Amplifier Bias for Minimum Noise Figure in Thermally Constrained Systems

The noise figure of low noise amplifiers (LNAs) critically depends, amongst others, on the biasing conditions and the transistor temperature. The latter may be constrained by the performance of the cooling system, in particular its thermal resistance, and the ambient temperature. This paper reports as a case study how the noise figure can be minimized in such constrained systems by properly adjusting the biasing conditions. For this, the two-dimensional noise figure dependence on the temperature and the drain current is measured for a commercial LNA. For a systematic mathematical treatment, the measured data are interpolated by means of two black-box models. In contrast to the first one, which requires the noise figure to be measured in a climate cabinet, the second one only necessitates room temperature data in addition to commonly available data sheet values. It is shown, that, depending on the thermal resistance of the cooling system and the ambient temperature, i.e., two constraining system parameters, an optimal drain current setting may reduce the dissipated power by up to about 50 % compared to the nominal data sheet value. As this lowers the heat load and, in turn, the device temperature, the noise figure can be improved by up to 0.4 dB.