An apparatus for making a resistivity measurement of an underground formation surrounding a borehole is provided. The apparatus includes a conductive tool body, at least one current injector electrode positioned between the tool body and a wall of the bore hole, at least one current return electrode positioned between the tool body and the wall of the bore hole, an electrical measurement device configured to monitor a voltage or a current signal indicative of an impedance imbalance, a regulator connected to the electrical measurement device, whereby the regulator is configured to receive the voltage or the current signal indicative of the impedance imbalance and is configured to generate an adjustment signal based on the voltage or the current signal indicative of the impedance imbalance.