SYMPHONY, a mixed signal simulator which exploits the special characteristics of BiMOS mixed signal circuits is presented. SYMPHONY contains a fast simulator for digital circuits and several new techniques to efficiently simulate BiMOS circuits. The typical switching behavior of bipolar devices in digital setting is exploited by using a simplified model to approximate the bipolar device char acteristics. The problem of minimizing the worst case approximation error is formulated and a heuristic is pro posed to achieve this by using a PWL model with expanded Chebyshev points as the breakpoints. Dynamic circuit partitioning is combined with an event-driven approach to exploit the latency and multi-rate behavior. Experimental results demonstrate that SYMPHONY can yield 2x-250x speed-up over SPICE3e depending upon the amount of analog circuitry present in the design.