A continuous-time common-mode feedback circuit for a two-stage differential amplifier

Abstract

The invention discloses a common-mode feedback circuit for duration of a two-stage differential amplifier. The common-mode feedback circuit comprises a two-stage differential amplifier, a common-mode voltage detecting unit and a common-mode feedback control signal generating unit, wherein the common-mode voltage detecting unit is used for detecting the output common-mode voltage of the two-stage differential amplifier; the common-mode feedback control signal generating unit is used for controlling the first-stage current source loading of the two-stage differential amplifier to be stable at a pre-arranged direct-current operation point; two outputs of the two-stage differential amplifier are connected with the common-mode voltage detecting unit respectively; the output of the common-mode voltage detecting unit is connected with the common-mode feedback control signal generating unit; the two outputs of the common-mode feedback control signal generating unit are connected with the first stage current source loading of the two-stage differential amplifier respectively. According to the invention, a 'dead lock' condition is prevented from occurring to the two-stage differential amplifier when the two-stage differential amplifier is in closed-loop application, and a prodigious transient value is emerged in the input common-mode signal.

Description

technical field [0001] The invention relates to the field of analog integrated circuits, in particular to a continuous-time common-mode feedback circuit for two-stage differential amplifiers. Background technique [0002] As the two-stage differential amplifier has the advantages of large output swing, good common-mode noise suppression, and can effectively suppress even-order harmonic distortion, it has been widely used. The common-mode feedback circuit is an indispensable key circuit in the fully differential operational amplifier. Its purpose is to stabilize the DC level of each node inside the differential amplifier, ensure that the transistors are all working in a saturated state, and precisely control the output common-mode voltage of the differential amplifier. level to optimize the output swing of the amplifier. The common-mode feedback circuit is also beneficial to suppress the common-mode fluctuation of the amplifier and improve its common-mode rejection ratio. ...

AI Technical Summary

Problems solved by technology

In practical applications, such as figure 1 As shown, when the common mode signal at the input of the two-stage differential amplifier has a large transient value, the M101 and M102 tubes of the two-stage differential amplifier are turned off, the current flowing through the M105 and M106 tubes is 0, and the M105 and M106 tubes Working in the deep linear region (assuming that the common-mode voltage detection circuit 101 has not transmitted the detected output common-mode voltage to the common-mode feedback control signal generation circuit 102), the drain terminal voltage of M105 and M106 tubes is pulled to close to ground (0V), causing the M107 and M108 tubes to be turned off, so that the output common-mode voltage of the two-stage differential amplifier is close to the power supply voltage VDD, so that the M111 tube is turned off, and the gate voltage of the M113 tube is pulled down, thereby making the The M105 and M106 tubes are turned off, causing the internal common-mode negative feedback loop to be disconnected, so that the open-loop gain of the external positive feedback loop is greater than 1, which meets the positive feedback conditions. At this time, even if the common-mode signal returns to normal, due to The outer loop has formed positive feedback, and the output voltage of the two-stage differential amplifier will continue to remain close to the power supply voltage, showing a "locked" state, making the amplifier unable to work normally

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