An analog circuit includes a first stage comprising a first stage transistor, wherein the first stage transistor is configured to receive an input signal; and a second stage coupled to the first stage, the second stage comprising a pull-up transistor and a pull-down transistor, the pull-up transistor and the pull-down transistor having the same channel type as the first stage transistor. One of the pull-up transistor and the pull-down transistor receives the input signal and the other of the pull-up transistor and the pull-down transistor receives an output of the first stage transistor. The first stage and the second stage are configured to generate a sigmoid function.