 # Facts you should know about feedback amplifier to score high

Introduction

1. In today’s article, we are going to study about ‘ Facts you should know about feedback amplifier to score high ‘ How beneficial it is and what is feedback. We will also read about some more topics like basic information about feedback, positive feedback, negative feedback, principle of feedback, equation for feedback, advantage of negative feedback etc in Detail.  So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

# Feedback amplifier

The process of injecting a fraction of output energy of some device back to the input is known as feedback. There are two types of feedback amplifier.

There are two types of feedback amplifier-

1. Positive feedback
2. Negative feedback

## Positive feedback

When the feedback energy is in same phase with the input signal aids (support) it. It increases the gain of amplifier but it has disadvantage of increased instability and increased distortion.

## Negative feedback

When the feedback is out of the phase with the input signal and thus opposed it, called negative feedback. It reduces the gain of an amplifier but the stability increases. And reduction in distortion and improves in input and output impedance. And also increase the band width.

### Principle of feedback Amplifier is shown by A and the gain of amplifier is A. Small part or whole output voltage is applied on input with the help of another circuit. This fraction of output voltage is called feedback ratio and denoted by β n/w –

• there should be no change in the amplifier
• Input signal voltage should be transmit only by the amplifier.
• Feedback voltage should be passed only by the β n/w.
• No extra load should be applied on input and output n/w due to β n/w.

So that resultant voltage at input is given by,

Vif = Vi ± Vf        …(1)

Vif = Vi + Vf   (positive feedback)

Vif = Vi – Vf   (negative feedback)

I.e., in general we take equation (1).

If A is the gain of an amplifier (without feedback)

A= Vo/ Vi

For feedback circuit – If Vif is the I/P voltage and Vo’ is the output voltage then gain will be,

A’ = A= Vo’ /Vif

Vif= Vo’/ A    …(2)

Lets suppose feedback voltage Vf= β Vo’     …(3)

From equation (1),

Vif = Vi + Vf

Vif = Vi + β Vo’

Vo’/ A = Vif = Vi + β Vo’

Vo’ = Vi / ((1/A)- β)

And hence, Vo’ = Vi A/ 1-Aβ

If we suppose that original amplifier and feedback circuit behaved as a system in combined form then resultant gain is given by-

Af = Vo’ / Vi

( ∴ ƒ = with feedback)

Af = A/ 1- Aβ

This is called general feedback equation.

Here, Aβ → loop gain

Af = closed loop gain with feedback

A= open loop gain without feedback gain

1- Aβ = return difference

1. If |1- Aβ| > 1 ( ∴ Aβ is negative feedback) then |Af|< A. I.e., this is the condition of negative feedback.
2. |1- Aβ| < 1 (∴ Aβ → positive) then |Af|> A. This is the condition of positive feedback.
3. If |1- Aβ|= 0, when Aβ= 1. Then, |Af| = ∞ is the condition of Oscillator.

1. Stability of gain by negative feedback

Without feedback stability factor of an amplifier is given by,

S= dA/ A    …(1)

With feedback,

Sf= dAf/ Af    …(2)

We know gain of an Amplifier with feedback is

Af = A/ 1- Aβ       …(3)

Differentiate this equation with respect to A,

dAf / dA = (1- Aβ + Aβ)/ (1- Aβ)^2

dAf / dA = (1)/ (1- Aβ)^2

And, dAf / dA = (1)/ (1- Aβ).  (1)/ (1- Aβ)

dAf / dA = (1)/ (1- Aβ). Af/ A

dAf / dA = (1)/ (1- Aβ). A

Sf = S/ (1- Aβ)           …(4)

For the stability of an amplified, value of stability factor should be minimum. When feedback is negative then, (1- Aβ) > 1. So that Sf < S

For positive feedback, (1- Aβ) < 1. So that Sf> S

Conclusion

So our article is finished and after completely reading this article, one can easily tell what is feedback. And we have also discussed some more topics like basic information about feedback, positive feedback, negative feedback, principle of feedback, equation for feedback, advantage of negative feedback etc.

‌So one can say that they got a detailed information about feedback, and basic information about other topics.

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