Joachim your practical metod it is a good rough aproximation. I'll try that in the next days.Why I was asking about this problem ? Because I have in the input stage of the amplifier a parasitic capacitance and that absorb a lot of current in the high frequency region. If the opamp will function in the B-class region at the high frequency , then I 'll have a crossover distorsion and switch-off distorsion in that region of frequency, where the negative feedback it is minimal and can not correct this. How I can't modify (in this moment) the input stage of the amplifier for the lesser parasitic capacitance , I'll try to biasing the opamp in the A-class region, or XD-class.