4 The drop-in solution

There is also a solution when you don’t want extra compensation circuitry, or it’s not feasible to add it. TI’s OPA994 device family has a special compensation structure that is stable across capacitive loads, which is possible because the bandwidth of the device changes when the output sees different capacitive loads. Keeping the bandwidth constantly lower than the pole introduced by the output impedance and capacitive load will maintain the stability of the amplifier, regardless of what capacitor you place on the output. Figure 5 illustrates the phase margin for different values of load capacitance with no external compensation resistor, taken from the OPA994 data sheet.

Figure 5 Phase margin over various capacitive loads in unity gain.

Every design decision comes with a cost in addition to a benefit, and the OPA994 device family is no different. A more complex design results in a larger device, which can be more costly than simpler devices. Additionally, this means that the device cannot fit into TI’s smallest packages, such as the 0.64mm²extra-small outline no-lead (X2SON) package. This design is currently only available in a bipolar amplifier, so if you require the low-input bias current of a complementary metal-oxide semiconductor (CMOS), this device may have too high of an input bias current.

There are many benefits associated with a bipolar amplifier, including lower noise and more bandwidth, for less quiescent current than CMOS devices. The full trade-off of bipolar vs. CMOS can be weighed on a circuit-by-circuit basis [1]. Overall, the OPA994 can in many cases serve as a drop-in solution for stability.