The most common application of a MOSFET—switching

Switch function

1. As shown in the figure: when the control pin G of PQ27 is at a low level, PQ27 is cut off; at this time, the control pin G of PQ26 is at a high level, and PQ26 is turned on.

When the control gate G of PQ27 is at a high level, PQ27 turns on; at this time, the control gate G of PQ26 is at a low level, causing PQ26 to turn off.

The MOS switches described above implement Signal Switching (high-to-low level switching).

2. MOS switches achieve voltage switching.

3. Common Connection Methods for MOSFETs in Circuits

NMOS transistor: Drain connected to the input; Source connected to the output.

PMOS transistor: Source terminal connected to the input; Drain terminal connected to the output.

What would happen if we connected it the other way around? See the diagram below:

Due to parasitism Diode It conducts directly, so the source voltage can reach the drain without restriction, and the MOSFET loses its switching function.

4. Switching Conditions of MOSFETs

When the gate voltage is “? V,” the MOSFET turns on (in saturation); when the gate voltage is “? V,” the MOSFET turns off. This issue pertains to the operating principles of MOSFETs, which we will not discuss here—let us simply state the result:

For both N-channel and P-channel MOSFETs, the gate voltage is compared to the source voltage.

N-channel: It turns on when UG > US. (For simplicity, we can assume it turns off when UG = US.)

P-channel: Conducts when UGS is on. (For simplicity, assume) It cuts off when UG = US.

But by how many volts must UG exceed US (or fall below US) for the MOSFET to enter saturation and turn on? The exact threshold depends on the specific MOSFET; different devices require different gate‑source voltage differences.

Common MOSFETs used for signal switching: the gate‑source voltage (UGS) only needs to exceed the source voltage (US) by 3–5 V; in practice, as long as the device is turned on, it does not have to be in deep saturation. For MOSFETs employed for high‑voltage on/off switching, the gate‑source voltage should be greater than 10 V, and during turn‑on the device must operate in the saturation region.

5. The following are several examples: