When a load is connected between the source and ground of an NMOS transistor, can the control gate still control the on and off states?

Let's take an NMOS as an example.

For an N-channel MOS transistor, the conduction condition is generally that VG is greater than VS voltage.

Suppose a 5V voltage is applied to the gate, the MOS transistor is turned on at this moment.

The reason is that at this time VG=5V, VS=0V, which satisfies the conduction condition VGS is greater than 0.

After the MOS transistor is turned on, VS will become VCC, which is 12V. At this time, the VGS voltage will become 5V, subtracting the original 12V, VGS becomes -7V, and the MOS transistor will be turned off at this moment.

But in some circuits, the load must be placed under the source.

When the base of the transistor is connected to a high level, the transistor conducts, which means the gate of the MOS transistor is connected to 0V, so the MOS transistor is turned off.

However, at the same time, this capacitor will charge to 12V. During this charging process, the source voltage will gradually decrease from 12V to 0V. So when the base of the transistor is placed at a low signal, the transistor is turned off, and at this moment the gate voltage of the MOS transistor is raised by the capacitor.

Therefore, the voltage applied to the gate is 12V from the capacitor plus 12V from VCC, so it is 24V.

Finally, the GS voltage is 24V, subtracting the previous 12V, it is 12V, so the MOS transistor is in the on state.

Diagram of MOS transistor load switch circuit.