Page 40 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 2: Logic Gates

Notes
Types of Logic Components
One widely used logic family is the transistor-transistor logic (TTL). Another family is the
complementary metal oxide semiconductor logic (CMOS), which performs similar functions
at very low power levels but at slightly lower operating speeds. Several other, less popular
families of logic circuits exist, including the currently obsolete resistor-transistor logic (RTL)
and the emitter coupled logic (ECL), the latter used for very-high-speed systems.
Logical Gates

The elemental blocks in a logic device are called digital logic gates.
An AND gate has two or more inputs and a single output. The output of an AND gate is true
only if all the inputs are true.
An OR gate has two or more inputs and a single output. The output of an OR gate is true if
any one of the inputs is true and is false if all of the inputs are false.
An INVERTER has a single input and a single output terminal and can change a true signal
to a false signal, thus performing the NOT function.

An NAND gate has two or more inputs and a single output. The output of an NAND gate is
true if any one of the inputs is false and is false if all the inputs are true.

An NOR gate has two or more inputs and a single output. The output of an NOR gate is true
if all the inputs are false and is false if the inputs are different.
An EXCLUSIVE OR gate has two or more inputs and a single output. The output of an
EXCLUSIVE OR gate is true if the inputs are different and is false if the inputs are the same.
Obs: You can easily observe how the NAND gate can be emulated by two other gates (AND,
NOT). The output of the AND gate is connected to the input of the NOT gate.
The output of the NAND gate is true if one of the inputs is false. Now let’s verify: We put
true and false on the inputs of the AND gate. This gate now returns false answer (0).This false
answer is the input of the NOT gate. This gate returns true (opposite of false). You can now
see that the answer is similar to the one witch NAND returned.
Questions:
1. Give the cause through which digital logic circuit was developed.
2. What are the logic components?

Self Assessment

Choose the correct answer:
8. The universal gates only are NOT, AND, OR.
(a) True (b) False

9. How many two-input AND and OR gates are required to realize Y=CD+EF+G ?
(a) 2,2 (b) 2,3
(c) 3,3 (d) none of these
10. A universal logic gate is one which can be used to generate any logic function. Which of
…………….the following is a universal logic gate?
(a) OR (b) AND

(c) XOR (d) NAND

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