| 1. A voltage-clamping device used in an operational amplifier, wherein the operational amplifier comprises a first transistor,
a cross-voltage between a gate and a source of the first transistor is near to a specific voltage and a cross-voltage between
a drain and the source of the first transistor is not equal to zero so as to generate a substrate current, the voltage-clamping
device comprises:
a second transistor, wherein a source of the second transistor is coupled to the drain of the first transistor, a gate of
the second transistor receives a bias signal so that the second transistor is biased in saturation region, and the voltage
at the source of the second transistor is made equal to the difference between the bias signal and the threshold voltage of
the second transistor, thereby reducing the cross-voltage between the drain and the source of the first transistor, so as
to reduce the substrate current accordingly.
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