All good. Reading what you wrote earlier, it seems there is a misunderstanding in your circuit analysis. A quote from something you've posted earlier:
That's the way it is with these circuits because the transistor provides an increase in loop again, and it could be a lot, and that means the internal compensation of the op amp is no longer effective in keeping the circuit stable. This i could guess is the major problem.
The MOSFET or transistor, has a gain <1. It's the voltage across the source resistor which is being fed back into the op amp. The only thing the buffer does is providing current gain (hopefully a lot) which doesn't change the loop gain at all in this case.
For the voltage across the resistor (R3 in the circuit I posted) we get: A= Vin/Vout= (Gm x Rs)/ (1+ Gm x Rs). So, no matter what Gm is and Rs might be, the gain is very close to 1 but never >1. And that's what the inverting input of the op amp is being fed with. What remains is getting rid of the pole pole introduced by the MOSFET capacitance and it's driving source.
Also, TopQuark does know what his talking about.
Hello again,
Not sure why you are harping on this when i posted a better circuit diagram that you apparently ignored.
There's more to it than that. That appears to be what you and your friend seem to be overlooking.
If it was that simple, then why did it not work for you the first time and why so many posts talking about such a "simple" concept.
Almost 40 posts now. Gee what a simple thing to do, let's create a mosfet current sink and then figure out why it doesnt work over multiple posts (chuckle) and at the same time still say it's simple. BTW i like to add a little levity now and then to keep the discussion a bit lighter in spirit.
It's time to put your simulator where your mouth is (ha ha).
Simulate a circuit you think will work, then post here. It's time for proof, not words alone.
See, i can say a rock and a racoon will form the best current sink (har har). See how easy that was? It's only when we prove it.
It may be that a mosfet circuit will in fact work one way or another, but the way it appeared in the first diagram in this post will not be as stable as a real common source circuit. That's because the transistor adds GAIN. Apparently it is gain that you dont seem to see is there.
That's ok i guess, but you should look at what happens in the drain when the transistor drives the mosfet.
As i said you may get it to work, but then get it to work and stop knocking other designs when you still havent gotten one to work, and also you should go back and compare the circuits and see what one is the MOST stable.
Notice that there has been a lot of rocket testing recently, and many of the projects have failed (not all though). Why is that. It's because the designers believed it would work but it didn't in actual practice.
Where did the Space Shuttle go. It took years to figure out it was not as practical as thought.
Elon Musk is a pretty smart mofo, yet he has had many failures. Testing and proving, that's what it is all about in the real world.
If you want the MOST stable circuit, then you go with the most stable design. That's why i am trying to point out to you.
I am not forcing you to use any particular circuit you are certainly free to use any design you care to. Im just suggesting a better circuit and it comes from some 50 years experience, not only in general electronic analysis and professional design work with companies and universities, but in power supply design, very sophisticated power supply design that involved not only these simple DC supplies but also in pure sine converters that had to be very low in THD and also very high in efficiency (90 percent plus).
What i am trying to do is share those years of work and study and with my huge technical library with you so you can get a good working circuit. It's always, always up to you if you want to use a given design or any given advice but now you know where this is coming from.