Simulator Based Device Sizing Technique For Operational Amplifiers

In this work, design of CMOS operational amplifiers (opamps) is discussed using a new technique of transistor sizing, called as Potential Distribution Method (PDM). PDM is a technique proposed to simplify analog design and is primarily based on voltage and current distribution at different nodes of a circuit. Apart from being technology independent, PDM is also free from complex mathematical expressions governing the devices and the circuit. Instead of relying on traditional analytical methods, PDM directly uses the simulator as a device sizing tool to meet the desired performance from the circuit. This is achieved by first designing the circuit with moderate performance by logically allocating node voltages and currents, and then modifying the potentials and currents (dc operating points) to meet the target specifications. A fully differential folded cascode opamp is thus designed to illustrate the design methodology. The effect of node voltages and branch currents on response of the opamp is also examined. Thereafter, using these dependencies, the guidelines for tuning the performance parameters to achieve the target specifications are discussed. The methodology demonstration is carried out using UMC 180 nm CMOS technology at 1.8 V supply and the simulation results are presented. Key–Words: Analog design, CMOS, operational amplifier, transistor sizing, simulation, fully differential folded cascode.