Lecture 2 Feedback Amplifiers

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Lecture 2 Feedback Amplifiers

  1. 1. Lecture 2 Feedback Amplifier <ul><li>Introduction of Two-Port Network </li></ul><ul><li>Negative Feedback (Uni-lateral Case) </li></ul><ul><li>Feedback Topology </li></ul><ul><li>Analysis of feedback applications </li></ul><ul><ul><li>Close-Loop Gain </li></ul></ul><ul><ul><li>Input/Output resistances </li></ul></ul>
  2. 2. 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
  3. 3. Two-Port Network ( y -parameters) (Short-Circuit Admittance) Short-circuit input admittance At port 1 Short-circuit reverse transadmittance At port 2 Short-circuit forward transadmittance Short-circuit output admittance
  4. 4. Two-Port Network ( h -parameters) (hybrid) Short-circuit input impedance At port 1 Open-circuit reverse voltage gain At port 2 Short-circuit forward current gain Open-circuit output admittance
  5. 5. Two-Port Network ( g -parameters) (inverse-hybrid) Open-circuit input admittance At port 1 Short-circuit reverse current gain At port 2 Open-circuit forward current gain Short-circuit output impedance
  6. 6. 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) Z 12 = Z 21 (reciprocal circuit) (4) Z 12 = Z 21 and Z 11 = Z 22 (symmetrical and reciprocal circuit)
  7. 7. y -parameter example
  8. 8. 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?
  9. 9. General Feedback Structure A : Open Loop Gain A = V o / V   : feedback factor  = V f / V o
  10. 10. Negative Feedback Properties <ul><li>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 </li></ul><ul><ul><li>Desensitized gain : gain less sensitive to circuit component variations </li></ul></ul><ul><ul><li>Reduce nonlinear distortion : output proportional to input (constant gain independent of signal level) </li></ul></ul><ul><ul><li>Reduce effect of noise </li></ul></ul><ul><ul><li>Control input and output impedances by applying appropriate feedback topologies </li></ul></ul><ul><ul><li>Extend bandwidth of amplifier </li></ul></ul><ul><li>All of these properties can be achieved by trading off gain </li></ul>
  11. 11. Gain De-sensitivity <ul><li>Feedback can be used to desensitize the closed-loop gain to variations in the basic amplifiler. </li></ul><ul><li>Assume  is constant. Take differentials of the closed loop gain equation gives, </li></ul><ul><li>Divided by A v , the close loop gain sensitivity is equal to, </li></ul><ul><li>This result shows the effects of variations in A on A CL is mitigated by the feedback amount. </li></ul><ul><li>(1+ A  ) is also called the desensitivity amount. </li></ul>Differential respected with A
  12. 12. 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)
  13. 13. Feedback Structure (Series-Shunt) <ul><li>Voltage amplifier voltage-controlled voltage source </li></ul><ul><li>Requires high input impedance, low output impedance </li></ul><ul><li>Voltage-voltage feedback </li></ul>Voltage Gain Calculation:
  14. 14. Input/Output Resistance (Series-Shunt) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  15. 15. h -parameter Modeling <ul><li>Only uni-lateral case </li></ul><ul><li>will be considered : </li></ul><ul><li>NO reverse dependent signal found in the amplifier network. |h 12a | = 0 </li></ul><ul><li>NO reverse dependent signal found in the feedback network. |h 21f | = 0 </li></ul>
  16. 16. Uni-lateral
  17. 17. Series-Shunt Example Equivalent circuit <ul><li>It is observed that: </li></ul><ul><li>Series connection in input ports </li></ul><ul><li>Shunt connection in output ports </li></ul><ul><li> Series-Shunt connection </li></ul><ul><li>h -parameter should be used. </li></ul>
  18. 18. h -parameter analysis 1
  19. 20. Feedback Structure (Series-Series)
  20. 21. Input/Output Resistance (Series-Series) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  21. 22. Series-Series Example CE amplifier with an un-bypassed emitter ac small signal equivalent circuit
  22. 23. Feedback Network with z -parameter Reduce equivalent circuit
  23. 24. Close loop analysis
  24. 25. Final R in and R out
  25. 26. Feedback Structure (Shunt-Shunt)
  26. 27. Input/Output Resistance (Shunt-Shunt) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  27. 28. Shunt-Shunt Example CE amplifier ac small signal equivalent circuit Shunt-Shunt connection found!  y-parameter
  28. 29. Feedback Network y -parameter modeling
  29. 31. Feedback Structure (Shunt-Series)
  30. 32. Input/Output Resistance (Shunt-Series) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  31. 33. Summary y -parameter Shunt-Shun g -parameter Shunt-Series z -parameter Series-Series h -parameter Series-Shunt Parameter used Output impedance Input impedance Close loop gain Feedback Structure
  32. 34. Supplementary Find the input and output resistance from - Two port network, and - Circuit theory
  33. 35. Circuit Theory
  34. 36. Two Port Network 

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