Nonlinear modelling of GaAs MESFETS for power amplifier design

An empirical method has been used to develop a nonlinear model for the GaAs MESFET. The model is derived solely from S-parameter measurements over a range of bias points and frequencies. Linear equivalent circuits for the MESFET are derived from small-signal S-parameter measurements using TOUCHSTONE over a range of bias points. The linear elements in the equivalent circuit are by definition constant with bias change, whereas the nonlinear elements change with both gate and drain biases. The change with bias in each nonlinear element is expressed as a 2-dimensional Chebyshev polynomial in V_ds and V_gs. This approach to modelling avoids the simplifications inherent in many SPICE (or SPICE derived) models which allows nonlinear elements to be fixed functions of only a single voltage. For example, C _gs and C_dg are sometimes modelled as reverse-biased diodes with capacitances a fixed function of V_gs or V_dg respectively. With this modelling approach, the channel current characteristics are based on high frequency S-parameter measurements and the effects of frequency dispersion in the transconductance and output conductance are not encountered. The channel current is derived from the nonlinear transconductance and output conductance and, in order to do this, a bias dependent resistor is added in parallel with the current source I_ds