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3rd January 2020, 20:06 #1
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3rd January 2020, 20:54 #2
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
It may not be. Two ways to know.
1. generate a Nyquist plot.
2. Run a transient analysis to check the step response for oscillations.
Also, you may try to move the zero towards lower frequencies and thus not let the phse drop so much.
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3rd January 2020, 22:50 #3
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
Why Phase starts from 180 degree ? Is it an Inverter Amplifier ??
If it's so, the danger is much behind from where the Gain drops down to 0.
While the Gain is higher while the Phase is close to 0, so it will close to oscillate..
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4th January 2020, 01:10 #4
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
The bode plot is simple enough (monotonous gain, only one crossing of the 0 dB line) to see that the circuit is unconditional stable. Nyquist criterion can be applied, but it's not necessary to assess stability in this case.
The phase margin is 72 degree when the gain is 0dB. However, the phase margin is close to 0 before the frequency reaches the unitygain bandwidth.
Although the amplifier is stable, the "sagging" phase will probably show in the transient response.
Why Phase starts from 180 degree ? Is it an Inverter Amplifier ??
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4th January 2020, 04:51 #5
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
On the face of it  it is quite stable  adding other delays to bring the total delay to 360 deg is where it will start to oscillate  then the feedback reinforces the input  at 180deg the feedback still subtracts from the input ...
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4th January 2020, 12:18 #6
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
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4th January 2020, 17:06 #7
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
Thank you all of you. Yes, it is an inverting amplifier. Thank FvM to give the simulation comparison.

5th January 2020, 15:42 #8
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
Is this a fixed frequency inverter (50 or 60hz for example)? If so PR (Proportional Resonant) control is another good option.
PR control adds a tuned resonant element to the feedback that works like an AC integrator. Any error signal at the tuned frequency is amplified creating very high gain at a that single frequency.
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10th January 2020, 19:54 #9
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
It is an opamp, not a fixed frequency inverter. Thank you for the information.

11th January 2020, 00:48 #10
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
Can I ask what the application is? I’ve experimented with this compensation scheme but have only recently found other examples of it in custom audio opamps.
I speculate that putting a limit on the “extra” pole will limit the large signal impact while maintaining the small signal benefit for locking in output accuracy at mid range frequencies.
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11th January 2020, 23:27 #11
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
It is a tranimpednace amplifier (TIA) used for converting the input current into output voltage.
There are two poles, P1 and P2, and a zero, Z1. P1 is the dominant pole, and Z1 samller than P2. Z1 compensates P2.
I am not sure about the influence of the poles and zero to middle range frequency signal.

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12th January 2020, 15:44 #12
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
I am not sure about the influence of the poles and zero to middle range frequency signal.
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12th January 2020, 18:17 #13
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
I was implying that the added open loop gain in the 1100hz range would result in lower closed loop error. Looking at these closed loop simulation that's not true however due to the early onset of the peaking. The peaking means the phase delay stays lower at higher frequencies in case that matters.
I expect that if the extra pole is moved left away from crossover the peaking will be reduced.
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13th January 2020, 16:24 #14
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
Oh, I see. Yes, the higher gain leads to an accurate settling, and the seperate pole improves the phase margin.
The plots from FvM are used to illustrate that it is still stable even if the phase drops close to zero before the unit gain bandwidth given that the phase margin is large enough at the unit gain bandwidth.
Thank you.
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27th January 2020, 09:58 #15
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
When the phase shift is close to 180 degrees, you still have a lot of open loop gain. This means that the denominator of G/1+GH can still be written as ~GH which would mean that the transfer function can be simplified as ~ 1/H. Of course there can be a lot of exceptions. But in most cases, it means that the closed loop transfer function does not become infinity at a frequency and therefore it is stable.
Only where the loop gain comes close to 1 (or 0dB) and the phase shift is close to 180 degrees, will there be a chance that the closed loop TF goes to infinity and therefore you will have oscillations.
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27th January 2020, 15:36 #16
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Re: Is it stable that phase drop to close to 0 but rise to 75 degree when gain is 0dB
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