Test: Effects of Feedback - Electronics and Communication Engineering (ECE) MCQ

Return ratio is Aβ, where β=feedback factor, and A=open loop gain.
Amount of feedback is A_F/A = 1 + Aβ
In decibels, amount of feedback = -20log₁₀(1+Aβ) = -20log₁₀(221) = -46.88 decibels.

Feedback factor, β = 10.
A_F = A/(1 + Aβ) = 50
A = 20 x 50 = 1000
Output is thus 5000.

A_F = A/(1+Aβ)
Aβ = 24
A = 8
Relative change in gain = dA_F/A_F = dA/A(1+Aβ)
dA_F/A_F = 2/8 x 25 = 0.01.

We can simply use the ratio of 0.1 to find the answer.
Loop gain Aβ = 9
1+Aβ = 10
Desensitivity = 1/S = 1 + Aβ = 10.

Greater desensitivity indicates better stability in gain. More desensitivity means gain becomes smaller, but stable.
For circuit Q, desensitivity = 1/S = 10
Circuit R has higher desensitivity, hence most stable.

D_HF = D_H/1 + Aβ
1 + Aβ = D_H/D_HF = 10/5 = 2
Sensitivity = 1/2 = 0.5.

A_F = A/1 + Aβ
A = A₁ x A₂ = 1200
Thus 1 + Aβ = 1200/200 = 6
Aβ = 5
β = 5/A = 0.0041.

Closed loop system is the system with the feedback and this can be positive or negative feedback and having feedback have less dependence on parameter variation.

Feedback control system can be positive and negative but positive feedback systems less widely used as the positive feedback systems are more sensitive to parameter variations but negative feedback are less sensitive to change in G than change in H.

Feedback can either make a system stable if not stable previously or may be it can cause instability as it reduces the gain of the system and hence the system can become stable if unstable or vice versa.