The open loop transfer function of a unity feedback control system is G(s) = K / ((s + 2)(s + 1)(s^2 + 6s + 25)). Determine the range of gain 'K' for the unity feedback control system to be stable.
- A. +200 < K < 666.25
- B. -200 < K < 112.17
- C. -200 < K < 666.25
- D. 0 < K < 200
Answer: D. 0 < K < 200
Explanation: Characteristic equation s⁴ + 9s³ + 45s² + 87s + (50+K) = 0; Routh-Hurwitz stability criterion gives 0 < K < 200 (Official ISRO key option d).