E-notes on feedback amplifiers and its Types

1. Lecture Notes on Feedback
Amplifiers and its Types
for
Open Educational Resource
On
Electronic Devices and Circuits (EC201)
by
Dr. Piyush Charan
Assistant Professor
Department of Electronics and Communication Engg.
Integral University, Lucknow

2. Contents
Basic concept of feedback
General Characteristics of negative feedback amplifiers
Classification of feedback
Voltage/Current shunt and series feedback
stability of feedback amplifiers
Multistage Amplifiers, and
Tuned Amplifier.
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3. Feedback
• Feedback implies feeding back (i.e., returning back) a part of the
processed signal to the input side so as to enhance or diminish the
input signal.
• When the input signal (current or voltage) is diminished, it is
considered as negative feedback.
• When the input signal is enhanced, it is known as positive feedback.
• Negative feedback is employed in amplifying systems to achieve
certain special characteristics that are not obtainable from the basic
amplifier.
• Positive feedback is employed to produce signal generator, such as
oscillators.
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4. Feedback
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5. Types of Feedback
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6. Feedback Amplifier
• An amplifier which involves negative feedback is known as the Feedback
amplifier
• Introducing feedback varies the performance parameters of amplifier.
• Effects of Negative Feedback:
1. Improves stability of the circuit.
2. Input and output resistance can be changed with feedback network.
3. Improves frequency response of amplifier.
4. Reduces the percentage of harmonic distortion.
5. Improves the signal to noise ratio(SNR).
6. Reduces the gain of the circuit.
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7. Block Diagram of Feedback
Amplifier
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8. Types of Sources
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9. Amplifiers
• Based on the magnitudes of the input and output
impedances of an amplifier relative to the source
and load impedances, respectively, amplifiers can
be classified into four broad categories as:
(a) Voltage amplifier,
(b) Current amplifier,
(c) Trans-conductance amplifier, and
(d) Trans-resistance amplifier.
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10. Voltage Amplifier
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11. Current Amplifier
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12. Transconductance Amplifier
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13. Transresistance Amplifier
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14. Input and Output Impedances of
Basic Amplifiers
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15. Sampling Network
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16. Mixer Network
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Series Mixing Shunt Mixing

17. Feedback Network
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18. Feedback Topologies
The four basic feedback topologies: (a) voltage-mixing voltage-sampling (series–shunt) topology; (b) current-mixing current-sampling
(shunt–series) topology; (c) voltage-mixing current-sampling (series–series) topology; (d) current-mixing voltage-sampling (shunt–shunt)
topology.
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19. Examples
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20. Examples
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21. Effects of Negative feedback
• Reduces the transfer ratio
• Increases Input resistances
• Increases gain stability
• Increases Bandwidth
• Decreases noise and distortion
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22. Reduction in Transfer Ratio
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