i made this feedback amplifiers presentation for engineering students who are pursuing 2nd year i hope it'll be helpful for them.

1. FEEDBACK AMPLIFIERS
Presented By Harit Mohan
B-tech (CSE-CCVT) 3rd sem
University of technology and
Management
Shillong

2. OUTLINE
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Definition
Classification of Basic Amplifiers
Amplifier without Feedback or with Feedback
Basic Concept of Feedback
General characteristics of negative-feedback amplifiers
Effect of negative feedback on input-output resistance
Method of identifying Feedback

3. Definition
Feedback is defined as the process in which a
part of output signal (voltage or current) is
returned back to the input.
The amplifier that operates on the principle of
feedback is known as feedback amplifier.

4. CLASSIFICATION OF AMPLIFIERS
•
•
Voltage Amplifier- An amplifier provides a voltage output
proportional to the voltage input and the proportionality
factor does not depend on the magnitudes of the source
and load resistance
Current Amplifier- An amplifier which provides an output
current proportional to the signal current .
•
Transconductance Amplifier- An amplifier in which, the
output current is proportional to the signal voltage,
independent of the magnitudes of source and load
resistance.
•
Transresistance Amplifier- An amplifier in which output
voltage is proportional to the signal current of the
magnitudes of source and load resistance.

5. Amplifier Without Feedback
• The most important thing to understand from Fig. is
that the output and input terminals of this amplifier
are not connected to each other in any way.
• Therefore the amplifier of Fig. is an amplifier without
any feedback,
Gain without feedback.

6. Amplifier With Feedback
• Refer to Fig. Here the same amplifier with a gain A is
being used along with a mixer network, sampling
network and a feedback network.
• The voltage gain of the feedback amplifier is given by,

7. BASIC CONCEPT OF FEEDBACK

8. • In each one of these circuits we may sample
the output voltage or current by means of a
suitable sampling network and apply this
signal to the input through feedback two-port
network.
• At the input the feedback signal is combined
with the external (source) signal through a
mixer network and is fed into the amplifier
proper.

9. Types of feedback
1. Positive feedback
2. Negative feedback.
 If the original input signal and the
feedback signal are in phase, the
feedback is called as positive feedback.
 However if these two signals are out of
phase then the feedback is called as
negative feedback.

10. General characteristics of negative
feedback amplifier
Stabilization of voltage gain
Decreasing output
impedance
Increasing input impedance Decreasing distortion

11. Effect of negative feedback on inputoutput resistance
• Input resistancefeedback amplifiers ,series mixing at the input
tends to increase the input resistance and shunt
mixing tends to decrease the input resistance.
• Output resistance
feedback amplifiers, voltage sampling at the
output tends to decrease the output resistance and
current sampling tends to increase the output
resistance.

12. Method od identifying feedback topology
Step1 -To find the type of sampling network
• By shorting the output if feedback signal becomes
zero, then it is called “voltage sampling “
• By opening the output loop if feedback signal
becomes zero, then it is called “current sampling”.
Step 2- To find the type of mixing network
• If the feedback signal is subtracted from the
externally applied signal as a voltage in the input
loop , it is called “series mixing”.
• If the feedback signal is subtracted from the
externally applied signal as a current in the loop it is
called “shunt mixing”.

13. Thus by finding the type of sampling network
and mixing network ,type of feedback amplifier
can be determined.

14. Thank you…