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LME49720 & LME49600 Composite Unity Gain Stability

Wes Young
Prodigy 30 points
Other Parts Discussed in Thread: LME49600, LME49720

Hello,

I am designing an output stage for a headphone amplifier, using an LME49600 inside the feedback loop of an LME49720 configured for unity gain. I would like to know if/what extra precautions should be taken to ensure stability when operating at unity gain?

The source will be a 10K potentiometer. Would it be worth adding a feedback resistor (R33 and R34 in attachment) of ~1.2Kohms to closer match the input impedances?

Many thanks

  • 0
    TI__Guru**** 189399 points

    Hi, Wes,

    Looks like an interesting application!

    I have asked my colleague to address your question.

    -d2

  • 0 in reply to Don Dapkus
    TI__Expert 3840 points

    Hi Wes,

    I haven't specifically tried to implement the solution this way, so I don't have a conclusive answer for you. We have implemented very low-gain solutions without issue, although there were gain setting resistors around the op amp.

    Do you have the ability to easily breadboard this up?

    You could also add spots for the gain setting resistors and stuff accordingly if not needed, ie 0 ohms and open. Maybe set the gain at 1.x?

    Also, you could try using an inverting op amp gain topology to get the gain of 1, albiet -1. 

    Let me know how it works out.

    Best Regards,

    JD

  • 0 in reply to John De Celles
    TI__Expert 3840 points

    Hi Wes,

    I quickly checked your desired topology (voltage follower) out in the lab and saw no issues; drove 1W into a 35 ohm load using +/-15V rails across frequency range with excellent distortion. 

    No global instabilities upon power up and clean clipping characteristics.

    Best Regards,

    JD

  • 0 in reply to John De Celles
    Prodigy 30 points

    Wow, thanks John, that's very helpful. I haven't had a chance to test anything yet, but I will do as you mention and add spots for the gain resistors so I can play around.

    Many thanks for your help,

    Wes