Consider a feedback circuit made of three AND gates with intermediate signals…
1999
Consider a feedback circuit made of three AND gates with intermediate signals defined as follows:
P = A·Y
Q = B·P
Y = C·Q
In a certain steady state, line Y is at 1. What are the possible values of A, B and C in this state?
a) A = 0, B = 0, C = 1
b) A = 0, B = 1, C = 1
c) A = 1, B = 0, C = 1
d) A = 1, B = 1, C = 1
- A.
d only
- B.
Both a and b
- C.
Both a and c
- D.
a, c and d
Attempted by 58 students.
Show answer & explanation
Correct answer: A
The correct answer is: d only.
The three AND gates give:
P = A·Y
Q = B·P
Y = C·Q
Substituting P and Q into the output equation:
Y = C·B·A·Y = A·B·C·Y
In the given steady state, Y = 1. Therefore:
1 = A·B·C·1
So A·B·C must be 1.
An AND product is 1 only when all inputs are 1. Hence A = 1, B = 1 and C = 1. Among the listed cases, only case d is possible.