From the figure it can be seen that the sigmoid function becomes flat when its argument is highly positive or highly negative.

San Jose State University

Sigmoid function defines the $\phi$ parameter of Bernoulli distribution. Sigmoid function can depict any real value in the range between 0 and 1.

$\sigma(x) = \frac{1}{1+e^{-x}}$


Logistic Sigmoid

Goodfellow, I., Bengio, Y., & Courville, A. (2016). $\mathit{Deep \ Learning.}$ MIT Press. Retrieved from [www.deeplearningbook.org](https://www.deeplearningbook.org) 

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