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The impulse response h(t) of a continuous time, linear time invariant system is described by h(t) = eat u(t) + ebt u(-t) where, u(t) denots the unit step function and a and b are real constants. This system is stable if 
  • a)
    both a and b are positive
  • b)
    a is positive and b is negative
  • c)
    a is negative and b is positive 
  • d)
    both a and b are negative
Correct answer is option 'C'. Can you explain this answer?
Most Upvoted Answer
The impulse response h(t) of a continuous time, linear time invariant ...
Concept:
An LTI system is stable if and only if its impulse response is absolutely integrable.
Calculation:
Given,
h(t) = eat u(t) + ebt u(-t)
h(t) = h1(t) + h2(t)
∵ τ > 0 therefore, for absolute integrability a < 0
∵ τ < 0 therefore, for absolute integrability b > 0
∴ h(t) should be in the form shown in the figure below:
Free Test
Community Answer
The impulse response h(t) of a continuous time, linear time invariant ...
Understanding System Stability
In continuous-time linear time-invariant (LTI) systems, stability is determined by the behavior of the impulse response h(t) as time approaches infinity. The given impulse response is:
h(t) = e^(at)u(t) + e^(bt)u(-t)
Where u(t) is the unit step function.
Analyzing Each Term
- First Term: e^(at)u(t)
- Active for t ≥ 0.
- For the system to be stable, this term must decay to zero as t approaches infinity.
- This occurs when a < />.
- Second Term: e^(bt)u(-t)
- Active for t < />
- For stability, this term must also decay to zero as t approaches negative infinity.
- This occurs when b < /> (i.e., b must be negative).
Conclusion on Stability
For the system to be stable:
- a must be negative (ensuring decay in the positive time region),
- b must also be negative (ensuring decay in the negative time region).
Thus, the correct condition for stability of the system is when a is negative and b is positive, confirming that the system will not exhibit unbounded output for bounded input.
Final Answer
Hence, the correct answer is option C: a is negative and b is positive.
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