A valve actuator, valve, and method of actuation of a valve are disclosed. The valve actuator includes a shape memory alloy element attachable to a valve body, capable of use with a valve to cause movement of the valve between first and second positions responsive to at least a threshold amount of temperature change of the shape memory alloy element. The valve actuator further includes a negative-differential spring applied to the shape memory alloy element, thereby reducing the threshold amount of temperature change needed to cause movement of the valve between the first and second positions.