12 Rail-to-Rail Output and Open-Loop Gain Dependence

As mentioned previously, the traditional output stage uses a common emitter topology and the rail-to-rail output stage uses a common collector topology. The common emitter configuration gain is approximately unity, whereas the common collector has a significant gain that is dependent on the load impedance. The open-loop gain for an op amp is the product of all the gain values for each amplifier stage. So, for a rail-to-rail amplifier, the DC open-loop gain changes when the amplifier load changes. Thus, for rail-to-rail devices, connecting a low-resistance load on the output of the op amp causes open loop gain to drop compared to the unloaded case. Conversely, the loading impact of loading on a traditional common emitter output stage has a minimal effect on open-loop gain. Table 12-1 shows an excerpt from a data sheet showing how open-loop gain is impacted by load resistance. Figure 12-1 shows how this loading impacts the A_OL curve. Although this discussion focused on bipolar amplifiers the same concepts apply to CMOS devices.