Effect of gain expansion on power HBTs

This paper describes the effect of gain expansion on power heterojunction bipolar transistors (HBTs). Gain expansion improves the power-added efficiency of power amplifiers in the high-input-power region under the CDMA criteria because the gain expansion compensates for the gain compression in the high-input-power region. RF simulation showed that the gain expansion of HBTs is caused by increased maximum stable gain in the low-collector-current region. Class-B operation was found to enhance gain expansion due to the transconductance drastically increasing with the collector current in the class-B operation region.