A common design uses an incandescent lamp or a thermistor in the feedback circuit. This type of feedback circuit evolved into what is now known as a "phase-locked loop". "Open-loop regulators" do not have a feedback circuit. This, in turn, would increase the cirrus cloud cover, effectively creating a positive feedback circuit. A feedback circuit monitors the output voltage and compares it with a reference voltage. This directs a feedback circuit, activating a heater located on the sample cell. The feedback circuit passes current into the cell to reduce the error signal to zero. The feedback circuit from the cerebellum to the cerebral cortex is also a two-stage system.