1. SAFFRONY INSTITUTE TECHNOLOGY
FEEDBACK AMPLIFIER
SUBMITTED BY : Suchit Darji
GUIDED BY: Prof. Nipa Modi

2. EE3110 Feedback Amplifiers
Introduction of Two-Port Network
Negative Feedback (Uni-lateral Case)
Feedback Topology
Analysis of feedback applications
Close-Loop Gain
Input/Output resistances
Ref:080130HKN
EE3110 Feedback Amplifiers

3. Two-Port Network (z-parameters)
(Open-Circuit Impedance)
Open-circuit
input impedance
At port 1
Open-circuit reverse
transimpedance
At port 2
Open-circuit forward
transimpedance
Open-circuit
output impedance
Ref:080130HKN
EE3110 Feedback Amplifiers

4. Two-Port Network (y-parameters)
(Short-Circuit Admittance)
At port 1
At port 2
Short-circuit forward
transadmittance
Short-circuit
input admittance
Short-circuit reverse
transadmittance
Short-circuit
output admittance
Ref:080130HKN
EE3110 Feedback Amplifiers

5. Two-Port Network (h-parameters)
(hybrid)
At port 1
At port 2
Short-circuit forward
current gain
Short-circuit
input impedance
Open-circuit reverse
voltage gain
Open-circuit
output admittance
Ref:080130HKN
EE3110 Feedback Amplifiers

6. Two-Port Network (g-parameters)
(inverse-hybrid)
At port 1
At port 2
Open-circuit forward
current gain
Open-circuit
input admittance
Short-circuit reverse
current gain
Short-circuit
output impedance
Ref:080130HKN
EE3110 Feedback Amplifiers

7. EE3110 Feedback Amplifiers
z-parameter example
Note: (1) z-matrix in the last circuit = sum of two former z-matrices
(2) z-parameters is normally used in analysis of series-series circuits
(3) Z12 = Z21 (reciprocal circuit)
(4) Z12 = Z21 and Z11 = Z22 (symmetrical and reciprocal circuit)
Ref:080130HKN
EE3110 Feedback Amplifiers

8. EE3110 Feedback Amplifiers
y-parameter example
Ref:080130HKN
EE3110 Feedback Amplifiers

9. y-parameter example (Cont’)
Note: the y-matrix is equal to the sum of two former ones.
Therefore, y-parameters is normally used in analysis of shunt-shunt circuits
What connection should be for h- or g- parameters?
Ref:080130HKN
EE3110 Feedback Amplifiers

10. General Feedback Structure
A : Open Loop Gain
A = Vo / V
 : feedback factor
 = Vf / Vo
Ref:080130HKN
EE3110 Feedback Amplifiers

11. Negative Feedback Properties
Negative feedback takes a sample of the output signal and applies it to the input to get several desirable properties. In amplifiers, negative feedback can be applied to get the following properties
Desensitized gain : gain less sensitive to circuit component variations
Reduce nonlinear distortion : output proportional to input (constant gain independent of signal level)
Reduce effect of noise
Control input and output impedances by applying appropriate feedback topologies
Extend bandwidth of amplifier
All of these properties can be achieved by trading off gain
Ref:080130HKN
EE3110 Feedback Amplifiers

12. EE3110 Feedback Amplifiers
Gain De-sensitivity
Feedback can be used to desensitize the closed-loop gain to variations in the basic amplifiler.
Assume  is constant. Take differentials of the closed loop gain equation gives,
Divided by Av, the close loop gain sensitivity is equal to,
This result shows the effects of variations in A on ACL is mitigated by the feedback amount.
(1+A) is also called the desensitivity amount.
Differential respected with A
Ref:080130HKN
EE3110 Feedback Amplifiers

13. Basic Feedback Topologies
Depending on the input signal (voltage or current) to be amplified and form of the output (voltage or current), amplifiers can be classified into four categories. Depending on the amplifier category, one of four types of feedback structures should be used.
(Type of Feedback) (Type of Sensing)
(1) Series (Voltage) Shunt (Voltage)
(2) Series (Voltage) Series (Current)
(3) Shunt (Current) Shunt (Voltage)
(4) Shunt (Current) Series (Current)
Ref:080130HKN
EE3110 Feedback Amplifiers

