Chapter 5: FEEDBACK IN AMPLIFIER Dr. Gopika Sood, PG Govt. College For Girls, Sector -11, Chandigarh

OUTLINE  Feedback in Amplifier  Advantages Of Negative Feedback  Stability of gain  Noise reduction  Reduction in Non-linear and Phase Distortion  Effect on Input and Ouput Impedance  Improvement of Frequency Response  Emitter Follower  Operational Amplifier

FEEDBACK IN AMPLIFIER FEEDBACK : It is a process in which a portion of the output energy (voltage or current) is transferred to the input of the system. Feedback in amplifier is of two types :  Positive, Direct or regenerative feedback  Negative, Inverse or Degenerative feedback Positive feedback : If the feedback energy is in phase with the input energy, it reinforces or enhances the input signal energy, then it is called positive feedback. Negative feedback : When the feedback energy is in out of phase (180 o phase) with the input energy, it reduces or diminishes the input signal energy, the feedback is called positive feedback. Classification of Feedback Voltage Feedback : It is the feedback in which the feedback energy is proportional to the output voltage of the amplifier. Current feedback : It is the feedback in which the feedback energy is proportional to the output current of the amplifier. Compound feedback : It is the feedback in which the feedback energy is partly proportional to the output voltage of the amplifier and partly proportional to the output current.

A - gain of the amplifier with no feedback V o – Output of amplifier V i – Input voltage of amplifier V s – Input signal voltage to the feedback amplifier V f - Feedback voltage A f – Voltage gain with feedback THEORY FOR FEEDBACK

The feedback fraction can be written as  = V f /V o V i = V s  V f V i = V s – V f Voltage gain with feedback A f = V o /V i V o = A V i V o = A (V s - V f ) V o (1+A  ) = AV s V o /V s = A/(1+A  ) A f = A/(1+ A  ) THEORY FOR FEEDBACK

A f = A/(1+ A  ) The feedback fraction  of output can be positive, negative or real number or a complex function of frequency. The following cases may arise from the above equation  If | 1+A  | > 1, we have |A f | < |A| then feedback is negative feedback which is used in amplifiers  If | 1+A  | = 0, we have |A f | =  implying that amplifier can give the output without input signal, the amplifier becomes an oscillator.  If | 1+A  | |A| then feedback is positive feedback. Feedback causes instability in the oscillations in the operation of the transistor CONDITIONS NECESSARY FOR FEEDBACK

ADVANTAGES OF NEGATIVE FEEDBACK Negative feedback produces various advantageous effects to improve the performance of amplifiers  Gain of the amplifier is stabilized against the variations I the values of trnsistor hybrid parameters like h fe  It reduces the non-linear distortion and improves the signal handling capacity of the amplifier  The noise level in the output of the amplifier is reduced  The phase distortion is reduced  The bandwidth of the amplifier is increased and the frequency distortion is reduced  Input and output impedances of the amplifier may be changed by combining the feedback signal with the externally applied signal

STABILITY OF GAIN Overall gain of the amplifier is given by A f = A/(1+ A  ) The stability of the gain can be reduced by differentiating the eq log A f = log A – log (1+ A  ) dA f /A f = dA/A ((1+  A -  A)/ /(1+ A  )) = dA/A (1/(1+ A  )) S = (dA f /A f )/(dA/A) = (1/(1+ A  )) Desensitivity, D = 1/S = (1/(1+ A  )) dA f /A f = 1/D (dA/A)

REDUCTION IN NON-LINEAR DISTORTION

REDUCTION IN NOISE Negative feedback reduces the noise level in the output signal V no = noise output without feedback V n = input noise voltage without feedback A = voltage gain of the internal amplifier V no = AV n V’ no = noise output with feedback V’ n = input noise voltage with feedback V’ n = V n -  V’ no V’ no = AV’ n = A(V n -  V’ no ) V’ no = AV n - A  V’ no V’ no (1+A  ) = AV n V’ no = 1/(1+A  ) AV n

IMPROVEMENT IN FREQUENCY RESPONSE

REDUCTION IN PHASE DISTORTION