Sigmoid Activation Function: A Key Ingredient in Neural Networks


In the world of neural networks and deep learning, activation functions are the secret sauce that empowers these networks to learn complex patterns and make predictions. One of the most essential and historically significant activation functions is the sigmoid function. In this guest post, we will dive into the world of the sigmoid activation function, exploring its properties, applications, and why it remains relevant even as deep learning advances.


The sigmoid activation function, also known as the logistic sigmoid function, is a widely used mathematical function in the field of artificial neural networks, machine learning, and statistics. It’s characterized by its S-shaped curve and is primarily used to introduce non-linearity into a model. The sigmoid function maps any real-valued number to a value between 0 and 1, making it valuable for problems that involve binary classification and tasks where you need to estimate probabilities.


The sigmoid activation function, with its smooth non-linearity and probability interpretation, remains a vital component in the toolbox of neural network activations. While it may have lost favor in hidden layers of deep networks, it continues to play a pivotal role in binary classification, recurrent networks, and as a building block in understanding neural network training.


The Basics of Sigmoid Activation

The sigmoid activation function, often referred to as the logistic sigmoid, is a mathematical function that maps any real-valued number to a value between 0 and 1. Its formula can be expressed as:







e represents Euler’s number, and

x is the input to the function.

The sigmoid function takes a real number as input and squashes it into the (0, 1) range, making it useful for problems where we need to model probabilities. It exhibits an S-shaped curve, and this characteristic is key to its applications.


Properties of the Sigmoid Function


    • Non-Linearity:

    •  The sigmoid function is non-linear, which means that it can model complex, non-linear relationships between inputs and outputs. This nonlinearity is essential for neural networks to learn and represent a wide range of functions.
  • Output Range: 

    • The sigmoid function’s output is bounded between 0 and 1, making it suitable for binary classification problems. It can be interpreted as the probability of a particular input belonging to one of the two classes.
  • Smooth Gradients: 

  • The function has a smooth gradient across its entire range, which is crucial for efficient optimization during the training of neural networks using techniques like gradient descent.

Applications of Sigmoid Activation

  • Binary Classification: 

    • The sigmoid function is commonly used as the final activation function in binary classification problems. In this context, it outputs the probability that a given input belongs to the positive class. A threshold is typically applied to make the final classification decision.
  • Recurrent Neural Networks (RNNs): 

    • Sigmoid activations are used in recurrent layers of RNNs to control the flow of information through time. The gates in Long Short-Term Memory (LSTM) networks, for example, use sigmoid functions to determine what information to store or discard.
  • Vanishing Gradient Problem: 

  • Although not often used as an activation function in hidden layers of deep neural networks due to the vanishing gradient problem, the sigmoid’s derivatives were crucial in understanding and addressing this issue. This led to the development of more sophisticated activation functions like the rectified linear unit (ReLU).

Limitations and Alternatives


While the sigmoid function has its merits, it’s not without limitations:

  • Vanishing Gradient: 

    • Sigmoid activations are prone to the vanishing gradient problem, making training deep networks challenging.
  • Output Range: 

  • The sigmoid function squashes its inputs into a small range (0, 1), which can cause vanishing gradients and slow convergence during training.

For many hidden layers in modern deep neural networks, alternatives like the ReLU and its variants are favored due to their ability to mitigate the vanishing gradient problem and promote faster convergence.



As deep learning evolves, the sigmoid activation function serves as a reminder of the rich history and continuing relevance of fundamental concepts in the field of artificial intelligence. Understanding the strengths and limitations of the sigmoid function is a crucial step in mastering the art of neural network design.

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