Power amplifiers are key components within the analog front ends of modern mobile communication devices. For the upcoming mass market applications in the sub-10GHz regime, the amplifying devices in such circuits will frequently be SiGe bipolar transistors due to their compatibility with silicon technology [1]. The modeling of these circuits requires large-signal transient circuit simulation, and the following important aspect for the design of SiGe bipolar microwave power amplifiers by circuit simulation will be addressed in this work: Time steps during transient circuit simulation are typically controlled such that the transient characteristics of the voltages and currents inside the circuit are well resolved. This is the method of choice, as long as the amplifying device is represented by a typical compact model modeling displacement currents at the device terminals by terminal charges, which are functions of the device terminal voltages. In the more complete modeling picture provided by mixed-level device/circuit simulation using a numerical device model [2] instead of a compact model, the displacement currents at the device terminals can be traced back to the time derivatives of the local electron and hole densities inside the device. Consequently, for minimizing time discretization error, these local time derivatives need to be considered as well by any proper time step control. Since power amplifiers reach their best efficiency typically in the compression mode, where high injection effects become important and considerably decrease internal device speed, it can be expected that the proper time resolution of the local carrier densities within the device will be especially important for such kind of circuits. This motivates this study, since the standard method of power amplifier design based entirely on compact models would be of questionable accuracy for cases, where the time resolution of the internal carrier densities turns out to be important for the overall circuit performance. Different power amplifiers operating in the compression mode will be simulated with and without considering local carrier densities for time step control, and the differences observable in the transient terminal voltage and current characteristics will be discussed.

* Supported by the German Ministery for Education and Research (BMBF) under contract no. 01 M 2416 C

[1] A. Schueppen, H. Dietrich, U. Seiler, H. von der Ropp, and U. Erben. A SiGe RF Technology for Mobile Communication Systems. Microwave Engineering Europe, June 1998, pp. 39-46.
[2] M. Stecher, B. Meinerzhagen, I. Bork, J. M. J. Kruecken, P. Maas, and W. L. Engl. Influence of Energy Transport Related Effects on NPN BJT Device Performance and ECL Gate Delay Analyzed by 2D Parallel Mixed-Level Device/Circuit Simulation. IEICE Trans. on Electronics, E77-C, 2, 1994, pp. 200-205.