1. FEEDBACK CIRCUITS CLASSIFICATION OF AMPLIFIER CONCEPT OF FEEDBACK
TYPES OF FEEDBACK
FEEDBACK TOPOLOGIES
Reference Books: 1.Electronic Devices and circuit theory, int. edition (Chapter 14) 2.Microelectronics circuits, 5th ed. Sedra and smith (Chapter 8) 3. Microelectronic circuits: analysis and design (Chapter 10)

2. Classification of Amplifiers
Before proceeding with the concept of feedback it is useful to classify amplifiers into 4 basic categories based on their input & output signal relationships.
Voltage amplifier
Current amplifier
Transconductance amplifier
Transresistance amplifier

3. Classification of amplifiers
1. Voltage amplifier if
then
and if
then,
hence
with
represent the open circuit voltage gain.

4. Classification of amplifiers
2. Current amplifier if
then
and if
then,
hence
with
represent the short circuit current gain.

5. 3.Transconductance amplifier
Classification of amplifiers
3.Transconductance amplifier if
then
and if
then,
hence
with
represent the short circuit mutual or
transfer conductance

6. 4. Transresistance amplifier
Classification of amplifiers
4. Transresistance amplifier if
then
and if
then ,
hence
with
represent the open circuit mutual or
transfer resistance.

7. Concept of feedback Valve example
As the water nears the specified level, the valve is closed.
Negative feedback is most commonly used to control systems.

8. Types of feedback What is feedback?
Feedback is a technique where a proportion of the output of a system (amplifier) is fed back and recombined with input.
There are two types of feedback amplifier.
Positive feedback
Negative feedback

9. Types of feedback 1.Positive Feedback
Positive feedback is the process when the output is added to the input, amplified again, and this process continues.
Example: In a PA system, you get feedback when you put the microphone in front of a speaker and the sound gets uncontrollably loud (you have probably heard this unpleasant effect).

10. Types of feedback 2. Negative Feedback
Negative feedback is when the output is subtracted from the input.
Example: Speed control. If the car starts to speed up above the desired set-point speed, negative feedback causes the throttle to close, thereby reducing speed; similarly, if the car slows, negative feedback acts to open the throttle.
The use of negative feedback reduces the gain. Part of the output signal is taken back to the input with a negative sign.

11. Negative Feedback Gain
Types of feedback
Negative Feedback Gain
The gain with feedback (or closed-loop gain) Af as follows:
The quantity A is called the loop gain, and the quantity (1+A) is called the amount of feedback.

12. Advantages of Negative Feedback
Stabilization of gain
make the gain less sensitive to changes in circuit components e.g. due to changes in temperature.
Reduce non-linear distortion
make the output proportional to the input, keeping the gain constant, independent of signal level.
Reduce the effect of noise
minimize the contribution to the output of unwanted signals generated in circuit components or extraneous interference.

13. Advantages of Negative Feedback (cont.)
4. Extend the bandwidth of the amplifier
Reduce the gain and increase the bandwidth
5. Modification the input and output impedances
raise or lower the input and output impedances by selection of the appropriate feedback topology.
Disadvantages of Negative Feedback
1. Circuit gain - Reduce
2. Stability – Tend to be oscillate

14. Basic structure of feedback amplifier
A : open-loop gain
B : feedback factor  xs xi xf xo + - A
Error signal
The gain of the feedback amplifier
If AB is very large, the overall gain becomes a function of the feedback network only.

15. Basic structure of feedback amplifier
Basic structure of a single - loop feedback amplifier

16. Feedback Network This block is usually a passive two-port network.
contain resistors, capacitors, and inductors.
Usually it is simply a resistive network.

17. Sampling Network
The output voltage is sampled by connecting the feedback network in shunt across the output.
Type of connection is referred to as voltage or shunt or node sampling.

18. Sampling Network (cont.)
The output current is sampled by connecting the feedback network in series with the output
Type of connection is referred to as current or series or loop sampling.

19. Comparator or Mixer Network
voltage - applied feedback .
identified as voltage or series or loop mixing.

20. Comparator or Mixer Network (Cont.)
current - applied feedback
identified as current or shunt or node mixing.

21. Feedback Topologies
Four basic feedback topologies based on the parameter to be amplified (voltage or current) and the output parameter (voltage or current).
The four feedback circuit can be described by the types of connections at the input and output of circuit.

22. 1. Series-shunt topology
Feedback Topologies
1. Series-shunt topology
Voltage-mixing voltage-sampling

23. 2.Shunt-series topology
Feedback Topologies
2.Shunt-series topology
Current-mixing current-sampling

24. 3.Series-series topology
Feedback Topologies
3.Series-series topology
Voltage-mixing current-sampling

25. Current-mixing voltage-sampling
Feedback Topologies
4.Shunt-shunt topology
Current-mixing voltage-sampling

26. Ideal series-shunt feedback
Feedback Topologies
Ideal series-shunt feedback
Rof
Rif Ri Ii Vi Vs Vf Vo
AVi Ro
Vo

27. Ideal series-shunt feedback (cont.)
Feedback Topologies
Ideal series-shunt feedback (cont.)
1. Input resistance with feedback
2.Output resistance with feedback

28. Practical series-shunt feedback
Feedback Topologies
Practical series-shunt feedback Rs Vs
Rif
Rin
Rout
Rof Vo

29. Ideal shunt-series feedback
Feedback Topologies
Ideal shunt-series feedback If Is Io
Rif
Rof Vi Ii Ri Ro
AIi
Io

30. Ideal shunt-series feedback (cont.)
Feedback Topologies
Ideal shunt-series feedback (cont.)
1. Gain of feedback amplifier
2. Input resistance with feedback
3.Output resistance with feedback

31. Practical shunt-series feedback
Feedback Topologies
Practical shunt-series feedback RL Io
Rout
Rin Is

32. Ideal series-series feedback
Feedback Topologies
Ideal series-series feedback
Rof
Rif
- Vf + Vs Io
Io Vi
Ri AVi Ro Ii

33. Ideal series-series feedback (cont.)
Feedback Topologies
Ideal series-series feedback (cont.)
1. Gain of feedback amplifier
2. Input resistance with feedback
3.Output resistance with feedback

34. Practical series-series feedback
Feedback Topologies
Practical series-series feedback RL Vs Rs
Rout
Rin Io

35. Ideal shunt-shunt feedback
Feedback Topologies
Ideal shunt-shunt feedback Ii Is If Vi Ri Ro
Rof
Rif Vo
AIi
Vo

36. Ideal shunt-shunt feedback
Feedback Topologies
Ideal shunt-shunt feedback

37. Practical shunt-shunt feedback
Feedback Topologies
Practical shunt-shunt feedback Rs Is
Rout
Rin RL Vo

38. Feedback relationship
Feedback Topologies
Feedback relationship
Gain
Input resistance
Output resistance
Without feedback A Ri Ro
Series-shunt
Series-series
Shunt-shunt
Shunt-series

39. Feedback Amplifier Topologies

40. FEEDBACK CIRCUITS Exercise
Consider the non-inverting op-amp circuit with parameters R1 = 10 k, R2 = 30 k, and A = 104. Assume Ri = . Determine the closed-loop voltage gain. If the open-loop gain increases by a factor of 10, what is the percent change in the closed-loop gain?
(Ans : , 0.036%)