PURPOSE: To eliminate variation in feedback loop gain when amplifiers are independent and coupled together in parallel, by providing a loss circuit loop to each amplifier in parallel, and breaking the loss circuit loop of one amplifier when the other amplifier is disconnected.

CONSTITUTION: Amplification part 3 and 3' are connected to an input coupling circuit 2 and an output coupling circuit 6, and part of the output of an output circuit 7 is fed back to an input circuit 1 through a feedback circuit 5 to constitute a no-break amplifier. Loss circuits 9 and 9' are added to amplification part units 4 and 4' to interpose the loss circuits through the other amplification part units respectively. Therefore, when only one unit is in use, neither loss circuit is inserted and when two units are connected in parallel, variation in feedback loop gain is absorbed by those amplification parts through the loss circuits by the amplification gain of the parallel amplification parts. Consequently, no influence of the variation upon the main feedback amount loop is exerted to use the amplification parts stable without reference to they are independent or in parallel.