The differential pair is one of the basic amplifying stages in amplifier design. Aiming at the the design of low-distortion amplifiers, a simple model for a differential pair is derived using the Volterra series. The parameters in this model correspond to the well-known small-signal parameters, furthermore the effect of the non-ideal tail-current source is made explicit in the model. The model gives insight in the way distortion is generated in a differential pair and it therefore allows the establishment of some simple design rules. Although being derived from a very simple model, the expressions give a good accordance with simulations.