Op amps… G=1 stable & decompensated

-April 07, 2013

You have voted. Unity-gain-stable op amps won in a landslide—they’re far more popular than decompensated op amps. What’s this all about?

Unity-gain-stable op amps (let’s call them UGS) are stable in the common G=+1 configuration, returning 100% of the output signal back to the inverting input. While it would be incorrect to call this truly a “worst case” for stability, you could reasonably call it a very common, testy case.

Decompensated op amps have smaller compensation capacitors that yield wider gain-bandwidth and faster slew rate. While higher speed normally demands more power, the same basic op amp can be significantly faster while operating on the same current. But they are not UGS—they must be used in noise gains significantly greater than unity.  My colleague, Soufiane, recently wrote about decompensated op amps—check it out here—but I have some additional observations.

Figure 1 shows the critical portion of the gain vs. frequency graph for an idealized pair of UGS and decompensated op amps. The decompensated version has five-times the gain-bandwidth—10MHz vs. 2MHz. Slew rate gets the same boost. Note that the unity-gain bandwidth of the UGS op amp is slightly less than its GBW, a common behavior. The unity-gain bandwidth of the decompensated amp is half its GBW. We have no business operating this amp with noise gain near the unity-gain bandwidth because a second pole at 3MHz greatly affects the gain/phase behavior in this region. Phase margin would be poor or non-existent.

Decompensated op amps seem to hold a certain mystery, leaving some users uncertain whether their circuits will be stable. And we occasionally handle application issues on our forums that can be traced to misuse of a decompensated op amp. A common misstep is shown in figure 2a. Though this amplifier is connected in a signal gain of -10, a feedback capacitor rolls off the response at high frequency. This capacitor can be a virtual short-circuit at high frequency where stability issues are a concern—unity gain. It’s okay to use a small feedback capacitor to compensate the feedback network for flat gain but a larger cap that rolls off response is sure to create problems.

Likewise, the multiple-feedback filter in figure 2b invites trouble regardless of the low-frequency gain of the filter. The integrator (figure 2c) is yet another application not suited to decompensated op amps.

We’ve improved our op amp designs. We’re smarter now and we have much better IC processes. With a few hundred microamps we can now make an amplifier that required a couple milliamps in the past. So sometimes a modern UGS op amp may come very close to, or even beat, the speed/power of an older decompensated amp. Still, a decompensated op amp may be the best solution for a demanding application.

Let me be clear. I’m not trying to urge selection of decompensated versus UGS op amps. Each has merits and you get to “vote” with your design choices. Whatever your selection, you should clearly understand the differences and issues. If you are unsure, help is available at our amplifier E2E forum.

Here are some examples of decompensated and UGS op amp pairs:

  • OPA228  (OPA227 UGS version)   Precision, Low Noise BJT Op Amp
  • OPA637  (OPA627 UGS version)   Precision, High Speed JFET Op Amp
  • OPA345  (OPA344 UGS version)   Rail-to-Rail CMOS Op Amp
  • LMP7717  (LMP7715 UGS version)  88MHz CMOS Op Amp

What’s your experience? Have you had success or difficulty with decompensated op amps?

Thanks for reading,

Bruce       email:  thesignal@list.ti.com

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