About the phase margin

[tony_taoyh]

HI, all,

If the magnitude at DC or 1Hz is 1, i.e, 0dB.

How to define the phase margin?

Is it correct to use -3dB point to define the phase margin?


Thanks a lot.

 

[always@smart]

HI, all,

If the magnitude at DC or 1Hz is 1, i.e, 0dB.

How to define the phase margin?

Is it correct to use -3dB point to define the phase margin?


Thanks a lot.

yes, but in this case ur circuit will be always stable.

regards,
smart

 

[mike_bihan]

If your phase margin is zero at DC, then it is going to oscillate.

 

[vistapoint]

Why use -3dB?

What's the phase margin of an ideal unit-gain inverter? If we connect the output to input will it oscillate? What if the inverter has a little bit delay?

HI, all,

If the magnitude at DC or 1Hz is 1, i.e, 0dB.

How to define the phase margin?

Is it correct to use -3dB point to define the phase margin?


Thanks a lot.

yes, but in this case ur circuit will be always stable.

regards,
smart

 

[bg21359]

I personally think we need to know what kind of frequency response this circuit has?
low pass, high pass or band pass? Then we can get the phase margin defined

 

[is_razi]

phase margin occurs in the frequency that the gain of open loop system is 1 (0db).
mean : |GH(jω)|=1 ,we find the ω from this equation.

so you can calculate the phasemargin by this equation:
ΦM=180+ <GH(jω).
the ΦM is the phasemargin.

for your question if this frequency is for open loop so it's will be your ω and you can substitute it in the second equation and calculate the ΦM.

 

[chinito]

if magnitude at DC is 0dB, your circuit is unconditionally stable. By the way, you usually get this kind of magnitude response for ckts make of passive components only.

 

[skymusic]

Define Phase margin at frequency when Av is equal to 0dB. PM = 180-phase

 

[zorro]

Think at the utility of the phase margin concept (more than at a definition). Phase margin is the additional phase that can be tolerated before the circuit becomes unstable when the feedback loop is closed. (A similar concept holds for the gain margin.)
So, it isn’t relevant to speak about phase margin if the gain is less than 1.

For a real circuit, the phase at DC can just be zero or 180 degrees. So, if gain is 1 at DC and decreases with frequency, phase margin is 180 deg or 0. But this a very special case (gain=1 at DC) because that “additional phase”, caused by any circuit element cannot have any value ad DC (just 0 or 180 deg), so the concept of “phase margin” lacks its sense.

Regards

Z

 

[Karthikeya]

Why use -3dB?

What's the phase margin of an ideal unit-gain inverter? If we connect the output to input will it oscillate? What if the inverter has a little bit delay?

HI, all,

If the magnitude at DC or 1Hz is 1, i.e, 0dB.

How to define the phase margin?

Is it correct to use -3dB point to define the phase margin?


Thanks a lot.

yes, but in this case ur circuit will be always stable.

regards,
smart

a single invertor is a single pole system hence it can provide a max of 90 phase change .....thus a total phase of -270• -it cannot act as osc if o/p is fed i/p.

regards