Derivative error with respect to bias in binary cross entropy I will do research using NN with 1 hidden layer. To calculate loss using binary cross entropy and for the activation function using sigmoid. I found the derivative formula from Sadowski, 2016 (link:   https://www.ics.uci.edu/~pjsadows/notes.pdf)  as follows:

A weight Wji connecting hidden layer unit j to output unit i has gradient:


The paper does not explain how the derivative of error with respect to bias. So, how is the formula for the derivative of error with respect to bias? Is there any other source that can explain it because I have to include the source. Thank you so much.

For bias, you can take the input as $1$ in these equations, i.e. $x_k=1$. So, that will be
$$\frac{\partial E}{\partial b^1_j}=\sum_i (y_i-t_i)w_{ji}(h_j(1-h_j))$$
where $b^1_j$ is the bias term of neuron $j$ in the hidden layer.
