5 ee415 - adv electronics - presentation - schwappach

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5 ee415 - adv electronics - presentation - schwappach

  1. 1. Feedback Instability By: Loren Schwappach Prepared for: Professor Monica Young CTU – EE415 – Advanced Electronic Design September 2010 1
  2. 2. Overview• Definition of Stability• Equations• Design Considerations• Unstable Design• Stable Design• Questions 2
  3. 3. Instability:In a negative feedback system, a portion of the outputsignal is subtracted from the input signal to produce anerror signal.However, this error may change as a function of frequencycausing subtraction to result in addition.Negative feedback becomes positive feedback resulting inan unstable system. 3
  4. 4. How to check for stability:Use a Nyquist frequency plot or verifymathematically.How to add stability:•Decrease gain•Add pole•Use compensating capacitor 4
  5. 5. Equations for OPAMP filter design. 1 1 1 = = = 1 1 + 1 + + 1 + = 1 + 1 + 1 1 + 2 2 2 + 2 + 5
  6. 6. The Design: Requirements: Design a 3 stage filter for an audio signal (voice components (0 to 4kHz), amplification is preferred but not a requirement. Design 1: Four pole filter, using non-inverting Amps for large gain at cutoff designed as: 1 1 = → = → = 5Ω → = 1.59 2 × × × 2 × × 20 × 1.59 Design #2: Drop gain to 1, and add pole to stabilize circuit at 0dB. To find new pole:−180° = − tan−1 −135 / + tan−1 −135 / + tan−1 −135 / 6
  7. 7. Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): PSpice Schematic - Audio Filter/Amplifer Designed by Loren K. Schwappach Designed for Cutoff of 20kHz Actual Cutoff = 6kHz Vcc 0 Gain-1 5Vdc Vin 7 uA741 3 V+ 5 Buffer-10 + OS2 R3 7 uA741 6 3 V+ 5 Buffer-2 1mVac R1 OUT + OS2 R4 7 uA741 0Vdc 2 1 5 1.59u 6 3 V+ 5 Buffer-30 - 4 OS1 C1 OUT + OS2 R5 7 uA741 1 V- 2 1 5 C2 6 3 V+ 5 Gain-4 - 4 OS1 1.59u OUT + OS2 R6 7 uA741 V- 2 1 5 C3 6 3 V+ 5 R2 - 4 OS1 1.59u OUT + OS2 V- 2 1 5 C4 6 0 - 4 OS1 R7 OUT 0 1.59u 50 V- 2 1 0 - 4 OS1 0 V- 1 0 100 R8 Vee0 -5Vdc 7
  8. 8. Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): 8
  9. 9. Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results 9
  10. 10. Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results 10
  11. 11. Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results 11
  12. 12. Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results PSpice Schematic - Audio Filter/Amplifier - Stable Designed by Loren K. Schwappach Designed for Cutoff of 20kHz 0Vdc 1mVac Actual cutoff 5.689kHz Vcc Vin 7 3 5 0 V+0 7 + OS2 R9 5Vdc Amp-1 6 3 5 V+ 7 R1 OUT + OS2 R3 2 1 70 1.59u B-4 6 3 5 V+0 7 - 4 OS1 C5 OUT + OS2 R4 1 5 uA741 V- 2 1 1.59u B-1 6 3 5 V+ 7 R2 - 4 OS1 C1 OUT + OS2 R5 5 uA741 V- 2 1 1.59u B-2 6 3 5 V+ 7 - 4 OS1 C2 OUT + OS2 R6 1 0 5 uA741 V- 2 1 1.59u B-3 6 3 5 V+ - 4 OS1 C3 OUT + OS2 0 5 uA741 V- 2 1 1.59u Amp-2 6 - 4 OS1 C4 R7 OUT 0 uA741 V- 2 1 0 - 4 OS1 0 1 uA741 V- R8 Vee 00 1 -5Vdc 12
  13. 13. Simple 4 pole LP Filter w/ OPAMP buffer’s (Stable): Results 13
  14. 14. Simple 4 pole LP Filter w/ OPAMP buffer’s (stable): Results 14
  15. 15. Simple 4 pole LP Filter w/ OPAMP buffer’s (stable): Results 15
  16. 16. Conclusions:• After reducing gain to 1, I was still in an unstablecondition forcing me to add another pole.•By adding another pole I was finally able to stabilizethe design.• If I was going to push forward with this design, Iwould need to either increase the LP cutoff frequency,recalculate/change the added pole, and add at least3dB gain. 16
  17. 17. Questions: 17

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