Experiment 10 - Open-Collector Output

The comparator LM311 has an open collector output. This has several consequences for the properties of the output.

With an open-collector output, the output of the internal comparator is not directly connected to the output of the integrated circuit. Rather, it is connected to the base of an NPN transistor. The emitter of the transistor is connected to ground, while the collector forms the new output of the integrated circuit...

The operation is simple... In aLOW state, the transistor is off. In a HIGH-Zustand, the transistor is on and the open-collector output is connected to ground. In dem HIGH-Zustand kann nun ein Strom durch den Open-Collector-Ausgang fließen. Particularly obvious is the operation when you replace the NPN transistor visually by a switch...

Open-collector output of a comparator.

Open-collector output of a LOW state. The LED remains dark.

Open-collector output of a HIGH state. The LED lights up.

The LOW state has a problem. What this problem is and how you can fix it, you find out in the next experiment...