Recently, interest hasbeen shown in YDFA. The reasons for this growth of interes include it's simple energylevel structure, no excited state absorption at either pump or signal wavelengths, noconcentration quenching, and it's broad spectrum both in absorption and emi:sion. In thispaper, the YDFA's which incorporates isolator, filter or fiber grating are investigatetheoretically, using a simple model. The optimum placement of the filters and isolatorsare als calculated so that the gain and noise performance can be improved simultaneously.