Method and apparatus for adjusting the common-mode output voltag

Details Method and apparatus for adjusting the common-mode output voltag Method and apparatus for adjusting the common-mode output voltag

1997-06-06

1999-06-22

Pascal, Robert

Amplifiers

With semiconductor amplifying device

Including differential amplifier

327 96, H03F 345, G11C 2702

Patent

active

059146389

ABSTRACT:
A fully differential sample-and-hold amplifier 60 includes a common-mode feedback circuit 100 for adjusting the common-mode input so that the common-mode output remains near a desired level. During a first switching state, the sample-and-hold amplifier samples voltages and is coupled to the feedback circuit 100 for the adjustment of the common-mode input level. During a second switching state, sample-and-hold amplifier performs its charge transfer and amplification function while the feedback circuit 100 is not coupled to the sample-and-hold amplifier. During the first switching state, the feedback circuit 100 receives as an input the common-mode voltage of the operational amplifier and outputs an amplification of the difference between the common-mode voltage and a regulating input voltage. The output of the sample-and-hold amplifier is coupled to the common-mode input of the operational amplifier. In this manner, differences between the common-mode voltage of the operational amplifier and the regulating voltage are used to adjust the common-mode input of the operational amplifier so that the common-mode output of the operational amplifier approximates the regulating voltage. The common-mode feedback circuit 100 includes a current source I1, a pair of differential input transistors M3 and M4, and a pair of load transistors M1 and M2. The gate of the input transistor M3 is coupled to the regulating voltage, while the gate of the input transistor M4 is coupled by a switching means to the output of the operational amplifier. The load transistors M1 and M2 are connected in an active load configuration. The output of the common-mode feedback circuit 100 is taken from the drain of the load transistor M1. The operational amplifier used in the sample-and-hold amplifier may be a telescopic cascode operational transconductance amplifier.

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