Feedback amplifiers

 

Feedback amplifiers

Working principle of feedback amplifier:

            In general sense feedback is the process to collect information about performance and provide corrective actions to improve the overall performance. Feedback is the process of giving back fraction of output back to the input. These kinds of systems are known as closed loop systems and are mainly used for controlling purpose. Feedback may be positive or negative and have its own advantages, disadvantages and applications. Lets take an example of AC (air conditioner) which is set to one particular temperature and if temperature increases or decreases, it regulates (controls) it by using feedback taken from temperature sensors. Another example is automatic iron in which temperature is set at which it iron automatically turns ON and OFF to operate at fixed temperature. One more example is automatic door that opens if someone is in front of it and closes if he/she passes through it. These systems take feedback from sensors and operate accordingly. Amplifiers and oscillators are typical systems that use feedback.

            There are main two types of feedback amplifiers

1.      Positive feedback

2.      Negative feedback

Sub types of negative amplifiers

-voltage series feedback amplifiers

-voltage shunt feedback amplifiers

-current series feedback amplifiers

-current shunt feedback amplifiers

 

Figure: Feedback Amplifier

Figure shows amplifier with feedback circuit, the gain of the amplifier is shown as A. Gain is the ratio of output voltage Vo to the input voltage Vi. The feedback circuit extracts a voltage Vf = β Vo from the output Vo of the amplifier. The voltage is added in case of positive feedback and in negative feedback, voltage is subtracted, from the signal voltage Vs. Now,

Vi = Vs+Vf

= Vs + βVo  (for Positive feedback)

Vi =Vs−Vf

= Vs−βVo  (for Negative feedback)

 

The feedback ratio or feedback fraction is denoted by β and is given by formulae

β = Vf /Vo

Generally positive feedback increases (addition operation) the input signal and hence output. On the other hand negative feedback decreases (subtraction operation) input signal and hence output. In next article, positive and negative feedback are explained in detail.