EFFECTS OF FEEDBACK

1) Stabilization of Gain:

The gain of the amplifier may change due to the changes in the parameters of the transistor or the supply voltage variation.
Negative feedback stabilizes the gain of the amplifier against these factors.


2) Reduction in distortion: The negative feedback reduces the non-linear distortion in the output signal.

3) Reduction in noise: It reduces the level of noise generated within the amplifier.

4) Change in input impedance: It increases the input impedance of the amplifier.

5) Change in output impedance: It decreases the output impedance of the amplifier.


6) Increase in Bandwidth: The negative feedback decreases the lower cut off frequency f while increases the upper cut off frequency f i.e. it increases the bandwidth of the amplifier.

These factors are discussed one by one:













  •     Equation (2) shows that the gain A of the feedback amplifier is independent of internal gain A and depends only on feedback fraction /3. [ in turn depends on the passive elements such as resistors.
  •     The values of resistors remain fairly constant because they can be selected very precisely with almost zero temperature co efficient of resistance.
  •     When there is a certain change in the internal resistance of the amplifier due to some reasons, we now can find the corresponding percentage change in the overall gain of the feedback amplifier.
  •     Differentiating equation (1) with respect to A , we get






























Therefore the sensitivity is defined as the ration of percentage change in voltage gain with feedback to the percentage change in voltage gain without feedback.
The reciprocal of the term sensitivity is called desensitivity, i.e. desensitivity is (l+Af3)

 REDUCTION IN NON-LINEAR DISTORTION:
  •     A large signal stage has non-linear distortion.
  •     The reason is that the voltage gain changes at various points in the cycle. The use of negative feedback in large signal amplifiers reduces the non-linear distortion.
  •     Let the amplifier with gain A produces a distortion D with feedback. Suppose when feedback is applied, the gain becomes Af and the distortion becomes D Now,
  •     Fraction of output distortion feedback to the input = j3D
  •     After amplification, the distortion output = A3D















  •     This result is of great importance in the design of high-power audio amplifiers. Feedback must be employed in such equipment to reduce the harmonic distortion to the low levels required.
  •      The required distortion is usually less than 1 percent of the amplifier power output.

 REDUCTION u NOISE:

  •     Noise is always present in voltage as there is a noise voltage in the amplifier.
  •     The output of the amplificr consists of output signal and noise voltage (say N).
  •     After the amplification of negative feedback, the noise voltage Nf is given by,






 CHANGE IN INPUT IMPEDENCE:

  •     Due to the application of negative feedback, the input impedance increases.
  •     High input impedance is desirable in an amplifier because it will not load the input voltage source.
























































CHANGE IN OUTPUT IMPEDENCE:


  •     The output impedance decreases due to the application of negative feedback.
  •     Output impedance is desirable in amplifier because it is capable of delivering














  •     Here Z is the output impedance of the amplifier without feedback. In order to find the output impedance of the amplifier with feedback, short circuit the input source V and connect a voltage source V at the output terminal. The output side has been replaced by an equivalent voltage source AI3VQ.
  •     Let I be the current from the applied source.
 
For the output circuit,



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