stimulus ----> A ----> B and A regulates itself
The steady state of B as a function of A is a signmoid curve.
Consider two cases:
1) A negatively regulates itself and A has a basal level of production. In this scenario, the steady state value of A will remain not-too-low and not-too-high (due to negative regulation). Therefore, in the sigmoid curve of B vs. A, A remains in the somewhat-linear region of the curve.
2) A positively regulates itself and has no basal level of production. In this scenario, the steady state of A is either low of high, so B is low or high (possible amplified).
case (1) is one where the stimulus can linearly control B, and case (2) is one where the stimulus has a threshold above which B is active.