What is the relationship between MOSFET gate voltage and source voltage?

Question

I understand that a Mosfet is a voltage controlled current device - but I was a bit surprised that the circuit below didn't yield a higher voltage at the Mosfet source pin (load).

If I were to use two voltage sources with a common ground, and supply a Mosfet with an ideal 32 volt supply and set the gate to 16 volts:

• Why is the voltage at the source-pin so low?
• How might I increase the voltage at the source-pin?

Note: I do not want to use the 32v supply for the gate because the 32v supply will be under heavy load and will encounter voltage swings of 2 - 10v.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

I understand that a Mosfet is a voltage controlled current device - but I was a bit surprised that the circuit below didn't yield a higher voltage at the Mosfet source pin (load).

Correct but the important thing you are not considering is that the driving control voltage is between gate and source. So, if you want a MOSFET to turn on properly, you need a signifant voltage between gate and source AND NOT gate to 0 volts.

You are using source followers and that will always mean that if you want the source to switch to the 32 volt rail, the gate has to be higher than the source by several volts i.e. 36 volts or above.

Alternatively, move your load so that it is in the drain connection of the MOSFET and connect source to 0 volts. Now that's a different story entirely and this should do what you want.