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APPLICATION INFORMATION (CONTINUED)

Capacitive Load and Stability

R

S

100

Ω

C

L

R

L

V

IN

V

OUT

50

µs/div

OPA336-EP

SINGLE-SUPPLY MICRO-POWER CMOS OPERATIONAL AMPLIFIER

MicroAmplifier™ SERIES

SCES658 – JUNE 2006

The OPA336 can drive a wide range of capacitive loads. However, all operational amplifiers, under certain

conditions, may become unstable. Operational amplifier configuration, gain, and load value are just a few of the

factors to consider when determining stability.

When properly configured, the OPA336 drives approximately 10,000 pF. An operational amplifier in unity-gain

configuration is the most vulnerable to capacitive load. The capacitive load reacts with the operational amplifier

output resistance along with any additional load resistance to create a pole in the response, which degrades the

phase margin. In unity gain, the OPA336 performs well with a pure capacitive load, up to about 300 pF.

Increasing gain enhances the amplifier's ability to drive loads beyond this level.

One method of improving capacitive load drive in the unity-gain configuration is to insert a 50-

Ω to 100-Ω

resistor inside the feedback loop (see Figure 3). This reduces ringing with large capacitive loads, while

these conditions (see Figure 5).

Figure 3. Series Resistor in Unity-Gain Configuration

Improves Capacitive Load Drive

Figure 4. Small-Signal Step Response

Using Series Resistor to Improve Capacitive Load Drive

9

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