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Experiment 12 - The Pull-Up Resistor

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...
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