Page 42 - Digital Electronics by harish

1.5 DIGITAL LOGIC FAMILIES

1.5.1 Logic families

Gates are manufactured using different technologies. The following are some of the
technologies (or logic families) available in the market today.

1. Resistor-Transistor Logic (RTL)
2. Diode-Transistor Logic (DTL)
3. Transistor-Transistor Logic (TTL)
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4. Integrated Injection Logic (I L)
5. Complementary Metal Oxide Semiconductor (CMOS) Logic
6. Emitter-Coupled Logic (ECL)

Out of these logic families, TTL and CMOS Logics are familiar.

1.5.2 Characteristics of Logic families

Some of the important characteristics of digital logic families are:

1. Power supply voltage
2. Fan-in
3. Fan-out
4. Propagation delay
5. Noise immunity and Noise margin
6. Power dissipation

Power supply voltage
It is the permissible power supply voltage range of the gate IC.

Fan-in
Fan-in of a logic gate is defined as the maximum number of inputs connected to a gate
without degradation of the input voltage levels.

Fan-out
Fan-out of a logic gate is defined as the maximum number of similar logic gates that can be
connected at the output without any degradation in output voltage levels.

Propagation delay
It is the time taken by gate for the output to change after inputs have changed. It is normally
represented in milliseconds or nanoseconds.

Power dissipation
It is the measure of the power consumed by a gate when all the inputs are fully driven. It is
normally expressed in milli watts or nano watts.

Noise immunity
It refers to the ability of a gate to tolerate noise without causing spurious changes in the
output voltage. It is also called noise margin.

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