Types of feedback: negative and positive feedback, advantages and disadvantages of negative feedback

 

Types of feedback:  negative and positive feedback, advantages and disadvantages of negative feedback

            As discussed in previous topic, there are two basic types of feedback, positive feedback and negative feedback. In positive feedback, input signal is increased due to addition operation and in negative feedback, input signal is decreased due to subtraction operation.

 

Positive Feedback: In positive feedback, feedback signal is in-phase with the input signal. First have a look on following diagram.

 

Figure: Positive Feedback

In positive feedback, feedback network is designed in such a way that, it will provide phase shift of 180⁰ which is in-phase with the input signal, hence there is addition operation which in turns increases input signal. The property of amplifier is to provide phase shift of 180⁰ and phase shift provided by feedback network is 180⁰, hence total phase shift in positive feedback is 360⁰. Positive feedback increases input signal as well as output signal hence they are used in oscillator circuits and signal generators.

 

Advantage of positive feedback:

1.      Increase in voltage gain.

2.      Increase in output voltage.

Disadvantages of positive feedback:

1.      Increase in noise

2.      Less stability

3.      Less bandwidth

4.      Non-linear operation i.e. less fidelity

5.      More harmonic, phase and frequency distortion.

 

Negative Feedback: in negative feedback, feedback signal is out-of-phase with the input signal. Let’s have a look on following diagram first.

 

Figure: Negative Feedback

In negative feedback, feedback network is designed in such a way that, it will provide phase shift of 0⁰ which is out-of-phase with the input signal, hence there is subtraction operation which in turns decreases the input signal. The property of amplifier is to provide phase shift of 180⁰ and phase shift provided by feedback network is 0⁰, hence total phase shift in negative feedback is 180⁰. Negative feedback decreases input signal as well as output signal hence they are used in amplifier circuits.

 

Advantages of negative feedback:

1.      Less noise

2.      More stable.

3.      Higher fidelity i.e. more linear operation.

4.      Highly stabilized gain.

5.      Increased bandwidth and better frequency response.

6.      Less amplitude distortion.

7.      Less harmonics, phase and frequency distortion.

8.      Input and output impedance can be modified as desired.

Disadvantages of negative feedback:

1.      Reduction in overall gain

2.      Reduced output voltage.

 

Comparison of Positive feedback and Negative feedback:

 

S. N.	Parameter	Positive feedback	Negative Feedback
1.	Total Phase shift in circuit	0◦ or 360◦	180◦
2	Feedback and input signal	In-phase	Out-of-phase
3	Input voltage	Increases	Decreases
4	Output voltage	Increases	Decreases
5	Voltage gain	Increases	Decreases
6	Output impedance	Increases	Decreases
7	Noise	Increases	Decreases
8	Bandwidth	Decreases	Increases
9	fidelity	Decreases	Increases
10	Stability	Decreases	Increases
11	Input impedance	Decreases	Increases
12	Amplitude distortion	More	less
13	Harmonic distortion	More	Less
14	Phase distortion	More	Less
15	Application	Oscillators	Amplifiers