FEEDBACK AMPLIFIER by infojustwin

VIEWS: 14 PAGES: 20

									ECE, ME – I SEM


   BASIC
ELECTRONICS
Feedback amplifiers
A)   Introduction
B)   Types of Feedback
C)   Effect of feedback (positive & negative) on Gain
     & Bandwidth
WHAT DO YOU MEAN BY
FEEDBACK?
  When a part or fraction of output is
  combined to input, feedback is said to be
  exist.
 Types of feedback
  a) Negative Feedback: When the Feedback
  voltage or current) is so applied as to weaken the
  input signal it is in phase opposition to the input
  signal), It is called negative, inverse or
  degenerative feedback.
  b) Positive Feedback: When the Feedback
  voltage or current) is so applied as to increase the
  input signal it is in phase same to the input
  signal), It is called positive, generative feedback.
Concept of amplifier feedback(NEGATIVE)




                      xo     A
               Af              , if A f  A, then negative feedback
                      x s 1  A
               A f : the closed  loop gain of the amplifier
               A : the open  loop gain of the amplifier
                : feedback coefficient
               A : loop gain

   If A  1, then A f  1 /  .
                                Thus, the closed-loop gain would be much
     more stable and is nearly independent of changes of open-loop gain

                                 A
      If A  1, x f  x s           x s , so xi  x s  x f  0.
                                                            Thus in a negative
                               1  A
      feedback amplifier, the output takes the value to drive the amplifier
     input to almost 0 (this is summing point constraints).
Positive feedback

        xo    A
 Af             , if A f  A, then Positive feedback
        xs 1  A
 A f : the closed  loop gain of the amplifier
A : the open  loop gain of the amplifier
 : feedback coefficient
 A : loop gain


If A  1, then A f  inf inite
Feedback Types
   If the feedback network samples the output voltage, it is voltage feedback.
    If it samples the output current, it is current feedback.
   The feedback signal can be connected in series or in parallel with the
    signal source and the amplifier input terminals, so called series feedback
    and parallel feedback.
   So, there are four types of negative feedback in amplifier circuits:
       Series voltage feedback
       Series current feedback
       Parallel voltage feedback
       Parallel current feedback
Voltage Series Feedback

  Voltage-sampling series-mixing (series-shunt)
  topology
Voltage Shunt Feedback
Voltage-sampling shunt-mixing (shunt-shunt)
 topology.
Current Series


 current-sampling series-mixing (series-series) topology
Current Shunt

  current-sampling shunt-mixing (shunt-series)
                     topology
Effect of Negative feedback

   Gain Desensitized

   Bandwidth increases
Gain Desensitivity


Feedback can be used to desensitize the closed-loop gain to variations in the basic
  amplifier. Let’s see how.
Assume beta is constant. Taking differentials of the closed-loop gain equation
  gives…


                                  A                               dA
Divide by Af           Af                            dAf 
                               1  A                         1  A 2

                         dAf            dA 1  A   1 dA
                                                 
                          Af        1  A 2 A 1  A A

This result shows the effects of variations in A on Af is mitigated by the feedback
  amount. 1+Abeta is also called the desensitivity amount
We will see through examples that feedback also affects the input and resistance of
  the amplifier (increases Ri and decreases Ro by 1+Abeta factor)
Bandwidth Extension in Negative
feedback
       AvL  f  
                       Avm
                   1  jf 1 / f

       Avmf  f  
                       Avm
                    1  Avm


       Avlf  Avl / 1  Avl

       AvLf  f   Avm /(1  jf 1 / f )                 Avm
                   1   ( Avm /(1  jf 1 / f )) 1  jf 1 / f  Avm




Notice that the midband gain reduces by (1+AMbeta) while the 3-dB
roll-off frequency increases by (1+AMbeta)
Divide the numerator and denominator by 1+Avmβ
Thus lower cut off frequency reduces.
NOTICE THAT THE MIDBAND GAIN REDUCES BY
(1+AMBETA) WHILE THE 3-DB ROLL-OFF FREQUENCY
INCREASES BY (1+AVMBETA)


     AvLf  f  
                     Avmf
                  1  jf 1" / f

     Avmf  f  
                     Avm
                  1   Avm

      f   1"    f 1 / 1   Avm
f2= upper cut off frequency


It is increased by (1+Avmβ )
f2 “ = f2(1+Avmβ )
Hence bandwidth increases
Advantage of Negative Feedback

    –Desensitized gain – gain less sensitive to circuit
     component variations .Gain is stabilized.
    –Reduce nonlinear distortion – output proportional to
     input (constant gain independent of signal level)
    –Reduce effect of noise
    – input impedance increases and output impedances
     decreases
    Extend bandwidth of amplifier.
DISADVANTAGES:

 A) less gain
 B) possible oscillation if not properly designed.
EFFECT OF POSITIVE FEEDBACK ON BANDWIDTH


   Bandwidth decreases.(derive by yourself as like
    in Negative Feedback)
DISADVANTAGES:

A) Very prone to oscillation.
B) Poor freq. response, more distortion, more drift.



ADVANTAGES:
A) High gain

								
To top