The operational amplifier is one of the most versatile and important units in the analog-to-digital conversion circuit. A fully differential op amp is an op amp with both input and output differential signals. Compared with ordinary single-ended output op amps, it has the following advantages: The output voltage swing is larger; it better suppresses common mode noise; more Low noise; even-order terms that suppress harmonic distortion are better. Therefore, high-performance op amps usually use a fully differential form.
In recent years, the higher unity-gain bandwidth frequency and larger output swing of fully differential op amps have made it more widely used in high-speed and low-voltage circuits. With the increasing data conversion rate, the demand for high-speed analog-to-digital converters is becoming more and more extensive, and high-speed analog-to-digital converters require high gain and high unity-gain bandwidth op amps to meet the needs of system accuracy and rapid establishment. Speed and accuracy are the two most important performance indicators of analog circuits. However, the requirements of these two are mutually restrictive and contradictory. So it is difficult to satisfy both requirements at the same time. Folding cascode technology can successfully solve this problem. An op amp of this structure has a high open loop gain and a high unit gain bandwidth. The disadvantage of a fully differential op amp is that the common-mode loop gain of its external feedback loop is small, and the output common-mode level cannot be accurately determined. Therefore, a common-mode feedback circuit is generally required.