Sigmoid Activation Function in Detail Explained

When it comes to artificial neural networks, the Sigmoid activation function is a real superstar! It might sound like a fancy term, but don’t worry; we’re going to break it down in a way that even your grandma would understand.

sigmoid activation function

What’s the Buzz About Activation Functions?

Before we zoom in on the Sigmoid activation function, let’s understand the bigger picture. Imagine you’re training a neural network to classify between cute kittens and adorable puppies in pictures. Each neuron in your network collects information and decides whether the image it receives is a kitten or a puppy. But how does a neuron make such a decision? Well, that’s where activation functions come into play!

The Sigmoid Function: From Analog to Digital

Our journey starts in the 1940s when scientists and mathematicians were paving the way for modern computing. A brilliant mind named Warren McCulloch came up with the idea of mimicking the behavior of a biological neuron using mathematics. His work, alongside the great Walter Pitts, laid the foundation for artificial neural networks.

Fast forward to the 1950s, and the Sigmoid function enters the scene. The Sigmoid function, often denoted as σ(x), is a smooth, S-shaped curve that maps any real value to a range between 0 and 1. Its smoothness and continuous nature made it ideal for modeling the behavior of biological neurons, and it quickly became a favorite among researchers.

The Sigmoid

The Sigmoid activation function takes an input, let’s call it ‘x,’ and applies the following mathematical magic:

                       σ(x) = 1 / (1 + e^(-x))

Let’s decode this equation step-by-step. The ‘e’ represents Euler’s number (approximately equal to 2.71828), and the ‘x’ is the input to the function. When ‘x’ is a large positive number, e^(-x) becomes very close to zero, making the denominator in the equation almost equal to 1. As a result, σ(x) approaches 1, which means the neuron fires or gets activated!

On the other hand, when ‘x’ is a large negative number, e^(-x) becomes a large positive number, causing the denominator to grow significantly. As a result, σ(x) approaches 0, and the neuron remains dormant or not activated.

The Real-Life Scenario

Imagine you’re training a neural network to predict whether a passenger survived the Titanic disaster based on their age. You have a labeled dataset where each entry contains the passenger’s age and a binary value (0 or 1) indicating whether they survived or not. Your neural network will use the Sigmoid activation function for this binary classification task.

Let’s say the network has learned the optimal weight for the age input. When it receives the age of a new passenger, it multiplies the input (age) by the learned weight and passes it through the Sigmoid activation function. The output, which ranges from 0 to 1, can be interpreted as the probability of survival.

For example, if the Sigmoid output is 0.75, it means the network is 75% certain that the passenger survived, and if the output is 0.20, the network believes there’s only a 20% chance of survival.

Pros and Cons of the Sigmoid Activation Function

Like everything in life, the Sigmoid function comes with its fair share of pros and cons.


1 – Smooth and Differentiable: The smooth nature of the Sigmoid function allows for gentle changes in output with respect to changes in input. This property makes it easy to work with during the training process, where we use optimization algorithms to find the best weights for our neural network.

2 – Output in a Bounded Range: The Sigmoid function guarantees that the output is always between 0 and 1. This property is particularly useful for binary classification problems, where we want a clear-cut decision.

3 – Historical Significance: The Sigmoid function played a crucial role in the early days of neural networks, and its legacy can still be found in certain models and architectures.


1 – Vanishing Gradient Problem: One of the biggest downsides of the Sigmoid function is the vanishing gradient problem. As the output approaches 0 or 1, the gradient of the function becomes extremely small. When training deep neural networks, this can hinder the learning process and lead to slow convergence.

2 – Not Suitable for All Cases: The Sigmoid function might not be the best choice for certain tasks, especially those involving multi-class classification or regression problems.

ReLU and Beyond

As machine learning and deep learning research progressed, researchers started exploring new activation functions that could overcome the vanishing gradient problem and accelerate convergence. One such hero that emerged from the shadows is the Rectified Linear Unit (ReLU) activation function.

ReLU represented as f(x) = max(0, x), is simple yet powerful. It outputs the input value ‘x’ if it’s positive, and zero otherwise. ReLU’s non-linearity and ability to avoid the vanishing gradient problem made it the new favorite among deep learning practitioners.


The Sigmoid activation function has been a significant part of artificial neural networks’ history. It’s known for its smooth S-shaped curve. Although it encountered some difficulties, it opened doors for more advanced activation functions like ReLU. These newer functions are widely used in today’s modern deep-learning systems.

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