Approximate calculation device for sigmoid function

The invention discloses an approximate calculation device for a sigmoid function. The approximate calculation device comprises a negative number input conversion unit, a constant multiplication unit,a shift unit, a special value generation unit, a carry-save adder and a negative number result conversion unit. The negative number input conversion unit is used for taking an absolute value of an input negative value x and outputting a binary source code of the absolute value; the constant multiplication unit calculates a value of 1.4375 * x and outputs an integral part k and a decimal part p ofan operation result; the shifting unit is used for shifting the input p; the special value generation unit is used for generating an approximate value lambda = sigmoid (kln2); the carry-save adder isused for realizing addition of three numbers; and the negative result conversion unit realizes a conversion function of a corresponding result when the input x is a negative number. According to the device, approximate calculat