Shown in the schematic below is a typical MOSFET driven by a BJT. I know the purpose of almost everything here, except for R4. I just can't figure out what the resistor on the MOSFET's drain is for Is it some sort of pull-down? And why might it be needed here?
Answer
Given that you have said that the output line goes to an array of LEDs, I think that R4 is there to quickly discharge the LED capacitance when the FET is turned OFF. This would minimize ghosting if the LED array is multiplexed quickly.
I used to run into that problem with some of the large LED arrays we used to build many years ago. Adding a low-value resistor to quickly discharge the charge stored in the LEDs reduced or eliminated the ghosting problem.
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