I am new to electronics and have the following doubt.

Say Vgs= 3V to a NMOS produces Id= 3A. Now if we place a 3A current source at the drain,will it cause a 3V at the gate, when the gate is open (assume source is grounded) ? Why? Also what happens when gate is also at ground potential?

Thanks in advance

  • \$\begingroup\$ What is "when the gate is open" supposed to mean? \$\endgroup\$ Sep 27, 2015 at 7:54
  • \$\begingroup\$ Your question would improve substantially when a circuit diagram is added. Click edit and hit Ctrl-M for a circuit editor. \$\endgroup\$
    – jippie
    Sep 27, 2015 at 8:14
  • \$\begingroup\$ If you apply a 3A current source to D-S with gate open and 0V initially the transistor will break down at tens or hundreds of volts and probably be destroyed. \$\endgroup\$ Sep 27, 2015 at 12:39

1 Answer 1


If you put 3V on the gate to source then the drain current will be zero until you apply a drain-source voltage (usually via current limiting device such as a resistor).

It doesn't convert input voltage to output current like a solar panel converts light to voltage. It doesn't convert energy in one form to energy of another (in case you thought it did).

Here's a typical graph: -

enter image description here

When 3V is on the gate (relative to source) and at Vds of zero volts the drain current is also zero but, if you increase the drain voltage the current rises from zero to a value dependent on the graph. Note that each MOSFET will have their own particular graph in their data sheet.


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