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Log-sigmoid transfer function




To use a logistic sigmoid activation for deep learning, use sigmoidLayer or the dlarray method sigmoid.

A = logsig(N) takes a matrix of net input vectors, N and returns the S-by-Q matrix, A, of the elements of N squashed into [0, 1].

Plot of the logistic sigmoid transfer function.

logsig is a transfer function. Transfer functions calculate a layer’s output from its net input.

dA_dN = logsig('dn',N,A,FP) returns the S-by-Q derivative of A with respect to N. If A or FP are not supplied or are set to [], FP reverts to the default parameters, and A is calculated from N.

info = logsig(code) returns information about this function. For more information, see the code argument description.


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This example shows how to calculate and plot the log-sigmoid transfer function of an input matrix.

Create the input matrix, n. Then call the logsig function and plot the results.

n = -5:0.1:5;
a = logsig(n);

Assign this transfer function to layer i of a network.

net.layers{i}.transferFcn = 'logsig';

Input Arguments

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Net input column vectors, specified as an S-by-Q matrix.

Information you want to retrieve from the function, specified as one of the following:

  • 'name' returns the name of this function.

  • 'output' returns the [min max] output range.

  • 'active' returns the [min max] active input range.

  • 'fullderiv' returns 1 or 0, depending on whether dA_dN is S-by-S-by-Q or S-by-Q.

  • 'fpnames' returns the names of the function parameters.

  • 'fpdefaults' returns the default function parameters.

Output Arguments

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Output vectors, returned as an S-by-Q matrix, where each element of N is squashed from the interval [-inf inf] to the interval [0 1] with an "S-shaped" function.


logsig(n) = 1 / (1 + exp(-n))

Version History

Introduced before R2006a

See Also