The OPA4991-EP features a resistive output stage capable of driving moderate capacitive loads, and by leveraging an isolation resistor, the device can easily be configured to drive large capacitive loads. Increasing the gain enhances the ability of the amplifier to drive greater capacitive loads; see Figure 7-5 and Figure 7-6. The particular op amp circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing whether an amplifier will be stable in operation.

Figure 7-5 Small-Signal Overshoot vs Capacitive Load (10-mV Output Step, G = 1)

Figure 7-6 Small-Signal Overshoot vs Capacitive Load (10-mV Output Step, G = –1)

For additional drive capability in unity-gain configurations, improve capacitive load drive by inserting a small resistor, R_ISO, in series with the output, as shown in Figure 7-7. This resistor significantly reduces ringing and maintains DC performance for purely capacitive loads. However, if a resistive load is in parallel with the capacitive load, then a voltage divider is created, thus introducing a gain error at the output and slightly reducing the output swing. The error introduced is proportional to the ratio R_ISO / R_L, and is generally negligible at low output levels. A high capacitive load drive makes the OPA4991-EP an excellent choice for applications such as reference buffers, MOSFET gate drives, and cable-shield drives. The circuit shown in Figure 7-7 uses an isolation resistor, R_ISO, to stabilize the output of an op amp. R_ISO modifies the open-loop gain of the system for increased phase margin.

Figure 7-7 Extending Capacitive Load Drive With the OPA4991-EP