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OPA171 - the improvement of phase margin and gain margin

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Replies: 4

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Hi all,

Would you mind if we ask OPA171?

We simulated the phase margin and the gain margain on Tina-TI following files.(using  Middlebrook method)

OPA171_stab05_for_test .TSC

The value of phase margin and gain margain is requried each followings;
-Gain margin: 10dB
-Phase margin: 60degree

We changed the value of feedback capacitances(C1, C2 from 1pF to 100nF) and C3, C4(1pF to 100nF) 
However, we could not get target value of gain margin and phase margin.

So, how much of value are good for C1, C2 to prevent oscillation?
If you have some advice, could you let know us?

Kind regards,

Hirotaka Matsumoto 

  • Hi team,

    We guess that your teams are so busy, but could you let know us it?

    Kind regards,

    Hirotaka Matsumoto

  • In reply to Hirotaka Matsumoto:

    Hi Matsumoto-san,

    Sorry I was out of office when you contacted me by email.

    I'm not very familiar with the Middlebrook method you've used / shown in your TINA-TI file. However, I've used another method of phase margin / gain margin simulation using TINA-TI. In the method I use, you look for the "loop gain" 0dB crossing and see its phase angle and that represents your phase margin. It uses an ideal op amp to develop the "noise gain" expression and then plots the "loop gain" by the ratio of open loop response (dB) and noise gain (dB). This method graphically shows the "noise-gain" plot and you can visually investigate how open loop gain and noise_gain intersect (you like to have a low rate of closure between these two):

    TINA-TI circuit for above picture is here:

    /cfs-file/__key/communityserver-discussions-components-files/14/OPA171-open-loop-analysis-E2E-1_5F00_21_5F00_15.TSC

    However, I'm not getting the OPA171 open loop response that I would have expected though since it does not cross 0dB at around 3MHz! I'm not sure what might be the root-cause. I copied / pasted the OPA171 in your simulation file, as-is:

    Therefore, I don't think I can use TINA-TI to find the best compensation value in this case.

    For TINA-TI to point out the optimum value of feedback capacitor and isolation resistor for a particular load capacitance, the OPA171 model should also be fairly accurate for "output impedance" value (I don't have any information whether it is or it is not?).

    Here is some information I found on the web with regard to the type of compensation (in-loop compensation) you are doing to help you zero-in on the rough values of Cx and Rx (feedback cap and series isolation cap):

    http://www.analog.com/library/analogDialogue/archives/31-2/appleng.html

    Regards,

    Hooman

  • In reply to Hooman Hashemi:

    Recommend you download 4 part PowerPoint Op Amp Stability tutorial at:

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/2645.solving-op-amp-stability-issues.aspx

     Appendix 13 covers this cap load stability technique.

    Tim Green, SMTS
    Precision Amplifiers Applications Manager
    
    		
  • In reply to Tim Green1:

    Hello Tim,

    That four part Power Point Op Amp Stability tutorial looks very good.  Does TI offer that course at any of its Technology Days or is it offered as a web class?  It would be beneficial to go through it where I can ask questions. 

    thank you, Rich

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