AC amplitude changing with phase

Discussion in 'General Electronics Chat' started by hunterage2000, May 25, 2012.

  1. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    Did you see post #11? The inputs are diff amplitudes but same phase, the gain is 21, the output is amplified with same phase. My circuit I made on breadboard outputs zero amplitude at zero phase but increases with increase in phase on the generator.
     
  2. t_n_k

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    Mar 6, 2009
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    Hello Hunterage2000,

    You can probably sense that, despite the best efforts of several members, we haven't quite got the gist of what you are doing.

    Perhaps I could venture a fuller elaboration on your behalf which may or may not be correct.

    Anyway here goes.... [Note: text in italics reflects an even greater level of fanciful speculation or uncertainty on my part]

    1. You are building an instrumentation amplifier with a notional gain of 21.
    2. The amplifier is intended to amplify the output of a bridge network - perhaps such as one would find with a load cell measurement [although the gain seems somewhat low for a load cell amplifier].
    3. The bridge excitation is from a low frequency 60Hz AC source.
    4. You first simulate the arrangement and it seems to work conceptually as you intend.
    5. You have then built the amplifier and want to see if it works in practice.
    6. [To test the amplifier you would presumably set up a signal source and a bridge network as the input but what you have actually done is unclear to me.]
    7. It seems for testing purposes that you bypass the bridge component & simply apply the two differential amplifier input signals from a function generator source - a variable phase oscillator - VPO.
    8. [You offer no schematic for the specific connection of the two signals from the VPO to the amplifier and post only a fuzzy picture showing four wires coming out and going to an unknown connection arrangement.]
    9. The two signals are of slightly different magnitude [as measured by an oscilloscope??] & the phase difference between the two signals can be adjusted from 0° to any desired value. The phase difference is initially set to 0° [per the VPO dial settings].
    10. At 0° phase difference setting there is apparently no output from the amplifier even though there is a small amplitude difference betweeen the two input signals. [Why no output with a difference input?]
    11. Confused by the outcome you [for unknown reasons - twiddling?] progressively change the phase difference setting between the two signals and as you do, you notice the amplifier output increases and then eventually clips.
    12. You are further confused by this stange outcome.
    Is that a [more or less] correct summary?
     
    Last edited: May 26, 2012
  3. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    That is correct yes. At 1st the phase wasnt at zero and there was an output then I changed the phase to zero and the amplitude was zero
     
  4. t_n_k

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    At the risk of being overly pedantic I would ask you to confirm that the VPO produces two sinusoidal outputs - one reference output and one phase adjustable output. Looking at the VPO picture the top output terminals appear to be the reference 0° output and the lower output terminals are the phase shifted output.

    It's difficult to understand why the amplifier output is zero with the two VPO signals having unequal magnitude values - albeit with zero phase difference.

    Can you confirm measuring the two specified amplitudes with a CRO?

    If you have a DMM I would check the difference voltage on lowest AC range directly between the VPO active [red] output terminals [with the common / ground terminals connected]. You should see a difference value with the unit set up as shown in the image.

    How did you connect the 4 wires shown exiting the VPO. Did you connect the common / ground wires together at the amplifier input common? Presumably the active leads were connected to the amplifier differential inputs.

    If the instrumentation amp output is definitely zero with the two input signals having different magnitude at 0° phase difference then I would suggest there is a problem in your circuit setup or a wiring error. This is why you need to confirm directly by measurement that there is a magnitude difference between the two VPO signals.

    I would also be pedantic once more and make the point that relative phase shifting the two VPO output signals [away from 0°] will produce an instantaneous difference in their values - even if they have identical peak-to-peak values at the same frequency. This instantaneous difference would be seen by the instrumentation amplifier and amplified to produce an output signal. If this point is not clear to you then you have a gap in your understanding which you need to resolve with some careful thinking.
     
  5. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    I havnt got my oscilloscope with me so I will check tomorrow and post the info.
     
