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Close loop gain and phase margin

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sys_eng

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Close loope gain and phase margin

Loop.jpg

it says at w0 loop gain is 0 db.

but I thought 0 db should be way down and I marked as "what's this point".
 

Re: Close loope gain and phase margin

You see loop gain indicated in an amplifier open loop gain diagram. The point "way down" is unity gain frequency.

Feedback factor < 1 is the missing link. Read the text completely beyond the quoted part.
 

Re: Close loope gain and phase margin

You always must made a distinction between three gain definitions: open-loop gain, loop gain, closed-loop gain.
The phase margin s defined at the frequency where the loop gain is 0 dB.
 

Re: Close loope gain and phase margin

FvM said:
You see loop gain indicated in an amplifier open loop gain diagram. The point "way down" is unity gain frequency.

Feedback factor < 1 is the missing link. Read the text completely beyond the quoted part.
Click to expand...

how many dB is that for unit gain frequency? Not 0 dB because it's already taken up there.
 

Re: Close loope gain and phase margin

how many dB is that for unit gain frequency?
Click to expand...
It's 0 dB open loop gain. Or -xx dB loop gain, depending on the feedback factor implemented in the circuit.
 

Re: Close loope gain and phase margin

FvM said:
It's 0 dB open loop gain. Or -xx dB loop gain, depending on the feedback factor implemented in the circuit.
Click to expand...

w0 =0 dB (at 3dB point) as indicated at diagram.

and now you saying at unity gain bandwith is also 0dB.
that doesn't make sense.
 

Re: Close loope gain and phase margin

Loop gain is not the same thing as open loop gain, and the frequency for loop gain of 0 dB isn't necessarily the same as the frequency for open loop gain of 0 dB.

Referring to the diagram in post #1, I'm only guessing the "other point" could mark open loop gain of 0 dB. Because the diagram is missing a clear annotation, it might be anything.

You should:
- review your text books about the difference between loop gain and open loop gain
- get (or sketch yourself) a completely annotated bode diagram
 

Re: Close loope gain and phase margin

FvM said:
Loop gain is not the same thing as open loop gain, and the frequency for loop gain of 0 dB isn't necessarily the same as the frequency for open loop gain of 0 dB.

Referring to the diagram in post #1, I'm only guessing the "other point" could mark open loop gain of 0 dB. Because the diagram is missing a clear annotation, it might be anything.

You should:
- review your text books about the difference between loop gain and open loop gain
- get (or sketch yourself) a completely annotated bode diagram
Click to expand...


I know the difference between a close loop gain versus open loop gain.
I also think like you, the other point "unity gain bandwidth or other point" should be 0 dB.
But there's no way the labeled at frequency, w0 with 0dB, it has to be some number > 0dB.
I think that first diagram is mislabelled. It should be 0dB/decade at W0, and it should be the closed loop gain.
ClosedLoopGain.png
 

Re: Close loope gain and phase margin

If you carefully look onto the first diagram you will notice that at wo the label is "loop gain" (neither open-loop gain nor closed-loop gain).
 

Re: Close loope gain and phase margin

LvW said:
If you carefully look onto the first diagram you will notice that at wo the label is "loop gain" (neither open-loop gain nor closed-loop gain).
Click to expand...

If neither open nor closed, what is it? what's definition of that?
 

Re: Close loope gain and phase margin

Loop gain = Open loop gain * feedback factor
 

Phase margin definition

Phase margin is an indication ofexcess phase before -180° phase shift at unity gain

phase1.jpg
Above picture match that description.



However, below picture doesn't at point X1, it's not at the unity gain, and its measured phase margin of 72.
It deviates from the above definition why?
phase2.jpg
 
Last edited by a moderator:

Re: Close loope gain and phase margin

Loop gain is open loop gain * feedback factor.

In case of an amplifier circuit with feedback factor of 1, both are identical.

- - - Updated - - -

I see that the second bode diagram in post #12 brings up similar confusion as the post #1 diagram. Problem is that the author prefers to draw different possible loop gain curves into an open loop gain curve by designing amplifiers with different feedback factor and closed loop gain.

The completely unrealistic point of this diagram is the assumed feedback factor of -85 dB respectively a closed loop gain of 85 dB (about 17500). I really wonder what kind of amplifier is discussed here? It might be just a theoretical example, but if so, it has little practical relevance.
 

Re: Close loope gain and phase margin

In the diagram in post#1 the axes are not labeled with numbers. We shouldn't automatically assume that the omega axis is at 0 dB. In a lot of Bode plots you see the omega axis at some arbitrary location far below anything else so it doesn't clutter up the diagram. It's a matter of convenience. This may be look odd if you're used to cartesian coordinates where the axes intersect in the point (0,0). The axes in a Bode plot don't intersect at 0 Hz either because you can't draw 0 Hz on a log scale.
In the diagram loop gain is labeled x and is drawn as the distance between the magnitude curve and the broken line. So 0 dB is where the broken line is.
The point marked "what's this point" is just where the omega axis in the diagram happens to be. The point marked "Loop Gain = 0 dB" is in the right spot.
 

Re: Close loope gain and phase margin

Axel_Germann said:
In the diagram in post#1 the axes are not labeled with numbers. We shouldn't automatically assume that the omega axis is at 0 dB. In a lot of Bode plots you see the omega axis at some arbitrary location far below anything else so it doesn't clutter up the diagram. It's a matter of convenience. This may be look odd if you're used to cartesian coordinates where the axes intersect in the point (0,0). The axes in a Bode plot don't intersect at 0 Hz either because you can't draw 0 Hz on a log scale.
In the diagram loop gain is labeled x and is drawn as the distance between the magnitude curve and the broken line. So 0 dB is where the broken line is.
The point marked "what's this point" is just where the omega axis in the diagram happens to be. The point marked "Loop Gain = 0 dB" is in the right spot.
Click to expand...



then why so many bode plots I run across shows 0dB at same level as the omega w axis? Look at diagram at post #8.
 

Re: Close loope gain and phase margin

then why so many bode plots I run across shows 0dB at same level as the omega w axis? Look at diagram at post #8.
Click to expand...
Having the magnitude axis origin at 0 dB is in fact the standard, but not guaranteed.

I agree with Axel_Germann that you shouldn't jump into conclusions about a diagram without clear annotation. The most likely explanation is however, that the #1 diagram does basically the same as the second #12 bode plot. It shows loop gain for specific frequency independent feedback factors inside an open loop gain curve. This way you get two magnitude axis, different by an offset.

The zero of the open loop gain magnitude axis is at the origin, as shown in the #12 diagram. I think, the thread can be marked as solved if you understand this point.
 

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