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Original question:

LMP7731, LMP7732 input bias current and CM range

LMP7731: Understanding input bias current direction

Thomas Sørensen
Prodigy 60 points
Part Number: LMP7731

Hi,

The below graph of the input bias current of LMP7731, I want to make sure I understand what direction these currents have.

Is it per default considered the current direction is into the (+) and (-) terminals?

Thank you

  • +1
    Guru 294720 points

    See figure 44:

    At extreme input voltages near the supplies, the cancellation circuits cannot work, and the base current flows into the inputs for high voltages, or out of the inputs for low voltages.

  • 0 in reply to Clemens Ladisch
    Prodigy 60 points

    Thank you fore the very quick answer.

    Just as a follow question, these current sources in I_BIAS CANCELLATION CIRCUIT, they are supplying bias for the NPN transistor pair, when the inputs are far from the supply rails. But if the input voltage is high then it is not enough and current starts to flow into the IN+ and IN- terminals?

  • +1 in reply to Thomas Sørensen
    Guru 294720 points

    Correct. (When the input is too high, the current source does not work, and all the base current must flow through the input pin.)

  • 0 in reply to Clemens Ladisch
    TI__Guru 64840 points

    Adding my two cents, since the input bias cancellation scheme is not exact, and may over or under compensate the actual base current required by NPN input transistors,  the actual direction of the current within the LMP7731 linear input voltage range may of either polarity, which is shown below.

  • 0 in reply to Marek Lis
    Prodigy 60 points

    Hi,

    OK thank you for the input, that maske sense to me.

    In my application, I could see that when my inputs came close to the positive rail, more current started to flow input the inputs and I got some offsets.

    But it all makes sense now.

    Thank you both the support.