Operational Amplifier Stability - Part 9 of 15: Capacitive Load Stability: Output Pin Compensation by Tim Green, Linear Applications Engineering Manager, Burr-Brown Products from Texas Instruments Incorporated
Part 9 of this series is the fifth verse of our familiar electrical engineering tune, "There must be six ways to leave your capacitive load stable." The six ways are: Riso, High Gain & CF, Noise Gain, Noise Gain & CF, Output Pin Compensation, and Riso w/Dual Feedback. In Part 9, here, we cover Output Pin Compensation. This stability technique is NOT the same as an output op amp snubber network, which is often used on the output of power operational amplifiers (with all-npn output stages) to stop undesired, high-frequency oscillations when driving capacitive loads. Details of the use of the snubber network will be discussed in a later part of this article series.
Sometimes, in the real world, we do not always have access to the -input and/or +input of the op amp to allow us to use other compensation tricks in our analog tool box. Here we will derive the Output Pin Compensation technique for both emitter-follower output op amps and also CMOS RRO op amps. The emitter-follower application will entail a reference output on a unique 4 - 20 mA building block integrated circuit. The CMOS RRO application involves a difference amplifier used in the feedback for a power supply. Both of these definition-by-example cases are real-world applications where we will conclude our only stability option is Output Pin Compensation. In addition to first-order analysis and TINA Spice simulation, real-world implementation has been completed with as-predicted results.
