Using the nonlinear Volterra series representation, analytical expressions for the third-order intermodulation distortion power and intercept point for a MESFET small-signal amplifier are derived when its equivalent circuit is bilateral and includes the gate-to-drain capacitance (C-gd) explicitly as a nonlinear element. Previously developed analytical expressions treated C-gd as a linear element or incorporated it as a part of gate-to-source and drain-to-source capacitances (C-gs and C-ds). These new analytical expressions are then compared with experimental data and good agreement is obtained. The analytical expressions are also used to study the variation of intermodulation distortion with input power and frequency, and the effect of the individual nonlinear elements in the MESFET's equivalent circuit. (C) 2002 Elsevier Science Ltd. All rights reserved.