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LMH6643: Input Bias Current Direction

Ben Rademaker
Prodigy 40 points
Part Number: LMH6643


Hello,

I'm analyzing the worst case output voltage error that could be due to the input bias current of the LMH6643. I believe the offset voltage error should be equal to the bias current flowing into IN- times the negative feedback resistor. I see the datasheet specifies a negative input bias current along with the note "Positive current corresponds to current flowing into the device."

My question is, for this particular op amp, could the input bias current flowing into IN- ever be a positive value? Or will the bias current for all LMH6643 parts be negative? Since IN- seems to connect internally to the base of a PNP transistor, it seems intuitive that the leakage current would always flow in one direction, but I want to be sure as both the + and - voltage error matter for this application.

Thanks,

Ben

  • +1
    Guru 48395 points

    Well ben, the clues here are this is a negative rail input VFA, so the inputs on indeed two pnp transistors so their bias or base current both flow out of the pin. 

    The definitions in the data sheet are odd sometimes, but the V+ current coming out into a source R will give a positive going output voltage, the V- current coming out will give a negative going output - bias current cancellation is possible here leaving you with an Ios*Rf error. 

  • 0 in reply to Michael Steffes
    Prodigy 40 points

    Thanks for the response! Our IN+ pin is currently tied directly to the output of an op amp, so my understanding was we don't get the cancellation effects of an input resistor, but that is an improvement I'm hoping to make in the future.

  • 0
    Prodigy 230 points

    Hi Ben,

    I think the statement "Positive current corresponds to current flowing into the device." only defines what current direction should be considered as positive. Since negatíve value is specified (for both inputs), the bias current always flows out of the device. You're right, that it is related to the fact that the input differential pair uses PNP transistors.

    Just a note: rail-to-rail input stage can have bidirectional bias current (the direction depends on the input common mode voltage), see page 9 and 10 and Figure 11 in Application of Rail-to-Rail Operational Amplifiers.

    Best wishes, Zoltan

  • 0 in reply to Ben Rademaker
    Prodigy 230 points

    I just see that your question has already been answered by Michael while I was preparing my answer :-)