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LMV712-N and LPV542 is used as a voltage follower

Other Parts Discussed in Thread: LPV542, LMV712-N

hi:

When one amplifier unit of LPV542 or LMV712-N is used as a voltage follower, the positive phase input of the unit is hung in the air, then the level of the output terminal is low or high? Or not sure?

thanks

  • Hi Sean,

    The output will be at the same level as the input, as it is a follower. There is no default "zero signal" position.

    The output will float to a point where the bias currents and leakage currents cancel. If the leakages on the input are higher than the bias currents, then it will tend to float to one of the rails, depending on the direction of the leakage.

    So in short, without knowing your circuit or layout, we cannot say what the response will be. The output will either be railed, or floating at some random voltage. There is no "default" output condition.

    Regards,

  • Paul:

    Thank for your reply!

    As the following circuit diagram, when the side of R1 is floating,the level of OUTA is low or high? Or not sure?

    Would you recommend the  amplifier from TI,that it is used as a follower, positive input of the unit is hung in the air, then the level of the output terminal is  defaulted?

    thanks 

    sean

  • Hello Sean,

    No. We cannot specify what direction a floating CMOS input can go. There are too many external influences.

    However, if you want it to go in a specific direction or level, you can place a high value resistor (>10MEG, or whatever will not interfere with your source voltage) to a potential that you want the output to go when it is "floated" (like a pull-up or pull-down resistor).

    So if you want the output to go to ground when the input is floating, place a high value resistor from the input to ground. If you want the output to go to 1V, then you would place a large value resistor between the input and a 1V source.

    Regards,