We report the efficient and stable operation at 1549 nm of an actively Q-switched fiber laser based on an Er3+/Yb3+ doped double-clad single mode fiber. It operates with a repetition rate from 45 to 120 kHz and pulse duration from 34 to 80 ns; the output pulse shape and peak pulse intensity are stable over hours of operation. For a repetition rate of 120 kHz and a maximum pump power of 8.1 W we obtained an average output power of 4.0 W with an overall efficiency of 50% and with a minimum pulse duration of 34 ns.