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TLV8541: TINA-TI Stability Simulation

Nick Masur
Prodigy 205 points
Part Number: TLV8541
Other Parts Discussed in Thread: TIDA-00489, LPV802

Hello.

I'm attempting to prove that this noninverting amplifier circuit is stable using the method outlined in https://training.ti.com/ti-precision-labs-op-amps-stability-spice-simulation?context=1139747-1139745-14685-1138805-13850.

TLV8541_Stability_Simulation.TSC

When I adjust the circuit to isolate the feedback components, the gain dives well below -280dB. This makes sense given the voltage divider, but makes it impossible to assess the phase margin at 0dB.

TLV8541_Stability_Simulation-modded.TSC

If I increase R2 to a large value, the gain information becomes useful.

1. Is this method only valid if there is no voltage divider in the feedback loop?

2. Is there are better or more accurate way to simulate stability for this circuit?

Thanks.

  • +1
    Guru 140200 points

    Hi Nick,

    the mistake is that you set a way too high DC gain. Even the least input offset voltage will make the output of OPAmp go into saturation then and by this leaving the range of linear operation.

    Before running a phase stability analysis you should check all DC potentials by perfoming a DC analysis first.

    Note that the OPAmp is not able to close the feedback loop in the above simu because of a too high DC gain: The both inputs of OPAmp are not sitting on the same potential.

    AC coupling in the feedback loop can do the trick:

    Kai

  • 0 in reply to kai klaas69
    Prodigy 205 points

    Understood, thank you, Kai!

    A couple of follow up questions -

    1. I was under the impression that phase margin was the difference between -180 degrees and the loop phase at unity gain. The analysis method above suggests that the phase margin is just the actual loop phase at unity gain. Are these the same for some reason that I'm not seeing?

    2. Does the phase margin as assessed above just need to be positive, or does it need to be greater than 45deg?

    Nick

  • +1 in reply to Nick Masur
    Guru 140200 points

    Hi Nick,

    don't ovberlook the additional 180° phase shift because of connecting VG1 to the inverting input of OPAmp instead of the non-inverting input.

    Greater +45° in this plot is the aim. The more the better. In my simulation the phase margin is +70° which is very good.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 205 points

    Kai,

    I had a seperate question, which was related to this one. The gentlemen in that post are giving me information contrary to this post. When I directed them to this, they stated that this measurement is wrong. Can you review and elaborate?

    e2e.ti.com/.../4028171

    Thanks.

  • +1 in reply to Nick Masur
    Guru 140200 points

    Hi Nick,

    I only wanted to focus on that your simulation is suffering from the output of OPAmp hanging in saturation and I was relying on that you do the simulation properly after watching the training video. Sorry, my mistake Relaxed

    I will respond in your other thread.

    Kai

  • 0 in reply to kai klaas69
    Guru 140200 points

    Here is what I answered in the other thread referring to the TIDA-00489:

    i Nick,

    I would do the phase stability analysis by including the JFET circuit of the PIR sensor and its output impdance and by including the input capacitances of the following comparators:

    nick_lpv802.TSC

    As you can see I get the same result as David.

    And the second LPV802 shows a phase margin of 70°:

    nick_lpv802_1.TSC

    Kai