This invention relates to an artificial neural network (ANN), particularly to a neuron circuit and its activation function including the derivative. The neuron circuit capable of generating an adjustable sigmoid-like function and a good approximation of its derivative, comprises: a current generator for generating a current; a current-controlled transistor for changing an output voltage according to the current from the current generator; and at least one differential pair of transistors for generating the adjustable sigmoid-like function output and the good approximation of its derivative by the changed output voltage.