In the corresponding circuit diagram you represented the transistor by a switch. Thereby the LOW state corresponds an open switch. The open-collector output is no longer connected to the comparator circuit. It is said, the output is "floating", that is, the voltage at the output is no longer defined...
Open-collector output of a comparator. |
To define the output in the LOW state, you add the resistor R3, which is referred to as a pull-up resistor. This way you connect the open-collector output to the circuit.
Circuit diagram open-collector output with pull-up resistor. |
Circuit open collector output with pull-up resistor. (Enlarge) |
At this point, you have met the most important aspects of the comparator. Now you go on to applications...
