Feedback amplifiers

1. Feedback amplifiers
Mrs.V.SrirengaNachiyar, AP (SG) /ECE
Ramco Institute of Technology
Academic Year:2017-2018 (Odd)

2. Introduction- Feedback
• Feedback is taking a part of the output back to
the input either in phase or out of phase with
the input signal.
• Feedback is classified into two types based on
how the feedback signal is feedback with
respect to the input signal.
• They are
Positive or regenerative feedback
Negative or Degenerative feedback

3. Positive or regenerative
feedback
The feedback signal is fed in phase with
the input signal

4. Negative or Degenerative
Feedback
• The feedback signal is fed out of phase with
the input signal.
• Negative feedback is employed in amplifiers
to improve their performance and positive
feedback is employed in oscillators to cause
instability and hence result in oscillations

5. Feedback amplifiers
• An amplifier which involves negative feedback is
generally defined as a feedback amplifier.
Effect of Negative feedback:
 Improves stability of the circuit
Input resistance and Output resistance can
be changed with feedback network
Improves the frequency response of the
amplifier
Reduces the percentage of harmonic
distortion
Improves the SNR

6. Block diagram of a feedback
amplifier

7. • Source:
The first functional block of a generalized
feedback system is the source
The source can be either a voltage source
or a current source
A voltage source is represented as a
Thevenin’s network and a current source is
represented as a Norton’s network

8. • Amplifier:
Amplifier is basically classified into 4
types based on the input given and output
obtained. They are:
Voltage amplifier
Current amplifier
Trans-conductance amplifier
Trans- resistance amplifier

9. • Sampling Network:
This functional block samples either the
output current or output voltage and gives to the
feedback network. Since it is available at the
output it affects the output resistance of the
feedback amplifier. Based on the sampled signal,
the sampler is classified as
a. Voltage sampler
b. Current sampler

10. • Mixer Network:
The mixer mixes the feedback signal with
the input signal. Mixing decides whether the
input resistance Ri increases or decreases.
Mixing also explains whether the feedback
signal mixes in-phase with the input signal or
out-of phase with the input signal. The mixer can
be divided into two types
 Series Mixer
 Shunt Mixer

11. References
• Electronic Circuits II- B.Sasikala & S.
Poornachandra
• https://www.sciencedirect.com/topics/engineer
ing/positive-feedback-loop
• https://www.electronics-
tutorials.ws/systems/negative-feedback.html
• https://electronicscoach.com/feedback-
amplifier.html