Phase Margin for fully differential OpAmp

Hi everyone,

I'm trying to size the miller cap for a fully differential OpAmp. Without the miller cap, the phase is around -230° at 0dB (open loop, differential output). I guess, the OpAmp is not stable, but I try to run some simulations and I'm not able to see it oscillating!
Anyway, I'm just wondering what should be the phase margin? 12° or 45°? (it will result in a lower gain, for sure).
Should I check the phase margin for each independant output (+ and -) or for the differential output (Vout+ - Vout-)??

Thank you for your help.

The Phase margin should not affect the DC gain at all.

The better the phase margin, you would see less ringing at the output with step inputs. 45° is a good number.

I used to check for the differential input to differential output.

Fabien said:

Without the miller cap, the phase is around -230° at 0dB (open loop, differential output). I guess, the OpAmp is not stable, but I try to run some simulations and I'm not able to see it oscillating!
.

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The phase margin needs to be determined from the loop gain - not from "open-loop" gain of the amplifier only.
What do you mean with "open loop"?

LvW said:

What do you mean with "open loop"?

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By "open loop", I mean nothing is connected between the output to the inputs.

Here is my test bench and the gain and phase results (without Miller caps).

nitishn5: for sure it doen't affect the DC gain, but it does for the GBW, so it does for the specifications. GBW should be 5 times the switching frequency.
I tried some pulse waves at the inputs, but no oscillations at the outputs!?

You need to put a feedback and close the loop before applying the AC signal.
There is a method using ideal inductors of huge values in the feedback path to set the DC operating point and using an ac source to get the loop transfer function plots. I don't remember it exactly but I have seen it in this forum.

phase margin should be between 45 to 60 degrees... to achieve good stability in closed loop configuration....

thanks

I agree but... if the OpAmp is stable in open loop, it should be in closed loop. As it should be used for SC cap, the OpAmp will work (in a little laps of time) in open loop, and the closed loop also depends on the caps I'll use.
Anyway, I'll check for the inductor method, but I still don't know how to check the OpAmp oscillations regarding to the phase margin...

Fabien said:

I agree but... if the OpAmp is stable in open loop, it should be in closed loop. As it should be used for SC cap, the OpAmp will work (in a little laps of time) in open loop, and the closed loop also depends on the caps I'll use.
Anyway, I'll check for the inductor method, but I still don't know how to check the OpAmp oscillations regarding to the phase margin...

Click to expand...

Fabien, I have the feeling you are not sufficiently familiar with the term "phase margin". Therefore, some basics:

*The phase margin applies to systems with feedback only. It is a measure for "safety" against oscillations in case of a closed loop - but it will be measured/simulated in OPEN loop conditions.
* Thus, you have to open the loop for applying a test signal ac source.
*But the problem is: In many cases, there will be no stable DC operating point under open loop conditions.
*Therefore: Open the loop and place a very large inductor (1 H or more) between both nodes (left and right). Thus, the loop is closed for dc but open for frequencies of interest.
* Then, inject a test voltage into the feedback path using a very large capacitor (1 F or more at one end of the inductor) and measure/simulate the voltage at the other end of the inductor.
* The voltage ratio gives you the loop gain response, which can be evaluated (magnitude and phase) to see the phase margin, which is the phase difference to -360 deg at the point where the magnitude is 0 dB.

LvW said:

*The phase margin applies to systems with feedback only. It is a measure for "safety" against oscillations in case of a closed loop - but it will be measured/simulated in OPEN loop conditions.

Click to expand...

Sure, that's the reason why I plot the Gain and Phase in open loop. In my simulation, the DC operating point seems to be stable, so if I understand, the inductor is not useful in this case.

The thing is: in OPEN loop, with no miller cap, the phase is far below -180° at 0dB. It seems the OpAmp is not stable! That's what I understand and what I can read in the books. If it's not stable, if I close the loop, the OpAmp may oscillate. So I put a pulse wave in this condition at the inputs, and nothing happen.
So the question is: how to see the system unstable ? Because with a phase margin of 12° (what I set with miller cap) the OpAmp should be stable but may have decreasing oscillations in closed loop. What I can't see.

May be I'm wrong...

Fabien said:

So I put a pulse wave in this condition at the inputs, and nothing happen.
So the question is: how to see the system unstable ? Because with a phase margin of 12° (what I set with miller cap) the OpAmp should be stable but may have decreasing oscillations in closed loop. What I can't see.

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What means "...and nothing happen"? Is the opamp dead? No change of the ouput voltage?
It would be best to show the circuit - otherwise, it is problematic to comment on a circuit that can`t be inspected.

i think, the op-amp's output starts oscillating and ends with decreasing oscillation and finally becomes DC...

Am i correct..? fabien..

kenambo said:

ithe op-amp's output starts oscillating and ends with decreasing oscillation and finally becomes DC...

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It should act like this, that's what I wanted to see but not able to.

LvW said:

What means "...and nothing happen"?

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It means nothing is oscillating at the output.

Anyway, guess it's OK, I re run a simulation with the inductor from input to output, and I got the oscillations. Here are the new snapshots.
Now, I think to define the phase margin, I should know what capacitors should be connected in the close loop, right? An then I can define 45°? Am I right?

But, nevertheless, the OpAmp should be unstable, so I expected to see infinite oscillation, not becoming DC!

Fabien said:

Anyway, guess it's OK, I re run a simulation with the inductor from input to output, and I got the oscillations. Here are the new snapshots.

Click to expand...

No - that´s not correct. You have performed a TRAN analysis using the inductors? WRONG!

The inductors are used ONLY for opening the loop ac-wise for the purpose of a loop gain simulation (AC analysis).
More than that, since the ac testsignal to be injected via a large capacitor must be the ONLY signal in the circuit, the "normal" input must be grounded during this simulation.