Direct capacitive loading reduces the phase margin of many op amps. A pole in the feedback loop is created by the combination of the op-amp output impedance and the capacitive load. This pole induces phase lag at the unity-gain crossover frequency of the amplifier resulting in either an oscillatory or underdamped pulse response. With a few external components, op amps can easily indirectly drive capacitive loads. Figure 6-2 shows an example.

Figure 6-2 LMC604x Noninverting Gain-of-10 Amplifier, Compensated to Handle Capacitive Loads

In the circuit of Figure 6-2, R1 and C1 serve to counteract the loss of phase margin by feeding the high-frequency component of the output signal back into the inverting input of the amplifier, thereby preserving phase margin in the overall feedback loop.

Capacitive load driving capability is enhanced by using a pullup resistor to V⁺ (Figure 6-3). Typically, a pullup resistor conducting 10μA or more significantly improves capacitive load responses. The value of the pullup resistor must be determined based on the current sinking capability of the amplifier with respect to the desired output swing. The open-loop gain of the amplifier can also be affected by the pullup resistor (see Section 5.7).

Figure 6-3 Compensating for Large Capacitive Loads With a Pullup Resistor