On the safe operating area of bipolar cascode amplifiers

This paper presents an investigation of the safe-operating-area boundary of bipolar cascode amplifiers in GaAs and SiGe technologies. A simple relation is derived to predict the instability onset due to electrothermal and avalanche effects. Circuit simulations and experiments performed on GaAs test structures are employed to analyze the influence of thermal coupling between transistors and the beneficial impact of base ballasting.