6 Summary

Amplifier circuits driving large capacitive loads become unstable due to the interaction between the open-loop output impedance and the load capacitance. When driving a large capacitive load, an optimal isolation resistor must be implemented to maintain closed-loop stability with minimal voltage error. For amplifiers with resistive open-loop output impedance, an equation was derived to quickly determine an optimal R_ISO value. The magnitude of R_ISO is dependent on the magnitude of the load capacitance, the gain bandwidth of the amplifier, and the open-loop output impedance. Amplifiers with low open-loop output impedance are best for driving capacitive loads.

The analysis in this document considers amplifiers with resistive output impedance over frequency. Certain amplifier architectures exhibit a complex output impedance, Z_O, across the bandwidth of the amplifier. Amplifiers with complex output impedance require further analysis and additional techniques to optimize capacitive load-drive stability. The scope of this article is limited to amplifiers that exhibit a resistive output impedance similar to the OPA392 output impedance shown in Figure 1-1.