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LOGIC GATES
Semiconductor Devices is one of the important and easy units in class XII CBSE Physics
syllabus. It is easy to understand and learn. Generally the questions asked are simple.
The unit carries 7 marks.In order to help you prepare this unit, I am starting a series of posts
and plan to cover all the important parts in theunit shortly in subsequent posts.
To begin with the easy thing first, I am posting Logic Gates as an important part of the MLL
(Minimum Level of Learning).A 2 to 3 marks question can be expected from this sub-unit in
the CBSE exam.
GET STARTED: HAPPY LEARNING!
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CBSE PREVIOUS YEAR QUESTIONS
Q. 1. You are given a circuit below. Write its truth table. Hence, identify the logic operation carried
out by this circuit. Draw the logic symbol of the gate it corresponds to.
[AI 2011]
Ans.
The gates used in the logic circuit are
(1), (2) NOT gate; (3) NOR gate
The logical combination of inputs and outputs for the circuits is as under:
A
B
X` = A j
Y` = B j
Z
( = X + Y)
0
0
1
1
0
1
0
0
1
0
0
1
1
0
0
1
1
0
0
1
The overall Truth-table is
A
B
Z
0
0
0
1
0
0
0
1
0
1
1
1
Hence it represents AND gate with logic symbol as above.
Q. 2. You are given a circuit below. Write its truth table. Hence, identify the logic operation carried
out by this circuit. Draw the logic symbol of the gate it corresponds to. [AI 2011]
Ans.
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As gate (1) is NAND gate with inputs shorted, it acts as a NOT gate.
The logic combination of inputs and outputs can be written as under:
A
B
X
0
0
1
1
0
0
0
1
1
1
1
0
The overall gate has a truth table represented as under:
A
0
1
0
1
B
0
0
1
1
Z
0
1
1
1
Y
1
1
0
0
Z
0
1
1
1
The truth table represents OR gate.
Its logic symbol is
Q.
3
You are given a circuit below. Write its truth table. Hence, identify the logic operation
carried out by this circuit. Draw the logic symbol of the gate it corresponds to. [AI 2011]
Ans.
Gate (1) and (2) are NOR gates with inputs shorted. So they act as NOT gates. Gate
(3) is a NOR gate.
The logical combination of inputs and outputs can be written as under:
A
B
X
Y
Z
0
0
1
1
0
1
0
0
1
0
0
1
1
0
0
1
1
0
0
1
The overall truth table realised by the logic circuit is:
A
B
Z
0
0
0
1
0
0
0
1
0
1
1
1
The truth table corresponds to AND gate. The circuit symbol of the gate is as under:
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Q.
R. 5 Draw the output waveform at X, using the given inputs A and B for the logic circuit shown below. Also,
identify the logic operation performed by this circuit. [ DELHI 2011]
Ans.
In the given circuit; gate (1) is NAND gate and gate (2) is NOT gate (formed by shorting the two inputs of
the NAND gate).
The inputs A and B in the given diagram and the corresponding values of output at Y and X are tabulated as
under.
Y ` = AB j
X =Y
Time interval
Input A
Input B
t1 – t2
0
0
1
0
t2 – t3
1
0
1
0
t3 – t4
1
1
0
1
t4 – t5
0
1
1
0
t5 – t6
0
0
1
0
t6 – t7
1
0
1
0
The corresponding output waveform X obtained from the logic circuit is as shown.
Q. 6. Write the truth table for the logic circuit shown below and identify the logic operation performed by
this circuit.
[DELHI 2011]
Ans.
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The logic gates connected in the logic circuit are as under:
(1) NOT gate
(2) NOT gate
(3) NAND gate.
The logical combination of outputs C, D and Y for different inputs A and B are tabulated below:
D= B
Y= C.D
1
1
0
0
0
1
1
0
1
1
0
1
1
1
0
0
1
A
B
0
0
1
C= A
The overall truth table is
A
B
Y
0
0
0
1
0
1
0
1
1
1
1
1
Hence the overall gate realised is OR gate
Q7.
Q8 (i)
Identify the logic gates marked P and Q in the given logic circuit. AI 2010
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Q. 9
DELHI 2010
..
Ans.
Q 10
CBSE Sample Paper
Ans.
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