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Bias Currenet

Larry Jorstad
Prodigy 10 points
Other Parts Discussed in Thread: OPA2107

In the OPA2107 the datasheet shows the bias current increasing with temperature. When I measure the bias current, I don't see this even up to 125c. It remains fairly flat. I understand that with protection diodes one might see leakage current increase with temperature, however, if there are two such diodes (one to V- and one to V+), they would cancel out and only the gate current remains. With the internal gate biased at around 0V and common mode at 0v, I would expect the bias current to not change that much over temperature. Does anyone out there have experience with this OPA2107? Thank you.

  • 0
    TI__Guru 64450 points

    Larry,

    OPA2107 utilizes JFET input transistors which rely on a reverse-biased p-n junction to function properly.  Therefore, there is always a net positive input bias current that doubles roughly every 10 deg C.  This is shown on the graphs below.

    In order to determine the input current, IB, at pico-amp (pA) range, you cannot use a resistor to measure the voltage across it since the resistor thermal noise would dominates such reading.  You must instead use a capacitor in series with each input terminal (see schematic below) and determine the input bias current by back-calculating it from the current equation:  IB=C*dVo/dt where C is the capacitor used and the dVdt is the rate of change of the voltage across the capacitor (you will need a scope to determine dVo/dt).