Bipolar amplifier bias technique for robust IM3 null tracking independent of internal emitter resistance

Bipolar amplifiers can be biased to give a deep null in third order non-linearity, with the potential for high IP3 amplifier stages. This requires maintaining a precise voltage drop across a small resistive emitter degeneration resistance, whose value is related to kT/q. To make such a scheme practical, the bias must not only take into account the change in kT/q with temperature, but must compensate for variations in the degeneration resistance. In this paper we present a bias technique for IM3 null tracking that can take account of temperature and resistance tolerances, and is also insensitive to the value of the internal emitter resistance. Simulations using a 27 GHz BiCMOS technology indicate that the bias of an amplifier can be maintained over temperature, representative element tolerances, and mismatch such that the IP3 performance is maintained within ±9.5 dBV of the optimum null condition. The technique is applicable for a range of bipolar BiCMOS technologies and is attractive for amplifiers where high IP3 is required with moderate noise figure.