Op amp circuit oscillations

Discussion in 'General Electronics Chat' started by rage_speed, Aug 30, 2009.

  1. rage_speed

    Thread Starter New Member

    Jun 29, 2009
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    Hi this was an interview question. I have uploaded the circuit

    This is an op-amp circuit with neg(r,r resistances) and pos(3r,r resistances) feedbacks. If because of noise the initial output is at Vsat, then will the circuit oscillate between vsat and -vsat?
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    From a quick thought and assuming no parasitic capacitance the output voltage will go to zero volts and stay there (with little oscillations around 0V due to delay in the amplifiers response).
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    It is a Schmitt Trigger, which means it is stable until a threash hold voltage is reached.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The net feedback is negative. I don't see a Schmitt trigger there, even if it had an input.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    You're right, the negitive feedback swamps the positive feedback. If you were to remove the negitive feedback it would still be stable, just bistable.
     
  6. rage_speed

    Thread Starter New Member

    Jun 29, 2009
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    0
    Hi,
    Thanks for the reply.
    I am not able to understand how it will go to 0v because when the output is at Vsat the voltage at the negative terminal is Vsat/2 and at positive terminal is Vsat/4, the difference is negative so the output voltage should be -Vsat.

    If the output is -Vsat then the net voltage difference at the terminals is positive so the output should go back to Vsat and the circuit should oscillate isn't it ?

    I am new to the field kindly excuse my ignorance.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    No problem, we like questions here. Figure out what would happen if the output of the op amp is high (max positive voltage), then work out what the voltage would be at the inputs (such as ½ the power supply voltage, or ¼ the power supply voltage), and follow through how the op amp would react to this. The answer will be obvious.
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    You are wise to not just accept the answer because there is a concern as to whether the circuit is AC stable. The wording of the question leads me to believe that the interviewer expects you to see that the net feedback is negative. However, he wants to know if you can tell if it is AC stable. The simple DC solution is 0V, but this DC solution is only correct if the circuit is AC stable. Otherwise the circuit can oscillate between +Vsat and -Vsat.

    In general, an AC analysis is needed to prove stability; however, experience tells most of us that the circuit is too simple to have a stability problem if the net feedback is negative. There just is not enough phase shift from components to get a net 180 degree phase shift (i.e. net positive feedback) at any frequency. A compensated OPAMP has a 90 degree phase shift for AC frequencies. So the circuit is DC stable and AC stable under reasonable assumptions. Those assumptions would be as follows:

    1. Use of a compensated (single-pole) opamp
    2. Good circuit board layout with low paracitics
    3. Good power supply decoupling with appropriate filter capacitors.
     
  9. Perfectboy

    New Member

    Sep 9, 2009
    9
    0
    Actually the analysis of #3 is perfectly matched with my classroom texts. I agree with him.
    [​IMG]
     
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