal is achieved.

But in case ICs belonging to the different logic family are used in a digital system than to ensure compatibility interfacing techniques must be used. And that is the reason why we must understand different logic families and use the best combination of ICs during the design of a digital system.

Now the question arises what might be the consequence of choosing wrong combinations of ICs. The answer is that it may not match the necessary capability needed.