Types of Feedback

Feedback in a control system is the process of comparing the output of the system back to its input, and using the difference (error) between them to adjust the system's input or process, so that the output reaches the desired value.

Feedback = Output is measured → compared with desired Input → Error is calculated → System is corrected.

There are 2 types of feedback:

(i) Positive Feedback

In a positive feedback system, the feedback signal is ADDED to the set point (input signal). This increases the error rather than reducing it.

• Positive feedback makes the system UNSTABLE.
• Rarely used in control systems; mainly used in oscillators and amplifiers.

Example: If desired temperature = 100°C and feedback = +20°C, the system reads 120°C and keeps increasing — this is unstable behaviour.

(ii) Negative Feedback

In a negative feedback system, the feedback signal is SUBTRACTED from the set point (input signal). This reduces the error and brings the output closer to the desired value.

• Negative feedback makes the system STABLE.
• Most real-world control systems use negative feedback.

Example: A room thermostat — if room temperature exceeds the set point, the A.C. increases cooling to bring it back down.

Positive Feedback	Negative Feedback
Feedback signal is ADDED to input	Feedback signal is SUBTRACTED from input
System becomes UNSTABLE	System becomes STABLE
Error increases over time	Error decreases over time
Used in: Oscillators, amplifiers	Used in: A.C., Refrigerator, thermostats