FEED BACK AMPLIFIERS

INTRODUCTION

  •     The Voltage gain, Input impedance, Output impedance, Bandwidth etc., are few important characteristics of an Amplifier.
  •     These parameters are nearly constant for a given amplifier. Sometimes, we are required to change these parameters. This can be achieved by a technique known as feedback.   

  •     When a part or fraction of output is combined to the input, feedback is said to exist. Thus the process of combining a fraction of output energy back into the input is known as feedback.
  •     The feedback technique is broadly classified as positive and negative feedback.

NEGATIVE FEEDBACK:

  •     If the net effect of feedback is to reduce the magnitude of the input signal, it is called as negative feedback. [ called as Inverse or Degenerative Feedback.] Negative feedback reduces the gain of the amplifier.

ADVATAGES OF NEGATIVE FEEDBACK:


  •     Stabilization of Gain
  •     Reduction in Distortion
  •     Reduction in Noise
  •     Increase in Input Impedance
  •     Decrease in Output Impedance
  •     Increases the range of Uniform Amplification.

POSITIVE FEEDBACK:

  •     If the net effect of the feedback is to increase the magnitude of the input signal, it is called as Positive.
  •      Direct or Regenerative Feedback. This positive f/b has the disadvantage of increased distortion and instability.
  •     So, positive feedback is seldom used in amplifiers.


TYPES OF FEEDBACK:

  •     The f/b amplifiers are classified into general classes:
  •     VOLTAGE FEEDBACK
  •     CURRENT FEEDBACK
  •     High Voltage Feedback, the voltage fed back to the input terminal is proportional to the output voltage.
  •     In Current Feedback, the voltage fed back to the input terminal is proportional to the current through the load.
GENERAL THEORY OF FEEDBACK:

The feedback amplifier has two parts i.e.,
  •     Amplifier
  •      Feedback circuit

The feedback circuit usually consists of resistors. This returns a fraction (say j3 ) of the output voltage back to the input.















  •     Let A be the gain of the amplifier i.e. the ratio of output voltage V to the input voltage V This is the gain of the amplifier without feedback.
  •     The feedback network extracts a voltage Vç jW from the output V of the amplifier. 
  •     This voltage is added(positive fib) or subtracted (negative Q’b) from the signal voltage V .  Now,






















FOR POSITIVE FEEDBACK:

  •     If the feedback signal 4 is in phase with input signal 4 then the net effect of the feedback will increase the input signal given to the amplifier i.e. = hence, the input voltage applied to the basic amplifier is increased thereby increasing 4 exponentially.
  •     This type of feedback is said to be positive or regenerative feedback.
  •     Gain of the amplifier with positive feedback is:













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