NMOS & PMOS: Useful Applications

Transistors, specifically MOSFETs, I would say are the second most important component to an EE designer next to the passive compoents (resistors, inductors, capacitors).

I will show you a few useful applications that can be applied using MOSFETs that anyone working in electronics should have in there arsenal.

First lets briefly look at the two variations of MOSFETs.

You have the N-Channel MOSFET below:

image-nmos

Then, you have the P-Channel MOSFET below:

Image - PMOS.png

They have some minor differences in the way they are made and how they are used but in essences they have the same function and operate the same.

Every MOSFET has a Gate-source threshold voltage \Huge (V_\text{GS}).  This is the most important part of the FET because in order to turn on the MOSFET you need to apply a voltage at the gate that exceeds this threshold voltage.  In the datasheet of the MOSFET, you can find this value.

Now, onto the main event:

1. Using them as digitally controlled switches.

One of the most common uses for these FETs is to use them as switches.  When working with electronics, especially with battery operated electronics, you sometimes need to control when certain sensors are turned on or control when certain components get powered on in order to conserve battery power.

Here comes the FETs to the rescue.

You have two ways of accomplishing this task, using either the NMOS or the PMOS.

Note: When using MOSFETs with digital electronics, make sure you get a logic level mosfet.  Meaning that the on voltage for Vgs is between 2V-5V.

Using the NMOS, this is considered LOW SIDE switching because the source pin is connected to ground.

image-nmos-switch

Using the PMOS, this is considered HIGH SIDE switching because the source pin is connected to the device/component instead of ground.

image-pmos-switch

2. Using them as logic level converters[1]

This one mostly applies to the NMOS MOSFET, I haven’t seen a configuration out there with PMOS.

Nowadays with DIY electronics being more and more popular and with a larger community of makers, you have a huge variety of microcontrollers and sensors to make the next big product. With this variant comes a variety of components with different supply voltages, ranging down from 1.8V up to 5V.  The issue with this is trying to match sensor supply voltages with your microcontroller when it comes to communicatong via I2C or SPI or even a single 1 or 0 signal. If you supply a sensor or IC with a higher signal voltage than it can handle you risk damaging the component and rendering it useless.

Here comes the NMOS to the rescue. With an NMOS you can fix this issue by making a 5V to 3.3V logic level shifter to communicate with each other while avoiding any damage.

image-bi-directional-switching

image-i2c-level-switch

3. Making a constant current source.

This is one is my favorite application using MOSFETs.  For me I do a lot of testing of DC DC Converters to make sure it meets my design specifications and one test requires I have a constant current that will not fluctuate.  Using this design I am able to set the desired current for my test.

Another application is used for LEDs.  LEDs are a lot different that traditional light bulbs in that instead of operating off a voltage rating, they operate at a current rating.  Essentially you need to maintain a constant current applied to the LED to achieve your desired brightness.

image-ccs

These are some of the three uses for MOSFETs that I normally deal with for my projects.  There’s plenty more but these were my top 3.

If you have any questions, input, corrections, please let me know in the comments below.

Enjoy building :)!

References:
1. Bi-Directional Level Shifting
2. Sparkfun Learning