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The open loop DC gain of unity negative feedback system with closed loop transfer function
\frac{s+4}{s^{2} + 7s + 13} is
(a) {4\over13}
(b) {4\over9}
(c)4
(d)13
Answer is ( b )
Concept
Solution :The closed loop transfer function is given by :{G(s)\over( 1 + G(s)H(s))}
= {G(s)\over 1 + G(s)H(s)} ... Eq(1)
= {s+4 \over s^{2}+7s+13}
H(s) = 1 for unity feedback
\therefore , Eq (1) reduces to {G(s)\over(1+G(s))} = {s+4\over(s^{2}+7s+13)} ... Eq(2)
To simplify this, take the reciprocal of Eq (2)
{(1 + G(s))\over G(s)} = {s^{2} + 7s + 13\over s+4 }
{1\over G(s)} = {(s^2 + 7s+13)\over(s+4)} -1
{1 \over G(s)} ={(s^2 +7s + 13 ) - (s+4)\over(s+4)}
{1\over G(s)} = {s^2+6s+9\over(s+4)}
G(s) = {(s+4)\over(s^2+6s+9)}
For DC gain , s = 0 \therefore, G(s) = 4/9
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