5/28/2023 0 Comments Siberian mouse wdupload.comIt's essentially impossible to build a repeatable FET-based current source as you have drawn. MOSFETs have a very wide spread of Vgs, unlike the (relatively) tight spread of practical Vbe of 0.7(ish) volts found in bipolars. You can also use a BJT in place of M1.The current is about 0.6V/R2.It works by Q1 reducing the gate voltage to M1 (or base current if a BJT) as the voltage across R2 approaches 0.6V or so and Q1 starts to turn on.It will drop out around 0.6-0.7V or so if the parts are well chosen, so a 3V LED can be maintained at fully brightness/current to about 3.7V input. It needs to turn on fully with about 2.7V to get the full compliance range. Here is an alternative method that is almost as simple and is more repeatable (and, probably at least as important, has a lot of compliance):– Schematic created usingUse a MOSFET rated to work on 2.5V drive if your logic input is 3.3V.
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