Taming Oscillations—the capacitive load problem

We’ve been looking at stability of op amps, considering how phase shift (or call it delay) in the feedback path can cause problems. Picking up from last week, stability with a capacitive load is a tricky case. If you’re joining us late, you may want to first read the previous two blogs, Why Oscillations and Taming Oscillations.

The trouble maker, open-loop output resistance (Ro) of the op amp, is not actually a resistor inside the op amp. It’s an equivalent resistance dependent on the internal circuitry of the op amp. There’s no chance to change it without changing the op amp. C_L is the load capacitance. If you want to drive a certain C_L, you are stuck with the pole created by Ro and C_L. A 1.8MHz pole inside the feedback loop of a 20MHz op amp in G=1 spells trouble. Check it out in figure 1.

Solutions to this issue have a common theme—they slow the amplifier down. Think about it… the loop has a fixed amount of delay, from Ro and C_L. To accommodate this delay, the amplifier must respond more slowly so that it does not speed past, overshooting a desired final value.

A good way to slow things down is to put the op amp in a higher gain. Higher gain decreases the bandwidth of the closed-loop amplifier. Figure 2 shows the OPA320 driving the same 1nF load but in a gain of 10. The response to a small step is dramatically improved, but still marginal. Increase the gain to 25 or more and it would look pretty good.

But here’s another trick. Figure 3 is still a gain of 10 but with Cc added, slowing things down a bit more in just the right way. Not enough Cc and the response looks more like Figure 2. Too much Cc and you are headed for trouble, more like figure 1.

Getting this compensation just right is solving a “rate of closure” issue—Bode analysis. It’s more than I can tackle in a blog so I’m trying to temp you. A bit of intuition is helpful with these problems but if you want to advance to the next level of phase compensation competence you need Mr. Bode.

My former colleague Tim Green wrote an article series on the subject of op amp stability and Bode analysis. And my colleague Collin Wells has done a great job distilling the message to its essence. If you are ready to take this topic a step further, I highly recommend that you start with Collin’s presentation attached below. And if you are lucky, you can see him present it live at a TI Tech Days somewhere near you.

Comments welcome and thanks for reading,

Bruce