Physics, Modeling and Design Implications of RF Correlated Noise in SiGe HBTs

Abstract

Accurate noise compact modeling and efficient noise extraction techniques are required for RF circuit design. Understanding the impact of noise sources and the noise propagation is also necessary for device and circuit noise optimization. In this work, we discuss RF noise physics, modeling, extraction and circuit design implications. The related compact model parameters are determined based on the experimental results of various SiGe HBTs.

After a review of previous noise model and their implementation in compact models, we develop a much improved physics-based compact noise model for use with any existing compact models. We investigate the impact of CB SCR transit time effect on noise parameters of bipolar transistors, together with the noise transport in the neutral base. A model suitable for compact model implementation is developed. The resulting frequency dependence and correlation of terminal current noises can be generated from independent white noise sources, which is important as the current standard simulators are unable to handle correlated noise sources.

We present a new compact modeling approach to extraction of intrinsic transistor terminal current noises and terminal resistance thermal noise. The extraction method are based on the transfer function which can be calculated using ac small signal simulation results. Thus this method is independent of specific compact model and avoids tedious element by element de-embedding procedure.

The relevant importance of noise sources are evaluated in various noise representations and proved to be varying from one representation to another. It is even unfair to claim the noise source's dominance within one single representation as it depends on each noise power spectre density and noise parameter. The base resistances are identified to be the most important elements of the extrinsic network that determine the intrinsic terminal noise current propagation towards the external terminals. Such propagation increases the $N\!F_{min}$ due to intrinsic noise currents considerably, making it much higher than the $N\!F_{min}$ due to $R_b$'s thermal noise. $R_b$'s role as an impedance element can be much more important than its role as a thermal noise source for practical SiGe HBTs. Analytical expression are derived to demonstrate that the well known effect of noise correlation is shown to be highly dependent on $R_b$. Consequently, $R_b$ reduction should continue for $N\!F_{min}$ improvement despite a nearly negligible $N\!F_{min}$ due to $R_b$'s thermal noise.

Impact of correlated RF noise on LNA design is also examined. The useful simultaneous noise and impedance matching conditionally holds based on the simplified analytical derivation results. Simulation results show that noise matching requires a considerably larger transistor and power consumption in the presence of intrinsic terminal current noise correlation. The actual noise figure of LNA designed using SPICE model are found to be overall comparable to that of the correlated model designed LNA, which is due to the small noise conductance tolerating significant noise mismatch.