Operational amplifiers are the heart of most voltage-mode analog circuits. They usually dictate the operation speed and the accuracy of the switched-capacitor (SC) circuits. They also consume most of the power in the SC circuits. High performance analog-to-digital (A/D) converters usually use the SC circuit technique. Therefore, the performance of the operational amplifiers determines the performance of the A/D converters.
For the SC circuits, the load is purely capacitive. Usually single stage operational transconductance amplifiers (OTAs) are preferred over multi-stage operational amplifiers. In OTAs, the capacitive load is used to create the single dominant pole, which usually yields high unity-gain bandwidth. The DC gain is usually moderate but can be improved by cascoding. For multi-stage operational amplifiers, internal miller capacitors and sometimes resistors are used to split poles and introduce zeros to compensate for the phase lag and the frequency response can be independent of the load. However, the unity-gain bandwith is usually lower than the single-stage OTAs, though the DC gain is higher due to the cascading of more stages. For high speed A/D) converters, usually single-stage architectures are preferred in that it is possible to achieve a single-pole settling and to have a very wide bandwidth. However, the gain is usually not enough for high accuracy A/D converters.
In for example the document U.S. Pat. No. 7,149,956 is shows a fully-differential operational amplifier for MOS integrator circuits, where the operational amplifier has one cascode transistor pair in the P-branch and one cascode transistor pair in the N-branch, see FIG. 5 in the said document.
USPTO Backgrounds
USPTO Backgrounds