In one embodiment, a circuit includes at least one transistor with a base and collector being electrically connected to a ground, and at least one current source being configured to apply four different currents (A, B, C, and D) to the emitter. A sum of the currents A and C are substantially equivalent to a sum of the currents B and D, or a sum of the currents A and D are substantially equivalent to a sum of the currents B and C. The circuit outputs first, second, third, and fourth voltage potentials between the emitter and the base during application of the currents A, B, C, and D, respectively.