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OPA387: Stability - lower Riso increases stability?

Tomasz Jastrzebski
Prodigy 32 points
Part Number: OPA387

Tying to fine-tune a simple amplifier based on on OPA387 I found out that the higher Riso resistor value the lower the stability - see the diagram below, phase-margin(Aol)=61.5°@Riso=100Ω.
When Riso is 51Ω, phase-margin (Aol) stays below recommended safe 45°. This is contrary to what I expected.
The circuit has been analyzed according to "10.3 TI Precision Labs - Op Amps: Stability - Spice simulation" presentation.
Am I missing something or what I observe is to be expected?

OPA387 open loop.TSC

  • +1
    TI__Mastermind 47796 points

    Hi Tomasz, 

    Please remove Cf of 420, which is missing "p" as picof and phase margin should go back close to 90. The unity gain BW is at approx. 5.7MHz/302 = 18.8kHz based on Gain Bandwidth Product. 

    Please let me know if you have further issues. 

    Best,

    Raymond

  • +1 in reply to Raymond Zhang1
    TI__Guru 60570 points

    Tomasz,

    I think you contradict yourself - you say:

    "Trying to fine-tune a simple amplifier based on OPA387 I found out that the higher Riso resistor value the lower the stability - see the diagram below, phase-margin(Aol)=61.5°@Riso=100Ω.  When Riso is 51Ω, phase-margin (Aol) stays below recommended safe 45°. This contrary to what I expected.

    Since you get 61.5 degrees phase margin with Riso of 100ohm but only 45 degrees with Riso of 51ohm, the higher Riso does increases stability as it should.

    Having said that, as Raymond pointed out, you simulated the stability with Cf of 420 Farad instead of 420pF.  If you simulate it with 420pF, you will get 83 and 69 degrees for Riso of 100ohm and 51ohm, respectively.  Thus, once again, the higher the Riso the greater stability.

    Just a note: AOL = Vo/Vfb, invBeta = 1/Vfb,  LoopGain = Vo

  • 0 in reply to Marek Lis
    Prodigy 32 points

    Thank you  and , the problem was missing "p" and my lack of proper understanding of phase-margin.
    To estimate phase-margin I should look at VO (=AOLβ), not VFB, and make sure it safely stays above 45° while the "rate of closure" at fC (where AOL=1/β) stays below 20dB.
    For future reference I attach the most relevant slides from "10.2 TI Precision Labs - Op Amps: Stability - Phase margin"
    and "10.3 TI Precision Labs - Op Amps: Stability - Spice simulation" training courses.