14. Feedback Structure (Series-Shunt)
Voltage Gain Calculation:
Voltage amplifier voltage-controlled voltage source
Requires high input impedance, low output impedance
Voltage-voltage feedback
Ref:080130HKN
EE3110 Feedback Amplifiers

15. Input/Output Resistance (Series-Shunt)
Input Resistance:
Output Resistance
(Closed loop output resistance with zero input voltage)
Ref:080130HKN
EE3110 Feedback Amplifiers

16. EE3110 Feedback Amplifiers
h-parameter Modeling
Only uni-lateral case
will be considered :
NO reverse dependent signal found in the amplifier network. |h12a| = 0
NO reverse dependent signal found in the feedback network. |h21f| = 0
Ref:080130HKN
EE3110 Feedback Amplifiers

17. EE3110 Feedback Amplifiers
Uni-lateral
Ref:080130HKN
EE3110 Feedback Amplifiers

18. EE3110 Feedback Amplifiers
Series-Shunt Example
It is observed that:
Series connection in input ports
Shunt connection in output ports
 Series-Shunt connection
h-parameter should be used.
Equivalent circuit
Ref:080130HKN
EE3110 Feedback Amplifiers

19. EE3110 Feedback Amplifiers
h-parameter analysis 1
Ref:080130HKN
EE3110 Feedback Amplifiers

20. EE3110 Feedback Amplifiers
Ref:080130HKN
EE3110 Feedback Amplifiers

21. Feedback Structure (Series-Series)
Ref:080130HKN
EE3110 Feedback Amplifiers

22. Input/Output Resistance (Series-Series)
Input Resistance:
Output Resistance
(Closed loop output resistance with zero input voltage)
Ref:080130HKN
EE3110 Feedback Amplifiers

23. Series-Series Example
CE amplifier with an un-bypassed emitter
ac small signal equivalent circuit
Ref:080130HKN
EE3110 Feedback Amplifiers

24. Feedback Network with z-parameter
Reduce equivalent circuit
Ref:080130HKN
EE3110 Feedback Amplifiers

25. EE3110 Feedback Amplifiers
Close loop analysis
Ref:080130HKN
EE3110 Feedback Amplifiers

26. EE3110 Feedback Amplifiers
Final Rin and Rout
Ref:080130HKN
EE3110 Feedback Amplifiers

27. Feedback Structure (Shunt-Shunt)
Ref:080130HKN
EE3110 Feedback Amplifiers

28. Input/Output Resistance (Shunt-Shunt)
Input Resistance:
Output Resistance
(Closed loop output resistance with zero input voltage)
Ref:080130HKN
EE3110 Feedback Amplifiers

29. EE3110 Feedback Amplifiers
Shunt-Shunt Example
CE amplifier
ac small signal equivalent circuit
Shunt-Shunt connection found!  y-parameter
Ref:080130HKN
EE3110 Feedback Amplifiers

30. EE3110 Feedback Amplifiers
Feedback Network
y-parameter modeling
Ref:080130HKN
EE3110 Feedback Amplifiers

31. EE3110 Feedback Amplifiers
Ref:080130HKN
EE3110 Feedback Amplifiers

32. Feedback Structure (Shunt-Series)
Ref:080130HKN
EE3110 Feedback Amplifiers

33. Input/Output Resistance (Shunt-Series)
Input Resistance:
Output Resistance
(Closed loop output resistance with zero input voltage)
Ref:080130HKN
EE3110 Feedback Amplifiers

34. EE3110 Feedback Amplifiers
Summary
Feedback Structure
Close loop gain
Input impedance
Output impedance
Parameter used
Series-Shunt
h-parameter
Series-Series
z-parameter
Shunt-Shun
y-parameter
Shunt-Series
g-parameter
Ref:080130HKN
EE3110 Feedback Amplifiers

35. EE3110 Feedback Amplifiers
Supplementary
Find the input and output resistance from
- Two port network, and
- Circuit theory
Ref:080130HKN
EE3110 Feedback Amplifiers

36. EE3110 Feedback Amplifiers
Circuit Theory
Ref:080130HKN
EE3110 Feedback Amplifiers

37. EE3110 Feedback Amplifiers
Two Port Network 
Ref:080130HKN
EE3110 Feedback Amplifiers

38. EE3110 Feedback Amplifiers
Ref:080130HKN
EE3110 Feedback Amplifiers

39. EE3110 Feedback Amplifiers
THANKYOU
Ref:080130HKN
EE3110 Feedback Amplifiers