Is it possible to build MOSFET current source similar to JFET current source like this:


simulate this circuit – Schematic created using CircuitLab

With MOSFET it should be something like this:


simulate this circuit

However, MOSFET current source doesn't work. As far as I know, it should be possible to do this, since JFETS are controlled in the same manner as MOSFETs.

Or am I wrong?:)


1 Answer 1


To let current pass with a regular (enhancement) N-FET, the gate needs to be at a higher voltage than the source. This is impossible in your schematic, because of the voltage drop at R1. The mosfet is always off.

However, this circuit works with a depletion FET. Depletion FETs let current pass when gate is at 0V, and turns off when the gate drops negatively below the source voltage (under a given threshold). This is like an enhancement FET, except that the gate threshold is negative instead of being positive (this makes it some kind of "normally connected" FET).

So, with your circuit and a depletion FET: if the current increases, it increases the voltage drop across R1 and therefore lowers the gate voltage, which will turn the mosfet off a little. If the current decreases, the mosfet turns on a little. This will always find equilibrium and you have your current source.

More info about the usage of depletion FETS here: Typical use of depletion MOSFET

  • 2
    \$\begingroup\$ It might be worth mentioning that depletion FETs are rather rare compared to enhancement FETs. They exist, but they are harder to find. On the other hand, JFETs have to be depletion mode, else the gate would be forward biased. (something kind of like an enhancement mode JFET did exist in the UJT, but there's a reason UJTs aren't used anymore) \$\endgroup\$
    – Hearth
    Commented May 14, 2019 at 14:47

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