opamp modelling in spice

Discussion in 'General Electronics Chat' started by rhlee, May 14, 2012.

  1. rhlee

    Thread Starter New Member

    May 5, 2012
    10
    0
    I'm trying to understand op amps through working with simulations in ng-spice.

    I decided to start with a very simple buffer circuit using the UA741 op amp from TI.

    [​IMG]

    I fed in a sine wave voltage. And I expected the same thing coming out. However the output was clipped.

    [​IMG]

    What am I doing to cause the output to be clipped?

    Here's the netlist:

    Code ( (Unknown Language)):
    1. * Spice netlister for gnetlist
    2. R3 0 4 100
    3. R1 1 5 100
    4. R2 2 3 100
    5. XOA1 3 4 5 0 4 UA741
    6. V2 0 2 sin(-2.5v 1v 1Hz)
    7. V1 1 0 5v
    8. .END
    And the schematic file if you're interested:

    Code ( (Unknown Language)):
    1. v 20110115 2
    2. C 40000 40000 0 0 0 title-B.sym
    3. C 43300 47000 1 270 0 voltage-3.sym
    4. {
    5. T 44000 46800 5 8 0 0 270 0 1
    6. device=VOLTAGE_SOURCE
    7. T 43800 46700 5 10 1 1 270 0 1
    8. refdes=V1
    9. T 43300 47000 5 10 1 0 0 0 1
    10. value=5v
    11. }
    12. C 45200 45000 1 90 0 voltage-1.sym
    13. {
    14. T 44700 45100 5 10 0 0 90 0 1
    15. device=VOLTAGE_SOURCE
    16. T 44700 45300 5 10 1 1 90 0 1
    17. refdes=V2
    18. T 45200 45000 5 10 1 0 0 0 1
    19. value=sin(-2.5v 1v 1Hz)
    20. }
    21. C 46000 47600 1 0 0 opamp-1.sym
    22. {
    23. T 46700 48400 5 10 0 0 0 0 1
    24. device=OPAMP
    25. T 46700 48200 5 10 1 1 0 0 1
    26. refdes=XOA1
    27. T 46700 49000 5 10 0 0 0 0 1
    28. symversion=0.1
    29. T 46000 47600 5 10 1 0 0 0 1
    30. value=UA741
    31. }
    32. N 43500 49000 48500 49000 4
    33. N 43500 46100 43500 45000 4
    34. N 43500 45000 48500 45000 4
    35. C 45100 45900 1 90 0 resistor-1.sym
    36. {
    37. T 44700 46200 5 10 0 0 90 0 1
    38. device=RESISTOR
    39. T 44800 46100 5 10 1 1 90 0 1
    40. refdes=R2
    41. T 45100 45900 5 10 1 0 0 0 1
    42. value=100
    43. }
    44. C 43300 44700 1 0 0 ground.sym
    45. C 46300 47300 1 0 0 ground.sym
    46. N 46500 48400 46500 49000 4
    47. N 47000 48000 47000 47000 4
    48. N 46000 48200 45000 48200 4
    49. N 45000 48200 45000 46800 4
    50. N 46000 47000 48500 47000 4
    51. N 48500 47000 48500 46500 4
    52. N 48500 45600 48500 45000 4
    53. C 43600 47000 1 90 0 resistor-1.sym
    54. {
    55. T 43200 47300 5 10 0 0 90 0 1
    56. device=RESISTOR
    57. T 43300 47200 5 10 1 1 90 0 1
    58. refdes=R1
    59. T 43600 47000 5 10 1 0 0 0 1
    60. value=100
    61. }
    62. N 43500 49000 43500 47900 4
    63. C 48600 45600 1 90 0 resistor-1.sym
    64. {
    65. T 48200 45900 5 10 0 0 90 0 1
    66. device=RESISTOR
    67. T 48300 45800 5 10 1 1 90 0 1
    68. refdes=R3
    69. T 48600 45600 5 10 1 0 0 0 1
    70. value=100
    71. }
    72. N 46000 47800 46000 47000 4
    I got the op amp spice model from here:

    http://www.ti.com/product/ua741

    Richard
     
  2. #12

    Expert

    Nov 30, 2010
    16,278
    6,791
    I think you are choosing an op-amp that was designed to work with +/- 15 volts and giving it a lot less to work with. Perhaps you could try an amplifier that is less than 40 years old? Or feed the one you have with the voltage it likes?
     
    rhlee likes this.
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The load on the output is too heavy. The 741 spice model can only source and sink 25mA.
    You are trying to source up to 35mA. Change the load to 200 ohms or more.
     
    rhlee likes this.
  4. rhlee

    Thread Starter New Member

    May 5, 2012
    10
    0
    Thanks guys,

    I used the low power op amp LM 358 and put a greater load on the output. All works now.
     
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