Biasing an HBT MMIC transistor for efficiency and output power enhancement

An optimization method for the variation of bias with input power, also referred to as dynamic biasing, is presented and applied to an unmatched InGaP 21-dBm heterojunction bipolar transistor (HBT). For a modulated signal with a crest factor of 6.5 dB, the optimization yielded two solutions for an output power of 14.8 dBm: one with 40% power added efficiency (PAE), the other one with 22%, which is twice the PAE of the static-biased transistor while holding nearly the same linearity levels. With dynamic bias it was possible to drive the transistor harder than with static bias: at an average output power corresponding to its single-tone 1-dB compression point, 21 dBm, a PAE of 38.4% was obtained with -36.6 dB adjacent channel power ratio (ACPR). Such an output power was unachievable with static bias due to breakdown. Varying the base current alone yielded a 7-point PAE improvement and 1 dB reduction in ACPR for an output power of 17.8 dBm.