A voltage-controlled current source containing one operational amplifier and used to drive an electrical impedance tomograph is described. The source is optimized to work at 50 kHz and maintains constant current through the entire range of loads expected in electrical impedance tomography. Because these loads may be grounded, the source can be used in parallel current injection systems. Because the source uses only one amplifier and does not require a driven shield, parts count is minimized. Because it can drive complex loads, a capacitor can be placed in its output path to prevent DC currents from flowing through the patient.