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a single FET won´t do. You need extra parts like resistors, and voltage source.
Maybe you need capacitors, zener diodes, opamps or even more parts.
it depends on your needs:
max voltage
current (if adjustable: min ... max)
precision
frequency range
...
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usually one has a resistor in the source path of the fet. the higher the current the more the voltage drop at the resistor.
Then a opamp regulates gate voltage in a way that the desired curent (voltage drop) is across the resistor.
Ok 1.8V but what's the rest of the context? Is this for an IC or a PCB or what?
Here is an option for a simple topology: broken link removed
The left is a depletion mode mosfet which isn't common, but works pretty well.
The right puts a constant voltage on a "normal" (enhancement mode) mosfet gate, and as the current increases, the voltage on the R increases which makes the voltage of the mosfet source go up. That decreases Vgs which shuts the mosfet off. So it ends up self regulating at a constant current depending on the voltage and the R.
The voltage source could be created with a simple zener diode pulled up to your Vcc. Though neither of the above is a particularly accurate current source (and low voltage zeners are terrible), and would require manual trimming on each circuit to get the current exactly on target. Though in simulation it will work great.
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