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As far as I understood from here, phase margin means the amount of phase that needs to be added in order to coincide with 0 db gain and -180 degrees phase. Likewise for the gain margin it means means the amount of gain that needs to be added in order to coincide with 0 db gain and -180 degrees phase. But i don't understand why do they both necessarily have to be positive for stable systems?

Re-stating my question. I want to know: Why gain crossover frequency must be lower than phase crossover frequency for stable systems?

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Relevant: Can a phase margin become bigger than 180° and is this bad?

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  • \$\begingroup\$ Positive gain and phase margin for stable systems is just convention. \$\endgroup\$ – Chu Mar 25 '18 at 19:00
  • \$\begingroup\$ You might also look up "stability margin" which is the shortest distance from the Nyquist curve to the critical point. It captures the other two ideas. \$\endgroup\$ – jonk Mar 25 '18 at 19:41
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    \$\begingroup\$ Look up Barkhausen's criterion. The old joke, is "my amplifiers oscillate, and my oscillators don't". \$\endgroup\$ – Whit3rd Mar 25 '18 at 20:05
  • \$\begingroup\$ Read about Harry Nyquist. \$\endgroup\$ – analogsystemsrf Mar 25 '18 at 22:06
  • \$\begingroup\$ @Chu I want to know the reasoning behind the convention \$\endgroup\$ – aadil095 Dec 29 '19 at 18:48

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