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Lecture 2 Feedback Amplifier

Lecture 2 Feedback Amplifier. Introduction of Two-Port Network Negative Feedback (Uni-lateral Case) Feedback Topology Analysis of feedback applications Close-Loop Gain Input/Output resistances. At port 1. At port 2. Open-circuit forward transimpedance. Open-circuit input impedance.

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Lecture 2 Feedback Amplifier

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  1. Lecture 2 Feedback Amplifier • Introduction of Two-Port Network • Negative Feedback (Uni-lateral Case) • Feedback Topology • Analysis of feedback applications • Close-Loop Gain • Input/Output resistances EE3110 Feedback Amplifiers

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

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

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

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

  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) Z12 = Z21 (reciprocal circuit) (4) Z12 = Z21 and Z11 = Z22 (symmetrical and reciprocal circuit) EE3110 Feedback Amplifiers

  7. y-parameter example EE3110 Feedback Amplifiers

  8. 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? y-parameter example (Cont’) EE3110 Feedback Amplifiers

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

  10. 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 EE3110 Feedback Amplifiers

  11. 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 EE3110 Feedback Amplifiers

  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) EE3110 Feedback Amplifiers

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

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

  15. 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 h-parameter Modeling EE3110 Feedback Amplifiers

  16. Uni-lateral EE3110 Feedback Amplifiers

  17. 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 EE3110 Feedback Amplifiers

  18. h-parameter analysis 1 EE3110 Feedback Amplifiers

  19. EE3110 Feedback Amplifiers

  20. Feedback Structure (Series-Series) EE3110 Feedback Amplifiers

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

  22. Series-Series Example CE amplifier with an un-bypassed emitter ac small signal equivalent circuit EE3110 Feedback Amplifiers

  23. Feedback Network with z-parameter Reduce equivalent circuit EE3110 Feedback Amplifiers

  24. Close loop analysis EE3110 Feedback Amplifiers

  25. Final Rin and Rout EE3110 Feedback Amplifiers

  26. Feedback Structure (Shunt-Shunt) EE3110 Feedback Amplifiers

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

  28. Shunt-Shunt Example CE amplifier ac small signal equivalent circuit Shunt-Shunt connection found!  y-parameter EE3110 Feedback Amplifiers

  29. Feedback Network y-parameter modeling EE3110 Feedback Amplifiers

  30. EE3110 Feedback Amplifiers

  31. Feedback Structure (Shunt-Series) EE3110 Feedback Amplifiers

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

  33. Summary EE3110 Feedback Amplifiers

  34. Supplementary Find the input and output resistance from - Two port network, and - Circuit theory EE3110 Feedback Amplifiers

  35. Circuit Theory EE3110 Feedback Amplifiers

  36. Two Port Network EE3110 Feedback Amplifiers

  37. EE3110 Feedback Amplifiers

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