Common mode sensing and control in balanced amplifier chains

Claim 1

A chain of fully differential amplifiers comprising: at least two amplifiers connected in cascade, each amplifier of the chain comprising an input differential pair of transistors each having a gate connected, respectively, to a first and a second input terminal of the amplifier, the respective sources of said transistors being connected in common; wherein at least the first or preceding one of said at least two cascaded amplifiers has two output terminals cascadedly connected to the following amplifier in the chain; means for sensing the output common mode voltage of said first or preceding amplifier; means for comparing the sensed output common mode voltage of said first or preceding amplifier with a preset reference voltage and for generating an error voltage signal proportional to the difference between said sensed output common mode voltage and said preset reference voltage; and means for maintaining said output common mode voltage of said first or preceding amplifier below a maximum value set by said reference voltage by applying said error signal to an output common mode voltage control terminal of said first or preceding amplifier; wherein said sensing means, said comparing means and said means for applying said error signal comprise an electrical connection between said commonly connected sources of said input differential pair of transistors of said amplifier which follows said first or preceding amplifier in the chain of cascaded amplifiers and a first input terminal of a differential input comparator having a second input terminal to which said preset reference voltage is applied by means of a level shifting circuit; and an electrical connection between an output of said differential input comparator and said output common mode voltage control terminal of said first or preceding amplifier.