Abstract: A closed loop operational amplifier circuit for providing a constant current in the closed loop for generating a precision constant current through an unknown resistance for measuring the resultant voltage drop across the resistance. The closed loop circuit having a compliance circuit with a high voltage diode for preventing damage to a transistor in the compliance circuit in one polarity direction and a current limiting circuit including the transistor for preventing damage to the transistor from high voltage in the opposite polarity direction, and a voltage comparison circuit responsive to a high voltage in the opposite direction for reverse biasing the high voltage transistor to prevent it from entering the secondary breakdown mode.