# LTC6228 phase margin

Hi,

I'm trying to calculate the phase margin using this circuit:

But I'm getting a strange phase response (I think):

Is this phase response right? If so does it mean that the opamp is always stable?

Thank you,

Regards,

Joao

• Hi Joao,

I tried to plot the phase margin for your circuit

In fact, this is an open loop gain. As an input signal, I take into account the differential voltage between the inputs. Try performing an analysis using similar considerations. I will also clarify that in your case the negative feedback circuit does not depend on the frequency and cannot affect the phase margin

Do you have another questions?

Regards,

Kirill

• Thank you,

I changed the method to get the loop gain (βA). I think it works now I just need an unreasonable large value for L,C. I tried it with 1T and it didn't work.

Regards,

Joao

• Hi Joao,

You surprised me. Yes, the simulator allows you to do such tricks, but it is not practical, in my opinion. If you want to plot the loop gain, you can calculate the feedback loop transfer coefficient and multiply it by the open loop gain. I have already mentioned that your feedback loop does not affect the phase, just the entire gain curve goes down.

Regards,

Kirill

• Hi,

Thanks you,

I was looking for a rebliable way of getting the loop gain, then if needed I would add a capacitor/s to adjust the phase. I have used this last method before I was just never needed to go above 1000 F,H.

I will try out your method. In that method I would need to calculate the open loop gain in the circuit like you showed in the previous answer and the feedback gain on a separate circuit just with the feedback network, right?

Regards,

Joao

• Hi Kirill,

I think the 2nd plot from Joao on May 11 (using huge L, and C) is a valid representation of loop gain. Not sure why you think that's not practical? I've used that trick / method with good results in the past.

To see phase margin, you read the phase when gain is 0dB.

Regards,

Hooman