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Analog Electronics Class 2: Ideal Op-Amp Analysis

Analog Electronics Class 2: Ideal Op-Amp Analysis. Sep 9, 2011. Ideal Op-Amp. Voltage Controlled Voltage Source. Ideal Op-Amp Characteristics. Open Loop Voltage Gain (A vol ) is infinite Input offset Voltage is zero Input bias current is zero No power supply limits

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Analog Electronics Class 2: Ideal Op-Amp Analysis

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  1. Analog Electronics Class 2:Ideal Op-Amp Analysis Sep 9, 2011

  2. Ideal Op-Amp

  3. Voltage Controlled Voltage Source

  4. Ideal Op-Amp Characteristics • Open Loop Voltage Gain (Avol) is infinite • Input offset Voltage is zero • Input bias current is zero • No power supply limits • Input impedance is infinite • Simple model is a voltage controlled voltage source with high gain.

  5. Close Loop Gain

  6. Feedback Analogy Scotty Increase the output voltage! The input offset voltage is not zero! I’ll try caption, but I’m giving her all she’s got!

  7. Simple Ideal Amp Analysis

  8. Simple Analysis for Inverting Amp Vin across Rin • Iin flows through Rf • Vout is the voltage across Rf

  9. Simple Analysis for Inverting Amp Find the same transfer function algebraically

  10. Non-Inverting Amp

  11. Op-Amp Buffer

  12. Current Reference

  13. Current Reference

  14. Current Reference VR1 = VREF = 10V IGND≠0A IB=0A The voltage from noninverting input to the output is zero (i.e. no Vosi).

  15. Simulating the DC Operating Point

  16. Probe Different Nodes You can probe any node. In this case I am probing V3. Voltage at meters is displayed. Vout = -4V

  17. Circuits to Memorize

  18. Common Amplifiers Buffer Vout = Vin

  19. Superposition

  20. Superposition principle • Used for circuits with multiple input sources. • Analyze the output response for one source at a time. • Short unused voltage sources • Open unused current sources • Repeat the analysis for each source • Add all the response from each analysis to get the overall system respones

  21. Superposition Example 2: Single Supply Amp (Noninverting)

  22. Superposition Example 1: Diff Amp

  23. AC Analysis

  24. Cf – Filter (High Gain) • At high frequency CF will short RF • High Freq Gain = 0/1k+1 = 1 • At low frequency CF acts like an open • DC Gain = (99k/1k)+1 = 100

  25. Low Frequency Gain = 100 (40dB) High Frequency Gain = 1 (0dB) Zero at f= 16.08kHz 40dB / 20dB/dec = 2 decades. -45o / decade +45o / decade

  26. Cf – Filter (Low Gain) • At high frequency CF will short RF • High Freq Gain = 0/1k+1 = 1 • At low frequency CF acts like an open • DC Gain = (2k/1k)+1 = 3

  27. Low Frequency Gain = 3 (9.54dB) High Frequency Gain = 1 (0dB) Zero -45o / decade +45o / decade

  28. Spice: Find the Transfer Function

  29. The inverting amp with Cf • At high frequency CF will short RF • High Freq Gain =- 0/1k = 0 • At low frequency CF acts like an open • DC Gain = -(2k/1k) = -2

  30. Comparing responses in Tina(Non-inverting vs. Inverting) • Use “control-C” to copy the graph. • Use the tabs (see bottom) to change plots. • Use “control-V” to past the response into a different plot

  31. Nodal Analysis

  32. Nodal Analysis • Sum of all currents at a node is zero • Assume all currents flow out of node • Assume node is the highest voltage potential • For Op-Amps • Ib = 0A • Vosi = 0V (voltage between op-amp inputs)

  33. Nodal Analysis on simple circuit

  34. Nodal Analysis: Transimpedance Amp

  35. Improved Howland Current Pump

  36. Mathcad: A New Tool

  37. Mathcad • Does symbolic analyses of transfer functions • Substitute, Simplify, and Solve Equations • Numerical evaluation of complex expressions • Document your work • Mathcad is not a number crunching software package like Matlab or Mathematica

  38. Select the expression and press control-C. Select the variable that you want to substitute. Use menu option: Symbolics\Variable\Substitute The expression with the substitution will appear below. This expression is simplified.

  39. Select the variable that you want to simplify. Use menu option: Symbolics\Simplify Result of simplification

  40. Evaluate an expression for specific values. Note the equal sign is different for evaluations then symbolic. “CTRL =“ Used for symbolic “:” used for numeric evaluation

  41. Past function here. Past independent variable here. Adjust axis scale as needed.

  42. Diodes and Transient Analysis

  43. Select sine wave, amplitude, and period. Click on Vg1 to edit signal. Click on “…” button to edit signal.

  44. Select transient analysis. Result window Adjust period to match signals period (5 x period in this example).

  45. Real Diode vs. Ideal Diode

  46. Add the diode into the circuit and run the same transient analysis.

  47. Analysis results.

  48. Half wave rectifier. Note: the forward voltage is zero. The op-amp makes the diode ideal.

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