Difference between Positive and Negative Feedback loop

In our previous article, we discuss the difference between open and closed loop control systems. The closed loop system comprises feedback. The feedback can be positive or negative depending upon the application. Let see what is the difference between positive and negative feedback loop control system.

The main difference between the two is that in a positive feedback loop control system, the effective input is the summation of reference input and feedback signal whereas, in a negative feedback loop control system, the effective input is the difference between the reference input and feedback signal.

We will see the differences between the two based on transfer function, effect on gain, accuracy, stability, applications, etc.

Difference between Positive and Negative Feedback loop Control System

Positive Feedback Control System Negative Feedback Control System
The transfer function for this type of system is

TF=\frac{G(s)}{1-G(s)H(s)}

The transfer function for this type of system is

TF=\frac{G(s)}{1+G(s)H(s)}

The effective input is the summation of reference input and feedback signal. The effective input is the difference between the reference input and feedback signal.
The gain of the system increases due to positive feedback. The gain of the system decreases due to negative feedback.
The stability of the system decreases due to positive feedback. The stability of the system increases due to negative feedback.
The bandwidth of the system decreases due to positive feedback. The bandwidth of the system increases due to negative feedback.
The accuracy of the system decreases due to positive feedback. The accuracy of the system increases due to negative feedback.
This is also known as Regenerative feedback. This is also known as Degenerative feedback.
Applications

In Oscillators

Applications

In Amplifiers

Conclusion

The positive feedback tends to make the system unstable whereas the negative feedback tends to make the system more stable. In other words, negative feedback brings the output towards the desired output whereas positive feedback takes the output away from the desired output.

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