Test: Realization of Logic Gates - Electronics and Communication Engineering (ECE) MCQ

Circuit A:

The output X is given as;

The output Y is given as;

Circuit B:

The output P and Q are given as;
P = A̅ 
Q = B̅ 
The output Y is given as;

For two input any gate one of the input can be used as control input and depending on output according to control input the gates can be used for different applications.
Example in XNOR gate, if one of the input is treated as control input, say  M  we observe that if M = 0, output, Y = A̅  means it act as inverter whereas when control input, M = 1, output, Y = A  means output just follow the input.

If control input,
M = 0, Y - A̅ 
If M = 1, Y = A 

NAND Gate:

• NAND gate represents the complement of the AND operation.
• The graphic symbol for the NAND gate consists of an AND symbol with a bubble on the output, denoting that a complement operation is performed on the output of the AND gate. 
• The logic NAND function can be expressed by the Boolean expression of, A.B.

Analysis:
Given A = 1100 and B = 1010

∴ The output will be 0111.

f = x̅ y z + w̅  x + w̅ 
= w̅  (1 + x) + x̅ y z
= w̅ + x̅ y z
∴ As the output of Gate 2 is not coming in the final simplified solution.
Gate 2 Is redundant.

The given circuit is redrawn as:

The output will be:

Analysis:

Gate 1 is buffer. Input to buffer is A̅ , Output = A̅ 
Gate 2 is buffer. Input to buffer is B̅ , Output = B̅ 
Gate 3 is OR gate. Input = A̅ and B, Output = A̅ + B
Gate 4 is OR gate, Input = A and B̅ , Output = A + B̅ 
Gate 5 is AND gate, Input = (A̅ + B) and (A + B̅), the final output:
x = (A̅ + B) (A + B̅)
x = A̅ B̅ + A B

Concept:
De Morgan’s law states that:

Analysis:


Only 3 NAND gates are required

In an all NOR gate realization of a combinational circuit,

• All Even level gates behave like AND gate
• All Odd level gates behave like OR gate

In an all NAND gate realization of a combinational circuit,

• All Even level gates behave like OR gate
• All Odd level gates behave like AND gate

In the given logic circuit, every gate has an identical non-zero delay.
Let the delay is t₀ msec.

Initially test signal was at logic LOW.
⇒ x = 0, y = 1

Initially output was HIGH
Let assume test signal is switched to logic high at t = 0 m sec
At, t = 0 m sec, x = 1
As there are three NOT gates, the delay of signal to reach y input is 3t0 msec.
At, t = 3t msec, y = 0 ⇒ remains as before

NAND gate also has delay of t₀ msec.
At, t = 4t₀ msec, f becomes high.
At, t = 0 msec, x = 1, y = 1
At, t = 100 msec,


Output pulses from HIGH → LOW → HIGH
At, 0 < t < t₀, f = HIGH
At, t₀ < t < 4t₀, f = LOW
At, t = 4t₀, f = HIGH

Concept:
De Morgans law:
The complement of the union of two sets is the intersection of their complements.
(x + y)’ = x’. y’
The complement of the intersection of two sets is the union of their complements.
(x.y)’ = x’ + y’
Analysis:


∴ OR gate