We present model calculations of gain at 1550 nm for Er-Yb double-clad amplifiers as a function of pump power, signal input power, fiber length, and temperature. The calculated gain is ~20 dB for a 6-W pump for a 20-mW input signal at 25 C. More than 10 W of amplified power can be obtained using an 8-m-long fiber with ~25 W of pump power. The gain decreases with increasing input signal. We show that a high-power Er-Yb codoped double-clad fiber amplifier also exhibits high gain for Yb transition near 1060 nm. This is not unexpected, since the input pump causes population inversion in Yb also.