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Dumb Question on Current Source

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engrguy42:
I'm also not ruling out a dumb breadboard error like I did the other day...

 |O

Real embarassing....

engrguy42:

--- Quote from: splin on May 26, 2020, 10:31:43 pm ---So, in the non-working case you are saying the gate-source voltage is 7.3V and the drain-source voltage is 10V - as measured directly at the MOSFET pins?

I have a vague recollection that some breadboards have power rails split half way along. Alternatively it may be down to dodgy contacts in the breadboard.

Can you be sure the opamp isn't oscillating? Changing the power supply connections may change parasitic capacitances enough to change the behaviour. Try adding a 100ohm resistor between the opamp output and the MOSFET gate.

Adding decoupling caps close to the opamp won't hurt either.

--- End quote ---

Yeah, measured right at the MOSFET. And I tried both a 100 ohm and a 1k ohm and no improvement. And yeah, it's got that power rails split but I have them all on one side. I even replaced both the MOSFET and the opamp and no improvement. Weird. Maybe dodgy contacts or something..

engrguy42:

--- Quote from: rfbroadband on May 26, 2020, 10:23:57 pm ---measure all DC voltages with a scope and look for instabilities or oscillations...

--- End quote ---

Bingo. There's an absolute crap-ton of switching noise.

engrguy42:
Thanks all. I threw a 1uF capacitor in the mix and it works fine now.  :-+

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