Optimal Design of a CMOS OpAmp via Geometric Programming
Rob A. Rutenbar, Georges E. Gielen, Brian A. A. Antao · 2009
We describe a new method for determining component values and transistor dimensions for CMOS operational amplifiers (op-amps), We observe that a wide variety of design objectives and constraints have a special form, i.e., they areposynomialfunctions of the design variables. As a result, the amplifier design problem can be expressed as a special form of optimization problem calledgeometric programming, for which very efficientglobal optimizationmethods have been developed. As a consequence we can efficiently determineglobally optimalamplifier designs or globally optimal tradeoffs among competing performance measures such as power, open-loop gain, and bandwidth. Our method, therefore, yields completely automated sizing of (globally) optimal CMOS amplifiers, directly from specifications.In this paper, we apply this method to a specific widely used operational amplifier architecture, showing in detail how to formulate the design problem as a geometric program. We compute globally optimal tradeoff curves relating performance measures such as power dissipation, unity-gain bandwidth, and open-loop gain. We show how the method can be used to sizerobust designs, i.e., designs guaranteed to meet the specifications for a variety of process conditions and parameters.