We demonstrate output pulse energies of 20nJ from an erbium-doped fiber oscillator that contains only positive dispersion fibers and is mode locked by use of nonlinear polarization evolution and stabilized with a birefringent filter. The fiber oscillator operates at a repetition rate of 3.5MHz with a central wavelength of 1550nm . The positively chirped output pulses have a duration of 53ps and are compressed to 750fs . The large positive chirp of the output pulses and the steep side edges of the pulse spectrum indicate dissipative soliton operation.