Power amplifiers (PAs) used in modern radio frequency transmitters for communications display significant nonlinear dynamic effects under typical operating regimes. Accurate and efficient implementations of behavioral models describing these effects are required in order to enable system-level simulation, or for guiding the design of effective linearization predistorters. This work proposes an efficient implementation of the widely adopted modified-Volterra model, resulting from a global rational functions approximation of the model nonlinear convolution kernels. The proposed implementation, tested by gallium nitride PA simulations, reproduces the model behavior with high-accuracy and features an extremely compact structure, comparing favorably with alternative descriptions.