Two major functions of MOS tube

Switch function

MOS tubes are usually used as switches in circuits. Let's use proof by contradiction to see how MOS tubes are connected when used as switches.

The switch connection methods of NMOS and PMOS are different.

NMOS tube: D pole connected to input, S pole connected to output (normal conduction)

PMOS tube: S pole connected to input, D pole connected to output (normal conduction)

 Disproof

NMOS tube: The S pole is connected to the input, the D pole is connected to the output , the parasitic diode is directly turned on, the S pole voltage can reach the D pole unconditionally, and the MOS tube loses its switching function;

PMOS tube: The D pole is connected to the input and the S pole is connected to the output , and it also loses its switching function.

 What are the switching conditions of the MOS tube?

In fact, whether it is NMOS or PMOS, the G pole voltage is compared with the S pole.

N channel: turned on when UG>US, and turned off when UG=US.

P channel: turned on when UG<US, and turned off when UG=US.

How many volts larger (or smaller) than US does UG have to be to achieve saturation conduction?

Common NMOS can be simply divided into two categories here: signal switching and voltage on and off .

Signal switching: UG is 3V--5V greater than US, as long as it is turned on, it does not need to be saturated. Common ones include VB162K;

 Voltage on-off: UG should be greater than US by more than 10V, and it must work in saturated conduction state when turned on. Common ones include VBQF1307 and VB7322.

 The opposite is true for PMOS here . When the voltage is on and off, UG is more than 10V smaller than US.

 In general, whether it is NMOS or PMOS, the cathode of the parasitic diode must be connected to the input terminal , and the anode to the output terminal or to the ground , otherwise the switching function cannot be realized.

This is the switch function of the MOS tube. In the next issue, we will talk about the second major function of the MOS tube: isolation .

Some of the above pictures and information are from "Study Help"