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Phase shift due to current in the output devices

Daniel Almeida
Expert 1010 points

Hello everyone,

I was doing some simulations in TINA and I've noticed that if the bias current for the output devices raises the phase shift also raises.

I think that LM4702 is already tested and have this issues in account, but I can't simulate this problems because in the model the current between source and sink pins are very low.

I've seen this effect while simulating circuits from Bob Cordell's book "Designing Audio Power Amplifiers".

I'm sorry for writing about a problem that is not directly related to a TI product, but I can't found answers in www.diyaudio.com and there are very little information about this problem.

I wanted to know if this phase shift is real or just a problem with models.

Thank you very much for your support,

Best regards,

Daniel Almeida

  • 0
    TI__Expert 3840 points

    Hi Daniel,

    I'm not sure I am familiar with the issue are bringing up.

    Can you please copy your simulation model here again? Additionally, is there an output simulation result that you can refer to exemplify this issue? What phase shift are you referring to? (frequency/phase response?)

    Also, is there a specific page or set of pages in Bob's book that you are referring to? I'm not sure I can help, but I will take a look.

    Thanks and Best Regards,

    JD

  • 0 in reply to John De Celles
    Expert 1010 points

    Thank you very much for your help,

    I can send you the schematic and the Bode plots if you need it.

    Best regards,

    Daniel Almeida

  • 0 in reply to Daniel Almeida
    Expert 1010 points

    Attached to this post I've two Bode plots one from an amplifier with 70mA bias current (Phase1), the other one is in cuttof (Phase2), the change I've made to the original design are Darlington bias spreader, short circuit circuit and IRFP240/9240, instead of BJTs but I've tried without this changes and the result is similar.

    The two circuits are attached to this file.

    This circuit is similar to the one found in figure 3.8 page 63 from Bob Cordell's book, "Designing Audio Power Amplifiers".

    I hope someone can help me,

    Best regards,

    Daniel Almeida

    6403.BC1.TSC

    8473.BC2.TSC

  • 0 in reply to Daniel Almeida
    Expert 1010 points

    Hello everyone,

    It seems that the power bandwidth of the amplifier raises when the bias current passing thru the output devices is higher.

    I don't know what is causing this problem.

    Thank you very much for your support

    Best regards,

    Daniel Almeida