Logic Gates and Circuits
NOT OR and AND Gates
Learning Objectives:
- Identify and draw the circuit symbol for NOT, OR, AND Gates.
- Determine the output of a single NOT, OR, AND Gate for a given input(s),
- Determine the output of multiple connected NOT, OR, AND Gates for a given input(s),
- Draw a truth table for NOT, OR, AND Gates.
AND gate
The AND gate is an electronic circuit that gives a high output (1) only if all its inputs are high. A dot (.) is used to show the AND operation i.e. A.B. Bear in mind that this dot is sometimes omitted i.e. ABOR gate
The OR gate is an electronic circuit that gives a high output (1) if one or more of its inputs are high. A plus (+) is used to show the OR operation.NOT gate
The NOT gate is an electronic circuit that produces an inverted version of the input at its output. It is also known as an inverter. If the input variable is A, the inverted output is known as NOT A. This is also shown as A’, or A with a bar over the top, as shown at the outputs.
Solved Example: 9979-01
In Boolean algebra, (A.A̅) + A =? (SSC JE EE Paper II- Sep 2019 Evening session)
A. A
B. 0
C. $\bar{A}$
D. 1
\begin{align*} (A.A̅) + A &= 0 + A \\ &= A \end{align*}
Correct Answer: A
Solved Example: 9979-02
A gate whose inputs are inverted is termed as: (
A. Drop Gate
B. Bubbled Gate
C. Converter Gate
D. Destructor Gate
Correct Answer: B
Solved Example: 9979-03
A small circle on the output of a logic gate is used to represent the: (URSC (ISRO) Tech Asst: Electronics Nov 2016)
A. Comparator operation
B. OR operation
C. NOT operation
D. AND operation
Correct Answer: C
NOR and NAND Gates
Learning Objectives:
- Identify and draw the circuit symbol for NOR, NAND Gates.
- Determine the output of a single NOR, NAND Gates for a given inputs,
- Determine the output of multiple connectedNOR, NAND Gates for a given inputs,
- Draw a truth table for NOR, NAND Gates.
NAND gate
This is a NOT-AND gate which is equal to an AND gate followed by a NOT gate. The outputs of all NAND gates are high if any of the inputs are low. The symbol is an AND gate with a small circle on the output. The small circle represents inversion.NOR gate
This is a NOT-OR gate which is equal to an OR gate followed by a NOT gate. The outputs of all NOR gates are low if any of the inputs are high. The symbol is an OR gate with a small circle on the output. The small circle represents inversion.
Solved Example: 9162-01
NOR gate is an OR gate followed by a _______ gate. (DSSSB Patwari Jun 2019 Shift I)
A. NOT
B. XOR
C. AND
D. OR
Correct Answer: A
Solved Example: 9162-02
All Digital circuit can be made using only ________ gates. (LMRC SCTO Electronics 2018)
A. AND
B. NOT
C. OR
D. NOR
Correct Answer: D
Solved Example: 9162-03
For a three input NOR gate, the possible number of states where output is 0 equals: (DSSSB JE Elec June 2022 Shift I)
A. 8
B. 1
C. 7
D. 5
Correct Answer: C
Solved Example: 9162-04
F = AB + CD + E will be implemented with how many minimum number of NAND gates? (UPPSC Polytechnic Lecturer Electronics Mar 2022 Paper I)
A. 3
B. 4
C. 5
D. 6
Correct Answer: D
EXOR and EXNOR Gates
Learning Objectives:
- Identify and draw the circuit symbol for EXOR and EXNOR Gates.
- Determine the output of a single EXOR and EXNOR Gate for given inputs,
- Determine the output of multiple connected EXOR and EXNOR Gates for given inputs,
- Draw a truth table for EXOR and EXNOR Gates.
EXOR gate
The ’Exclusive-OR’ gate is a circuit which will give a high output if either, but not both, of its two inputs are high. An encircled plus sign is used to show the EOR operation.EXNOR gate
The ’Exclusive-NOR’ gate circuit does the opposite to the EOR gate. It will give a low output if either, but not both, of its two inputs are high. The symbol is an EXOR gate with a small circle on the output. The small circle represents inversion.
Solved Example: 9980-01
Four statements are given below. Identify the correct statement. (SSC JE EE Sep 2019 Evening session)
A. XOR is universal gate
B. XNOR is a basic gate
C. XOR is derived gate
D. XOR is a basic gate
- AND, OR, NOT = basic gates.
- NAND, NOR, XOR, and XNOR = derived gates.
- NAND and NOR = universal gates.
Correct Answer: C
Solved Example: 9980-02
The 2-input EXOR has a high output only when the input values are: (ISRO Scientist CS 2017 Paper - I)
A. Low
B. High
C. Same
D. Different
Correct Answer: D