Ultra-Wideband, Low Power, Silicon Germanium Distributed Amplifiers

Abstract

<p> As modern digital communications evolve, the requirements imposed on the systems than are required to transmit/receive the signals involved become more stringent. Amplifiers are required to provide gain from low frequencies, sometimes down to DC, up to high frequencies in the order of few to tens of gigahertz. Not only is the gainbandwidth product to be enhanced, but also the amplifier should introduce minimal distortion to the signal and consume as low power as possible. </p> <p> Distributed amplification is a multi-stage broadband circuit technique that may provide such a function. In distributed amplifiers, inter-stage transmission lines provide the capability to reach higher operational frequencies by absorbing the parasitic capacitances of the transistors used. Unlike other broadband topologies that trade-off gain and bandwidth, distributed amplifiers do not, but rather, the trade-off is between gain and delay. As gain stages are added, the gain increases as the bandwidth remains the same but the signal delay is increased. </p> <p> This work considers the silicon germanium (SiGe) heterojunction bipolar transistor (HBT) implementation of distributed amplifiers. SiGe HBTs incorporate a thin SiGe base with Ge profiling to achieve high cut-off frequencies. SiGe BiCMOS processes are silicon based and hence have the major advantage of integrability to the low cost CMOS process unlike ill-V compound semiconductors. Hence, SiGe is a promising technology capable of bridging the performance gap between silicon and m-v semiconductors. </p> <p> The proposed amplifier achieves an approximately flat gain of 6.5 dB and a noise figure of 5.8-9 dB throughout the -3 dB passband of 10.5 GHz. The power consumed is 12.2 mW, significantly lower than previously published results by up to an order of magnitude is some cases. The group delay of the amplifier was found to be approximately constant in the passband at -60 ps. </p> <p> For the first time, temperature measurements are preformed on SiGe HBT DAs. Analysis show that the gain falls drastically with temperature increase due to deterioration in input matching caused by the significant change in the transistors input impedance with temperature. Similarly the NF, increases with temperature due to the decrease in gain. Moreover, noise analysis of SiGe HBT DAs is investigated, producing simulations predicting the NF of the proposed amplifier giving insight as to how noise may be reduced in future designs. </p>