Question 22.2: Figure 22.5 shows a folded cascode amplifier (see Fig. 20.45......
Figure 22.5 shows a folded cascode amplifier (see Fig. 20.45). Assuming that all MOSFETs are operating in the saturation region, estimate the minimum and maximum input voltage of the amplifier.
Note how the widths of M5-M6 are doubled to sink the additional current from the diff-amp. It’s important to understand how the sizes of the MOSFETs are adjusted in the current mirrors so that the currents sum correctly. The currents, as seen in Fig. 22.5, are labeled assuming V_{plus} = V_{minus}.
The maximum allowable input common-mode voltage is determined by
V_{C MM AX}=V D D-V_{S G}-2V_{S D,s a t}=5-1.15-0.5=3.35~VThe minimum input common-mode voltage is determined by noting the drain voltage of M5 and M6 (or M1 and M2) is a V_{S D,s a t}
V_{S D}\geq V_{S G}-V_{T H P}\to V_{D}\leq V_{G}+V_{T H P}and so
V_{C M MI N}=V_{D}-V_{T H P}=0.25-0.9=-0.65In other words, the input common-mode voltage can actually be below ground and the amplifier will still function correctly. This result is very useful. The folded-cascode amplifier with PMOS diff-amp can be used when the input voltage swings around ground. Note, however, that the output voltage of the amplifier will not swing below ground but is, rather, limited to 2V_{S D,s a t} above ground and 2V_{S D,s a t} below VDD if the MOSFETs are to remain in saturation (the high-gain region of operation).