1. A multiple-input and multiple-output amplifier having one reference terminal, n signal input terminals and n signal output terminals, where n is an integer greater than or equal to 3, the multiple-input and multiple-output amplifier comprising: n active sub-circuits, each active sub-circuit having a sub-circuit input terminal, a sub-circuit output terminal and a sub-circuit common terminal, the sub-circuit input terminal being connected to one of said signal input terminals and the sub-circuit output terminal being connected to one of said signal output terminals, each active sub-circuit being configured such that current flowing out of the sub-circuit common terminal and current flowing into the sub-circuit output terminal depend on a voltage between the sub-circuit input terminal and the sub-circuit common terminal, each said signal input terminal being connected to only one sub-circuit input terminal and each said signal output terminal being connected to only one sub-circuit output terminal; and a feedback network having a terminal connected to said reference terminal, the feedback network also having n other terminals each being connected to the sub-circuit common terminal of one of said active sub-circuits, the feedback network presenting, in a known frequency band, a non-diagonal impedance matrix, this impedance matrix being defined with respect to said reference terminal, the feedback network comprising two or more windings arranged in such a way that, in a part of the known frequency band, the mutual induction between the different windings of the feedback network has a non-negligible influence on the value of one or more non-diagonal components of said impedance matrix.