# AC amplitude changing with phase

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

1. ### hunterage2000 Thread Starter Active Member

May 2, 2010
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Hi, Im using a signal generator and I have 2 AC Signals going from it to a 3 opamp instrumentation amplifier. If I have the phase angle at 0, there is no output signal but as I increase it the amplitude increases to a point then clips. Can anyone suggest why this is?

2. ### crutschow Expert

Mar 14, 2008
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3,244
The instrumentation amp has a differential input so it rejects (does not amplify) common-mode voltages (any identical voltages on both inputs). It only amplifies differential mode signals (those different between the two inputs).

When the signal to the two inputs is in phase, the signals are identical so there is no amplification. As you vary the phase, the two signals become different and this difference is amplified by the instrumentation amplifier. That's perfectly normal behavior for such an amp.

3. ### hunterage2000 Thread Starter Active Member

May 2, 2010
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so for an AC bridge circuit with an excitation source of 50Hz, if there is an imbalance of voltage withe same frequency, how can it be amplified?

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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As long as there is an instantaneous amplitude difference between the two inputs (presumably ground referenced) you will have an instantaneous output. In the case of two equal amplitude signals exactly in phase there is no instantaneous difference - hence no output from a differential amplifier.

In the case of an ac excited unbalanced bridge there will be an instantaneous amplitude difference between the two bridge arms connected to the differential amp input and this would result in an output from the difference amplifier.

5. ### hunterage2000 Thread Starter Active Member

May 2, 2010
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yes, one input is 3Vpk-pk and the other 2.6. Not sure why this is happening. zero output at zero phase then the amplitude increases with increase in phase.

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Well one would also make sure there is no DC offset in either signal.

That being the case then ...

If the phase difference between the two signals is 0° then the difference between them will be 0.4V peak-to-peak or 0.141V rms.

If the phase difference between the two signals was 30° then the difference between them will be 3-2.6@30°=3.0-(2.25+j1.3)=0.748-j1.3=1.5@-60° pk-pk
or 0.53V rms

So changing the phase difference from 0° to 30° has increased the rms difference by 0.39V

This explains why the diff amp output increases as the phase difference increases - albeit with the two signals having fixed rms values.

7. ### hunterage2000 Thread Starter Active Member

May 2, 2010
400
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Why would changing the phase angle, change the amplitude though? The phase angle of both signals are the same, they come out of the same generator.

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
OK.

Perhaps if you post a schematic of your setup then it would make understanding the problem much easier.

You say there are two signals from a common source - but they have different amplitudes. Are these two signals derived from a bridge excited by the one AC source? It's entirely unclear what you are doing.

Post the schematic and explain what you are doing with the source.

9. ### hunterage2000 Thread Starter Active Member

May 2, 2010
400
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Yeah, here is the circuit and a pic of the signal generator im using.

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10. ### t_n_k AAC Fanatic!

Mar 6, 2009
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What relationship do the two images share?

The first is a picture of a physical oscillator. The second is a simulation schematic. You have explained nothing.

11. ### hunterage2000 Thread Starter Active Member

May 2, 2010
400
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The schematic shows the two inputs, V2 = 1.071Vrms, V1 = 928.571mVrms, the output shows 4.249Vpk. The signal gen is what I used for the 2 inputs, the top input is 3Vpk-pk=1.071Vrms and bottom is 2.6Vpk-pk=928.571mVrms. At the output on my oscilloscope is zero for zero phase the attached is what it should be.

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12. ### t_n_k AAC Fanatic!

Mar 6, 2009
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So your instrumentation amp has a gain of ~21 which is what one would expect with this arrangement.

13. ### hunterage2000 Thread Starter Active Member

May 2, 2010
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yeah just not sure why im getting zero at the output, any ideas?

14. ### BillB3857 Senior Member

Feb 28, 2009
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348
From what I see on the control panel of your signal generator, the two knobs on the right side control the relative amplitudes of the two signals. As stated previously, a phase difference into the amplifier will result in an output which is the summation of the instantaneous values of the two signals. I would first verify that the phase difference is indeed zero and remains so while varying only the amplitude of the signal generator outputs. One problem with test equipment is that it looses calibration over time. Even though the phase dial reads zero, the actual outputs may not be truly in phase.

15. ### hunterage2000 Thread Starter Active Member

May 2, 2010
400
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if the phase is zero then the summation is zero and the output should be the difference but theres zero amplitude at zero phase that increases with increase of phase.

16. ### BillB3857 Senior Member

Feb 28, 2009
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As it should! If it didn't motors AC motors wouldn't run.

17. ### hunterage2000 Thread Starter Active Member

May 2, 2010
400
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yeah I know AC motor sync speed is frequency dependent, but I dont see why the output is zero at zero phase for the inst amp

18. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
because it is a difference amplifier. If the difference is Zero, an amplified Zero is still Zero. As soon as you induce phase difference (key word DIFFERENCE) there is something to amplify.

19. ### hunterage2000 Thread Starter Active Member

May 2, 2010
400
0
the amplitude difference isnt zero though

20. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
If that is the case, I've lost track of the total situation. Good luck!