Typically op amp circuits contain many poles. In a folded-cascode topology, both the folding node and the output node contribute poles. For this reason, op amps must usually be compensated, i.e their open-loop transfer function must be modified such that the closed-loop circuit is stable and the time response is well-behaved.

The first approach requires that we attempt to minimize the number of poles in the signal path by proper design. Since each additional stage contributes at least one pole, this means the number of stages must be minimized, a remedy that yields low voltage gain and/ore limited output swings.

The second approach, on the other hand, retains the low-frequency gain and the output swings but it reduces the bandwidth by forcing the gain to fall at lower frequencies.

As shown in the figure, we’ve a PMOS current mirror which performs differential to single ended-conversion. We identify a number of poles in the signal paths. Path1 contains a high frequency pole at the source of M3, a mirror pole at node A1 and another high-frequency pole at the source of M7 whereas path 2 contains a high frequency pole at the source M4. These two paths share a pole at the output.

Since the output resistance of the op amp is much higher than the small-signal resistances seen at the other nodes in the circuit, we expect that, even with a moderate load capacitance, the output pole, ωp,out is the closest to the origin, Called the dominant pole, ωp,out usually sets the open-loop 3-dB bandwidth.

Let’s assume that the number and location of the non-dominant poles and hence the phase plot at frequencies higher than roughly 10ωp,out remain constant. Thus, we must force the loop gain to drop such that the gain crossover point moves toward the origin. To achieve this, we lower the frequency of the dominant pole in the vicinity of the gain or phase crossover points is close to 900 and relatively independent of the location of the pole. Following figure shows the translation of dominant pole toward the origin which affects the magnitude plot but not the critical part of the phase plot.