# What is Activation Function

##### The Role of Activation Functions in Neural Networks

In recent years, neural networks have become a popular approach to solving a wide range of problems, from image and speech recognition to natural language processing and autonomous vehicles. At the heart of neural networks are activation functions, which play a critical role in determining how information flows through the network and how the network learns from training data.

In this article, we’ll take a deep dive into activation functions and their importance in neural networks. We’ll cover what activation functions are, why they’re needed, different types of activation functions, and factors to consider when choosing an activation function for your neural network.

##### What are Activation Functions?

An activation function is a mathematical function that transforms the output of a neuron into its final output. In a neural network, a neuron receives input signals from other neurons or from external sources, calculates a weighted sum of those inputs, and applies an activation function to the sum. The output of the activation function is then passed to other neurons in the network as input, and the process continues until the final output of the network is produced.

The purpose of an activation function is to introduce nonlinearity into the output of a neuron. Without an activation function, the output of a neuron would be a linear function of its input, which would limit the ability of the network to model complex relationships between inputs and outputs. Activation functions allow neurons to produce outputs that are nonlinear, which enables them to model a wide range of relationships between inputs and outputs.

##### Why are Activation Functions Needed?

Activation functions are needed in neural networks for several reasons. First, they enable neurons to introduce nonlinearity into the output of the network. This is essential for modeling complex relationships between inputs and outputs that cannot be captured by a linear function.

Second, activation functions help to normalize the output of a neuron. In a neural network, neurons can receive input signals of different magnitudes and scales. Without an activation function, the output of a neuron could be much larger or much smaller than other neurons in the network, which could lead to unstable learning and training. An activation function that maps inputs to a limited range of outputs can help to ensure that the output of all neurons in the network is roughly normalized.

Finally, activation functions can help to introduce sparsity into the output of a neural network. Sparsity refers to the property of a neural network where only a small number of its neurons become activated for any given input. This can be useful for reducing the computational complexity of the network and improving its ability to generalize to new inputs.

##### Types of Activation Functions

There are several types of activation functions that can be used in neural networks. Here are some of the most common:

• Sigmoid Function: The sigmoid function is a nonlinear function that maps its input to a range of values between 0 and 1. It has a characteristic S-shaped curve, which means that small changes in its input can cause large changes in its output. The sigmoid function is often used in the output layer of a neural network to produce a binary output, such as a yes/no decision.
• ReLU Function: The rectified linear unit (ReLU) function is a nonlinear function that maps its input to a range of values between 0 and infinity. The ReLU function has a simple form and is computationally efficient, which makes it a popular choice for use in deep neural networks. However, the ReLU function can suffer from the problem of “dying ReLU”, where neurons can become inactive and stop producing any output.
• Tanh Function: The hyperbolic tangent (tanh) function is a nonlinear function that maps its input to a range of values between -1 and 1. It has a similar shape to the sigmoid function, but with a range that includes negative values. The tanh function is often used in the hidden layers of a neural network to introduce nonlinearity and normalize the output of its neurons.
• Softmax Function: The softmax function is a nonlinear function that maps its input to a set of values that sum to 1. It is often used in the output layer of a neural network when the network is being used for classification tasks with multiple classes. The softmax function ensures that the output of the network is a probability distribution over the possible classes.

There are other types of activation functions that can be used in neural networks, such as the exponential linear unit (ELU), the scaled exponential linear unit (SELU), and the parametric rectified linear unit (PReLU). The choice of activation function will depend on the specific requirements of the neural network and the problem it is trying to solve.

##### Factors to Consider when Choosing an Activation Function

When choosing an activation function for a neural network, there are several factors that should be considered:

• Nonlinearity: The activation function should be able to introduce the necessary nonlinearity into the output of the network to model complex relationships between inputs and outputs.
• Computational Efficiency: The activation function should be computationally efficient, especially if it is going to be used in a deep neural network with thousands or millions of neurons.
• Stability: The activation function should help to stabilize the learning process and prevent the network from entering unstable states during training.
• Sparsity: The activation function should be able to introduce sparsity into the output of the network, which can help to reduce computational complexity and improve generalization.
• Range: The activation function should map inputs to a range of outputs that is appropriate for the problem being solved.

By considering these factors, it is possible to choose an activation function that will help a neural network to learn effectively and produce accurate results.

##### Conclusion

Activation functions play a critical role in neural networks by introducing nonlinearity, normalizing output, and introducing sparsity. There are several types of activation functions that can be used, each with its own advantages and disadvantages. By choosing the right activation function for a neural network, it is possible to optimize its performance and ensure that it produces accurate results.