A simplified approach to optimum noise source impedance determination for GaAs FET amplifiers

A simple, analytical approach to determine the optimum noise source impedance of a GaAs FET amplifier in the 3-12 GHz frequency range is developed. The procedure can also be used in the 0-5—3 GHz range, but the model on which the procedure is based may be less accurate in this frequency range. The approach requires knowledge of the transistor's small-signal model parameters and its minimum noise temperature versus frequency. The approach is relatively insensitive to errors in the value of the GaAs FET small-signal parameters, but noise in the required minimum noise temperature data is potentially a source of nonlinear errors in the computed value of the optimum noise source impedance with respect to the error in the minimum noise temperature data The amount of error in the computed value of optimum noise source resistance is roughly proportional to the error in the minimum noise temperature data when the error at each data point is correlated, while the error in the computed value of the optimum noise source reactance is roughly three times the error present in the minimum noise temperature data. When the error is uncorrelated, the method does not yield acceptable results.