The gain performance and noise figure of ytterbium (Yb3+)-sensitized erbium (Er3+)-doped fiber amplifiers (EYDFAs) with a small numerical aperture of 0.16 are theoretically modelled. Taking into account the upconversion effect from the Ei(3+) levels (I-4(13/2) and I-4(11/2)), the Yb3+ <-> Er3+ energy transfer by cross-relaxation, the amplified spontaneous emission (ASE) as well as background losses, an amplifier model capable of obtaining a high signal gain and a low noise figure is investigated. The rate and power propagation equations of the amplifier are numerically solved under some pumping power configurations. The length of EYDFA is fixed to 0.77 m. At the pump wavelength of 1064 nm, the input pumping power is increased from 100 mW to 200 mW for a small signal power application of 1 mu W at the signal wave-length of 1530 nm. According to the numerical calculations, it is seen that 0.77 m-long Yb3+-sensitized Er3+-doped fiber amplifier shows a high gain of 45.4 dB and a low noise figure of 4.1 dB. (C) 2008 Elsevier B.V. All rights reserved.