Actual Noise Measure of Linear Amplifiers

Haus and Adler defined a noise measure Me and proposed to use the optimum value Me,opt of the noise measure as a valid measure of the absolute quality of amplifier noise performance. However, neither Me nor Me,opt includes the contribution from the noise originating in, and reflected back to, the load. It is the aim of this paper to propose a different noise measure M, which includes the noise contribution from the load, and hence enables us to compare the performance of two different amplifiers which are not necessarily optimized. It is shown that the optimum value Mopt of M is equal to Me,opt, and most of the conclusions Haus and Adler obtained for Me,opt hold equally well for Mopt. But the meaning of M is quite different from that of Me. A detailed discussion about this difference is given in the final section, and it is shown that for practical amplifiers, M is a more appropriate measure of the quality of the noise performance.