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question about conditional stability?

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wanily1983

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hi, guys
i am confused about the question that conditionally stable system is stable beacause the bode plot show its phase lag is less than 180 when magnitude is 0DB. but at the frequency before the fc, its phase lag may reach 180, then at this frequency, can the system oscilate?
with regards
wanily
 

Kral

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wanily1983,
No. The system as designed is stable. However, if the closed loop gain is changed so the the phase shift at the 0 db point is 180 deg, then the new system will be unstable and oscillate.
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Kral
 

ccw27

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One other question, I have a 3 stage amplifier and the loop gain phase at DC is 180. The system is oscillatory. What causes the system to start at 180?

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Kral

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ccw27,
Without seeing your circuit, my guess is that the 180 deg phase shift is due to a net inversion of the signal, rather than a phase shift. For example, if you connect 3 inverting stages in cascade, the output will be 180 degrees out of phase with the input at 0Hz (DC).
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Kral
 

wanily1983

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hi,Kral
can you explain why at the frequency which phase lag reach 180 degree, the opamp wouldn't oscillate.
i think at that frequency, the output voltage become larger and larger until the opamp remove the linear region. then its gain will decrease to 1 and it will oscillate.
 

Kral

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wanily1983,
This is pure speculation, based on the assumption that there is a net inversion at DC (0Hz). If this is the case, then at the frequency where the phase shift = 180 degrees, the totoal phase shift (including the inversion) between input and output would be 0 degrees, and the circuit would be stable.
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Kral
 

wanily1983

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Kral said:
between input and output would be 0 degrees, and the circuit would be stable.
Kral
between input and output would be 0 degrees, at that condition, it become positive feedback and if the loop gain is larger than 1, the circuit would be unstable.
i know that in engineering the condition stable circuit can work, but i just cann't understand it from the viewpoint of theory.
 

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