The signal flow graph of a closed loop system is shown in

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The signal flow graph of a closed loop system is shown in the figure, where TD, represent the disturbance in the forward path. The effect of disturbance can be reduced by—

1. increasing G₂(s)
2. decreasing G₂(s)
3. increasing G₁(s)
4. decreasing G₁(s)

Correct Option: C

The ratio of the output C(s) to the disturbance signal To(s) when R(s) = 0 is obtained by applying the signal flow gain formula to the figure, we get

C(s)	=	- G₂(s)
T_D	=	1 + G₁(s) G₂(s) H(s)

C(s)	≈	- 1
T_D	≈	G₁(s) H(s)

[assume G1(s) G2(s) H (s) >> 1] Thus it is seen that the introduction of feedback decreases the effects of disturbances and noise signals in the forward path of the feedback loop. The effect of disturbance can be reduced by increasing G1(s).