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Feedback Amplifiers Department of EE PowerPoint Presentation

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  • Slide 1 - Ref:080130HKN EE3110 Feedback Amplifiers 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
  • Slide 2 - Ref:080130HKN EE3110 Feedback Amplifiers 2 Two-Port Network (z-parameters) (Open-Circuit Impedance)
  • Slide 3 - Ref:080130HKN EE3110 Feedback Amplifiers 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
  • Slide 4 - Ref:080130HKN EE3110 Feedback Amplifiers 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
  • Slide 5 - Ref:080130HKN EE3110 Feedback Amplifiers 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
  • Slide 6 - Ref:080130HKN 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)
  • Slide 7 - Ref:080130HKN EE3110 Feedback Amplifiers 7 y-parameter example
  • Slide 8 - Ref:080130HKN EE3110 Feedback Amplifiers 8 y-parameter example (Cont’)
  • Slide 9 - Ref:080130HKN EE3110 Feedback Amplifiers 9 General Feedback Structure A : Open Loop Gain A = Vo / V  : feedback factor  = Vf / Vo
  • Slide 10 - Ref:080130HKN 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
  • Slide 11 - Ref:080130HKN 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
  • Slide 12 - Ref:080130HKN 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)
  • Slide 13 - Ref:080130HKN EE3110 Feedback Amplifiers 13 Feedback Structure (Series-Shunt) Voltage amplifier voltage-controlled voltage source Requires high input impedance, low output impedance Voltage-voltage feedback Voltage Gain Calculation:
  • Slide 14 - Ref:080130HKN EE3110 Feedback Amplifiers 14 Input/Output Resistance (Series-Shunt) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  • Slide 15 - Ref:080130HKN EE3110 Feedback Amplifiers 15 h-parameter Modeling
  • Slide 16 - Ref:080130HKN EE3110 Feedback Amplifiers 16 Uni-lateral
  • Slide 17 - Ref:080130HKN EE3110 Feedback Amplifiers 17 Series-Shunt Example Equivalent circuit It is observed that: Series connection in input ports Shunt connection in output ports  Series-Shunt connection h-parameter should be used.
  • Slide 18 - Ref:080130HKN EE3110 Feedback Amplifiers 18 h-parameter analysis 1
  • Slide 19 - Ref:080130HKN EE3110 Feedback Amplifiers 19
  • Slide 20 - Ref:080130HKN EE3110 Feedback Amplifiers 20 Feedback Structure (Series-Series)
  • Slide 21 - Ref:080130HKN EE3110 Feedback Amplifiers 21 Input/Output Resistance (Series-Series) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  • Slide 22 - Ref:080130HKN EE3110 Feedback Amplifiers 22 Series-Series Example CE amplifier with an un-bypassed emitter ac small signal equivalent circuit
  • Slide 23 - Ref:080130HKN EE3110 Feedback Amplifiers 23 Feedback Network with z-parameter Reduce equivalent circuit
  • Slide 24 - Ref:080130HKN EE3110 Feedback Amplifiers 24 Close loop analysis
  • Slide 25 - Ref:080130HKN EE3110 Feedback Amplifiers 25 Final Rin and Rout
  • Slide 26 - Ref:080130HKN EE3110 Feedback Amplifiers 26 Feedback Structure (Shunt-Shunt)
  • Slide 27 - Ref:080130HKN EE3110 Feedback Amplifiers 27 Input/Output Resistance (Shunt-Shunt) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  • Slide 28 - Ref:080130HKN EE3110 Feedback Amplifiers 28 Shunt-Shunt Example CE amplifier ac small signal equivalent circuit Shunt-Shunt connection found!  y-parameter
  • Slide 29 - Ref:080130HKN EE3110 Feedback Amplifiers 29 Feedback Network y-parameter modeling
  • Slide 30 - Ref:080130HKN EE3110 Feedback Amplifiers 30
  • Slide 31 - Ref:080130HKN EE3110 Feedback Amplifiers 31 Feedback Structure (Shunt-Series)
  • Slide 32 - Ref:080130HKN EE3110 Feedback Amplifiers 32 Input/Output Resistance (Shunt-Series) Input Resistance: Output Resistance (Closed loop output resistance with zero input voltage)
  • Slide 33 - Ref:080130HKN EE3110 Feedback Amplifiers 33 Summary
  • Slide 34 - Ref:080130HKN EE3110 Feedback Amplifiers 34 Supplementary Find the input and output resistance from - Two port network, and - Circuit theory
  • Slide 35 - Ref:080130HKN EE3110 Feedback Amplifiers 35 Circuit Theory
  • Slide 36 - Ref:080130HKN EE3110 Feedback Amplifiers 36 Two Port Network 
  • Slide 37 - Ref:080130HKN EE3110 Feedback Amplifiers 37

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