High frequency power transistor model
The development of a large-signal power transistor model applicable at radio frequencies is described. This model which has its basis in the classical large-signal models is valid for cut-off and active region operation but does not include saturation operation. The model is intended primarily for use in broadband linear radio frequency amplifier applications and is useful up to frequencies of the order of 1/15 f┬. The model is described by two first order nonlinear differential equations and a number of algebraic equations. Equation coefficients are determined from measurements made on the devices under study. Two methods are described for the solution of the model equations. The first and principal method is an iterative one requiring computer assistance whilst the second is analytical and depends upon piecewise linearisation of the device transfer characteristic. This analytical method whilst in some respects inadequate, e.g. distortion level predictions, is easy to implement and despite its limitations affords useful insight into output power capability and frequency limitations of specific devices. The model contains all transistor nonlinearities and parasitic elements of significance and an important feature is the inclusion of device temperature as a model variable, resulting in good accuracy over a wide range of operating conditions. A simplified input impedance representation is evolved and it is demonstrated that input impedance measurements provide a useful window on model structure and aid in the evaluation of parameter values.
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