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LM6132: Difference in bias current for Device Configuration

Oz Kocoglu
Prodigy 260 points
Part Number: LM6132

Hi there,

 

I have a question regarding the LM6132 bias current dependency on the OPAMP's configuration.

 

Is there a difference in the bias current for the LM6132 when it is used as voltage follower, where the positive and negative input have nearly the same voltage, or when it  is used as comparator, where there is a large difference in voltage between positive and negative input ?

 

The bias current is needed to calculate the offset voltage.  Can the maximum current of 300nA mentioned in the data sheet (table on page 5, LM6132AI) be used for the calculation when the device is configured as a voltage follower ?

 

Thanks and best Regards,

 

Oguz

  • 0
    TI__Guru 61605 points

    Hi Oguz,

    ****Edit****

    I missed the diode connected BJTs in Figure 30 that Ron pointed out.  Expect the input bias current to increase when the inputs are pulled apart.  Since there is not an absolute maximum differential voltage specification in the datasheet, the device should not be damaged as long as the input voltages are within the specified range based on the supply voltage.

    ****

    Many high voltage devices feature back-to-back diode clamps which prevent their operation as a comparator, or at least greatly increases the input current when used as a comparator.

    For a worst-case calculation the 300nA input current should be used to calculate errors due to the input bias current in a voltage follower configuration.

  • 0 in reply to Collin Wells
    TI__Guru*** 136326 points

    Oguz,

    The IIB in data sheet applies to both inputs being nearly the same (only separated by the devices' Vos) Notice that "Vo = Vs /2" default is on top of data sheet table.

    For some op amps IIB may be same regardless of input differential. Based on figure 30 circuit, if the inputs are not the same voltage then input(s) may have higher IIB. In fact I believe the polarity of IIB could be opposite.