Operational Amplifier Stability - Part 11 of 15: Modeling Complex Zo for Op Amps

by Tim Green, 
Senior Staff Systems Engineer, Apex Precision Power, a division of Cirrus Logic, Inc

Part 11 of this series will venture into the world of complex Zo inside operational amplifiers. As single supply applications of op amps have become more predominant at the board and systems level, semiconductor manufacturers have been challenged to create unique op amp topologies to provide rail-to-rail inputs and outputs along with high open-loop gains for accuracy and low noise on an ever-dwindling signal range (ie from ±10 V down to 0 – 5 V down to 0 – 3 V). These new and unique op amp topologies produce some very interesting and unique Zo characteristics which need to be understood and modeled in order to guarantee, by design, a stable circuit when driving reactive loads. We will look at a novel approach to modeling the complex output impedance of op amps. The building of an op amp Zo Block will allow us to move the op amp Zo outside of the traditional op amp SPICE macromodel and thereby separate the Aol curve from the effects of Zo interacting with reactive loads on the output. This will allow us, in a future Part, to simplify the challenges of stabilizing op amps with complex Zo characteristics.

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