UWB Active Balun Design with Small Group Delay Variation and Improved Return Loss

Abstract

Different from distributed baluns, active baluns have group delay variations in the lower bands related to inherent internal capacitances and resistance in transistors. A negative group delay (NGD) circuit is employed as a compensator of group delay variation for an ultra-wideband (UWB) active balun. First, three-cell NGD circuit is inserted into a simple active balun circuit for realizing both group delay compensation and return loss improvement. The simulated results show a group delay variation of 4.8ps and an input return loss of above 11.5dB in the UWB band (3.1-10.6GHz). Then, a pair of one-cell NGD circuits is added to reduce the remaining group delay variation (3.4ps in simulation). The circuit with the NGD circuits was fabricated on an InGaP/GaAs HBT MMIC substrate. The measured results achieved a group delay variation of 7.7ps, a gain variation of 0.5dB, an input return loss of greater than 10dB, and an output return loss of larger than 8.1dB in the UWB band.