Simulating Gain-Bandwidth—the generic op amp model

It may not always be obvious how the gain-bandwidth product (GBW) of an op amp may affect your circuits. Macro-models have a fixed GBW. Though you can look inside these models, it’s best not to tinker with them. What to do?

You can use a generic op amp model in SPICE to check your circuits for sensitivity to GBW. Most SPICE-based circuit simulators have a simple op amp model that you can easily modify. TINA’s is shown in figure 1.

First, set its DC open-loop gain to 1M (120dB). Then, a dominate pole frequency (entered in Hz) will create a GBW of the amplifier in MHz. In this example, a 10Hz dominate pole creates a GBW of 10MHz. Figure 2 shows the open-loop response for three different gain-bandwidths, 5MHz, 10MHz and 100MHz.

Note that that this simple model also includes a second pole (some folks call it a nuisance pole). In some cases, you may want to make this second pole a very high frequency such as 10GHz. This will create an ideal 90° phase margin for any reasonable GBW.  In this example, I set the second pole at 100MHz, equal to the highest GBW that I’m simulating. You can see the effect of this second pole in the 100MHz GBW response, causing the open-loop response to bend downward at 100MHz. It causes the unity-gain bandwidth to pull in to approximately 78MHz, similar to what you might see with a real op amp of this GBW. Unity-gain bandwidth and GBW of a real op amp are not necessarily the same number. (Does this need another blog?)

Active filter designs can be tricky to judge GBW requirements and are a good case for use of this technique. FilterPro, used to design the Chebyshev¹ filter in figure 3, provides GBW recommendations but its guidelines may be more stringent than needed in some circumstances. For this design, it recommends a 100MHz or greater GBW to achieve nearly ideal filter design characteristics. I simulated the design using the three gain-bandwidths shown in figure 2, 5MHz, 10MHz and 100MHz. With these results you might decide that a GBW less than 100MHz could be satisfactory. For final simulations, you should use the macro-model for the op amp you select.

I used the parameter stepping function in TINA, varying the dominate pole to change the GBW. Other simulators have similar capability. Of course, parameters could be changed manually, too. Either way, varying the GBW of a generic op amp model will give you insight on its effect in your circuits.

Have you used a generic op amp model to vary other parameters? Comments welcome.

Bruce       email:  thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

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Simulations use fee TINA-TI. Filter designs use free FilterPro.

Note 1:  Geek fact—Chebyshev (Чебышёв) was a Russian mathematician who died in 1894. Use of his polynomials to create equal-ripple filters came later.