The mixer amplifier includes an amplification stage having a current source circuit and a plug filter adapted to modify the current circulating in the current source circuit. The amplification stage and a mixer stage amplify an incoming signal and transpose the frequency of the signal to a predetermined frequency. Resistors pairs measure the imbalance between two branches and have a relatively high value (thus creating a high-pass filter). When the branches are perfectly balanced, the voltage tapped by the non-inverting terminal of the operational amplifier A is zero. During an imbalance, this voltage rises. The output of the amplifier drives the TNP transistor T9, causing current to flow into branches to solicit the transistors T7 or T8 of the current source circuits and thus return the two branches to balance. Accordingly, a balance is maintained between the two branches by providing a feedback within the mixer amplifier.