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The impact of substrate parasitic effects on the power gain relation between the CE and CB configurations of low-power SiGe HBTs has been analyzed. In comparison to large-area power SiGe HBTs, the speed and small-signal power gain of the device are much higher. Nevertheless, the relative parasitic effects on these small devices are much stronger than that on the large ones. It is found that due to the influence of substrate parasitic effects (input and output coupling between input and output pads), the power gain of CB SiGe HBTs can be dramatically degraded, while the power gain of CE SiGe HBTs is not affected much. The consequence of the different influences of parasitic effects on CE and CB SiGe HBTs is that the intrinsic higher power gain of the CB SiGe HBT than the CE device is lost, which is verified by measurements. Based on the detailed analysis of the substrate parasitic effects, an approach to restoring the superior power gain characteristics of CB SiGe HBTs is proposed.