# biasing of an NMOS

Suppose I have applied a constant current source(of value I) at the source of an NMOS ,the gate is grounded and the drain is connected to a voltage source with resistance R between them.

Equation of MOS in saturation region says that the potential of the source will be such that the current I flows through the MOS but what is happening physically that leads the source to that potential?

• Since the gate is grounded the source will have to be at a -ve potential in order to develop sufficient Vgs (>Vth) . Best is to simulate such circuits and to check your results. Mar 26 '17 at 9:27
• Please use the circuit editor (hit Ctrl-M while editing your question) to draw your circuit so it will be easier to understand what you're asking. In particular, which direction is the the current source driving current? Mar 26 '17 at 15:06

In order for this circuit to work, the current source simply needs to pull the MOSFET's source node below ground enough that $V_d-V_s > V_{th}$. Since it's an ideal current source, it has no problem doing this.