Ground the output of a Wien Bridge? What happens?

Discussion in 'Homework Help' started by Denny1234, Apr 16, 2009.

  1. Denny1234

    Thread Starter Member

    Feb 17, 2008
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    Nothing right?

    I got this question where after my simulation I have to ground the output of my wien bridge on multisim and then 'investigate the effect of grounding the output'.

    Its seems impossible, im right in saying that i just get ZERO OUTPUT :confused:!!??- And nothing interesting to report!?

    Thnx for any help
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    You forgot to post the schematic of your oscillator for us to see if it has anything that might be damaged when the output is shorted.

    If it uses an opamp then the opamp will limit the current to avoid damage.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    The input impedance will change also, and become a simple RC curve. I'm assuming this is not an oscillator.
     
  4. Denny1234

    Thread Starter Member

    Feb 17, 2008
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    Yeh its an oscillator, i have attached an image - its probably awful to look at - cant see what the question wants. Its not a biggy, just the last question for my report and I cnat find a way of looking into it using the Multisim Simus
     
  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    The output of the bridge is the + input of the opamp.

    If you ground the output of the bridge, the oscillator will stop oscillating. Maybe that's all they want you to say.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well I see that you have an unsymmetrical power supply +12, -5 volts.

    This places the output at +3.5 volts.
    The op amp shown is short circuit current limited at the ouput so will not suffer.
    So it is not 'nothing' that will happen on SC. You will be driving the op amp's current limit current to earth.
     
  7. The Electrician

    AAC Fanatic!

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    As I understood what he was to do, it was to ground the Wien bridge output, not the opamp output. Did you understand it differently?
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Not sure what you are getting at here?

    The Wien network feedsback the op amp output through the top 2μ7 capacitor (which itself is part of the Wien network). The network phase shifts frequencies selectively such that at the frequency of oscillation the feedback becomes positive promoting the oscillation.
    The other end of the Wien network is connected to ground.

    This whole arrangement is called a Wien oscillator.

    The output of this oscillator, from the op amp output, is shown as connected to the scopemeter input probe2. This is the output for both AC (the oscillation) and DC. The conditions are clearly noted in the list by the scopemeter, including the 3.3 volts DC offset.

    The op amp settles at DC half way between the power rails. Normally that would be at zero volts (earth) but in this case the rails are not symmetrical so there is a DC offset of 3.5 volts as I noted and shown on the diagram.

    Since there is no ouput blocking capacitor this DC also appears at the output.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The opamp does not have DC on its output (just a small input offset voltage multiplied by 3).
    The input is biased at 0V so the output is also 0VDC.
     
  10. studiot

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    Nov 9, 2007
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    Dunno what happened to my post,

    but why does the meter attached to probe 2 ( meter input and op amp output) in the simulation show 3.33v DC?
     
  11. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    If you'll tell me why the meter shows V(p-p) = 0 and V(rms) = 0, then I'll tell you why the meter shows 3.33v DC. :)

    Maybe the meter is broken; damaged by an overload? :)
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    But it also shows a sine wave?

    We don't know what its setting are of course; could it be set to dc?
     
  13. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    At the very least, it will ceast to oscillate, because your feedback path is now to ground. :)

    eric
     
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