The input does not control the output common-mode voltage. If the external circuit leaves V OCM disconnected, the IC biases V OCM to the midpoint between V CC and V EE using an internal voltage divider. The two complementary amplifier paths in Figure 3 share the same input differential pair, and their characteristics are well-matched. With symmetrical feedback, the architecture keeps the operating points close to each other. Therefore, distortion in the amplifiers is closely matched, resulting in symmetrical distortion of the differential signal.
Symmetrical distortions tend to cancel even-order harmonics.
Lab testing of the Texas Instruments THS4141 differntial amplifier at 1 MHz shows that the second harmonic at the output decreases by approximately 6 dB when measuring the signal differentially compared with measuring either output single-ended. The third harmonic remains unchanged between a differential and single-ended measurement. In a fully differential amplifier, two feedback paths, one for each side, are possible in the main differential amplifier. This arrangement naturally forms two inverting amplifiers, and you can easily adapt inverting topologies to fully differential amplifiers. Figure 4a shows how to configure a fully differential amplifier with negative feedback to control the gain and maintain a balanced amplifier.