I´m not sure if I´ve understood your question well - but, as you know, the transfer curve of a bipolar diff amplifier is a tanh (hyperbel-tan) function. Thus, a squarewave wave at the input is smoothed if the amplitude is app. around 80 millivolts. For a triangle input you get a sinusoidal wave with a THD of app. 1.5%.
Does this help?