This papers stems from the fact that, whereas there are passive models of transistors and tubes, a minimal passive model of the operational amplifier does not seem to exist. A new behavioural model is presented that is memoryless, fully described by its interaction ports, with a minimal number of equations, for which a passive power balance can be defined. The proposed model handles saturation, asymmetric power supply, and can be used with non-ideal voltage references. To illustrate the model in audio applications , the non-inverting voltage amplifier and a saturating Sallen-Key lowpass filter are considered.