# Things you should know about feedback techniques and methods

Introduction

In today’s article, we are going to study about ‘ Things you should know about feedback techniques and methods ‘ How beneficial it is and what is feedback methods. We will also read about some more topics like basic information about feedback, feedback techniques and methods, voltage feedback, current feedback,Voltage series feedback, Voltage shunt feedback,Current series feedback, Current shunt feedback circuit, Effect of feedback on output impedance,due to negative voltage feedback, Due to negative current feedback, Voltage and feedback current circuit, etc in Detail.  So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

# Feedback techniques and methods-

There are two types of feedback circuit

1. Voltage feedback
2. Current feedback

## Voltage feedback

In this method the voltage is feedback to the input of an amplifier and its proportional to the output voltage.

1. Voltage series feedback
2. Voltage shunt feedback

## Current feedback circuit

On this method the voltage is feedback to the input of an amplifier and its proportional to the current.

• Current series feedback
• Current shunt feedback circuit

### Effect of feedback on output impedance

1. Due to negative voltage feedback
2. Due to negative current feedback

Lets suppose that a amplifier of gain A and output resistance Ro is shown in fig A. To find the output impedance input signal is supposed to be zero as follows-

Thus Ro= Vo’ / Io’

(From figure B) without feedback

Now -βVo’ fraction of output voltage is feedback to the input terminal (negative sign shows negative feedback)

Thus, Vf= -βVo’ = Vi’    …(1)

From fig C voltage output terminals,

Vo’ = Io’Ro – Aβ Vo’

Vo’ (1+ Aβ) = Io’ Ro

And, Vo’/ Io’ = Ro/(1+ Aβ) = Ro’

Hence, Ro’ = Ro/ 1+ Aβ

Consequently by above equation we can say that

Ro’ < Ro.

#### Voltage and feedback current circuit

1) voltage feedback series input circuit/ emitter follower amplifier

Fig A is equivalent to above circuit with the help of h-parameter is as follows

Without feedback,

Vs = Rs Ii + hie Ii

Vs = Ii ( Rs + hie) …(1)

Similarly,

Vo = Re hfe Ii         …(2)

Put value of Ii from equation 1,

Vo= Re hfe Vs/ ( Rs+ hie)   …(3)

Gain of amplifier without feedback is,

A= Vo/ Vs

A= Re hfe Vs/ Vs ( Rs+ hie)

And, A= Re hfe / ( Rs+ hie). ..(4)

We know that feedback voltage,

Vf = βVo = Vo

I.e., β = 1

#### 1) Voltage gain

We know that,

Af = A/ (1+ Aβ)

(∴ for negative feedback)

Af= Re hfe / ( Rs+ hie)(1+ Re hfe/ Rs+ hie)

And hence, Af = Re hfe / ( Rs+hie+ Re hfe)

Af< 1

If Rs + hie < hfe Re

Then,

Af ≈ 1

#### 2) Input resistance

From equation 1,

(Without feedback) Ri= Vs/ Ii = (Rs + hie)

But we know input resistance with feedback is,

Ri’ = Ri (1+ Aβ)

Ri’ / (1+ Re hfe/ Rs+ hie) = (Rs+ hie)

And, Ri’ =  (Rs+ hie) (1+ Re hfe/ Rs+ hie)

Ri’ =   (Rs+ hie+ Re hfe)

#### 3) Output resistance-

For output resistance input signals are set to zero and Re is used as load resistance and for the determination of output voltage RL →∞ that means output terminals are open.

So that without feedback output resistance is taken as Ro≈ Re.

And with feedback output resistance,

Ro’ = Ro/ 1+ Aβ

Ro’ =lim Re→∞ Re/ 1+ Aβ

And hence,

Ro’=lim Re→∞ Re hfe/ 1+Re hfe/ Rs+ hie

Ro’ = lim Re→∞ Re +hfe/ (hfe+(Re+ hie)/ Re)

Consequently,

Ro’=( Rs + hie) / hfe

Emitter follower circuit.

Conclusion

So our article is finished and after completely reading this article, one can easily tell what is feedback techniques and methods. And we have also discussed some more topics like feedback, feedback techniques and methods, voltage feedback, current feedback,Voltage series feedback, Voltage shunt feedback,Current series feedback, Current shunt feedback circuit, Effect of feedback on output impedance,due to negative voltage feedback, Due to negative current feedback, Voltage and feedback current circuit, etc.

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