GPCAD: A Tool for CMOS Op-Amp Synthesis
M. Hershenson, S. Boyd, and T. Lee
Proceedings of the IEEE/ACM International Conference on Computer Aided Design (ICCAD), San Jose, CA, pp. 296-303, November 1998.
We present a method for optimizing and automating component and transistor sizing for CMOS operational amplifiers. We observe that a wide variety of performance measures can be formulated as posynomial functions of the design variables. As a result, amplifier design problems can be formulated as a geometric program, a special type of convex optimization problem for which very efficient global optimization methods have recently been developed. The synthesis method is therefore fast, and determines the globally optimal design; in particular the final solution is completely independent of the starting point (which can even be infeasible), and infeasible specifications are unambiguously detected. Also, because the method is very efficient, in practice it can be used to find the best op-amp topology out of a number of topologies. After briefly introducing the method, we show in detail how the method can be applied to a standard two-stage op-amp. We also show several example designs for other five op-amp topologies: two OTA op-amps, a two-stage cascoded op-amp, a folded-cascode op-amp and a telescopic op-amp. We also compute globally optimal trade-off curves relating power dissipation, unity-gain bandwidth and open-loop gain.