  6. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    I checked the circuit again and I got the amplified signal I expected, but now the problem is when I have V2 = 3V pk-pk and V1 = 2.6V pk-pk and vice-versa there is no 90 deg phase shift and both are inverted.
     
  7. BillB3857

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    Feb 28, 2009
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    Change the sync on your scope.
     
  8. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    both x and y axis is aligned for both on the oscilloscope. in the simulation, I get V2 > V1 as a positive output and V2 < V1 as a negative output.
     
  9. BillB3857

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    Looking back to your schematic, if the input to the top first stage amplifier is larger than the input to the lower first stage amplifier, what would you expect out of the 3rd amplifier with a larger input on the inverting input?
     
  10. t_n_k

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    Where is the 90° coming from? What are you referring to?

    Presummably you mean 180° when you swap the inputs.....???
     
  11. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    yeah 180 deg, I expected outputs like the attached but get inverted for both. Any ideas why?
     
  12. t_n_k

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    Unfortunately your posts are becoming increasingly cryptic. I've lost your train of thought.
     
  13. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    At the output, the oscilloscope is showing the attached for both V2 = 3Vpk-pk and V1 = 2.6Vpk-pk and vice-versa while the simulation shows what the outputs should be.
     
  14. BillB3857

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    Can you please identify which side of your bridge you are calling V1 and likewise, identify the side of the bridge you are calling V2. I assumed (yes, in this case it really does what it says, if you know what I mean) that V1 was being fed into the top op amp of your schematic and V2 to the lower op amp. Please provide more complete information. I will result in more complete help/problem determination/education/etc.
     
  15. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    the right side is V1 going into the top op-amp, the left side is V2 going into the bottom op-amp.

    The attached shows the simulations output when V2 = 3Vpk-pk and V1 = 2.6Vpk-pk on the left which is non-inverted and when V2 = 2.6Vpk-pk and V1 = 3Vpk-pk on the left which is inverted.

    For the signal generator I posted previously, I have the top output set as 3Vpk-pk going into pin 3 of the bottom op-amp and the 2nd going to ground, the 3rd output is set as 2.6Vpk-pk going into pin 3 of the top op-amp and the 4th goes to ground in which I get the non-inverted output I shouldnt get and vice-versa where I get an inverted output which is what I should get. This is shown in the attached image of digital oscilloscope output from the actual circuit that is wired correctly. Both are in phase at zero degrees.
     
  16. BillB3857

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    Your responses continue to confuse me. My fault, I'm sure. If the larger signal is going to the top left op amp of your circuit, the output of the far right hand op amp should be 180 out of phase of the input. If the larger signal is going to the lower left op amp of your circuit, the output should be in phase with the input. If you go back to your original post that shows the signal generator, schematic and scope representation, with the resistor values shown in the bridge, if the scope output shares a proper sync with the input, it appears to be correct. I really have no knowledge beyond that. Maybe others can better decipher what you are trying to communicate.
     
  17. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    thats correct, thats what I should be getting but for some reason Im not. I know that the placement and values of the resistors is correct and that the op-amps are being supplied with a dc voltage. Im back in the lab tomorrow so Im going to check that each op-amp is not fried but a bit confused that Im getting the correct amplitude for the output just not the correct phase for when V2 = 3 and V1 = 2.6V
     
  18. BillB3857

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    Phase relationships can be a real problem. Back in the dark ages, we had multiple 5 axis machine tools that were supposedly identical. Ironically, the motors on these machines were not directly interchangeable due the phasing of the internal feedback resolvers. It seemed that there was no real consistency of the wiring to the controls. You may just need to accept the results of your circuit and swap inputs to get the desired outputs.
     
  19. hunterage2000

    Thread Starter Senior Member

    May 2, 2010
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    when I swap both over I get the same inverted output, I dont know why, I wonder how they do it in industry? possibly they use an all in one integrated instrumentation amplifier chip, I dont know.
     
  20. BillB3857

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    If I understand properly, no matter which input is larger, the output amplitude is always the same phase (180) phase relationship with the input????
     
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