MOSFETs (n-channel)

Overview

A MOSFET is a type of transistor. FET stands for Field Effect Transistor. This lesson looks at how to use a MOSFET as a transducer driver and how to use the MOSFET equation to calculate the appropriate voltage and current in a MOSFET circuit. The transfer characteristics of a MOSFET are considered to aid understanding of what the MOSFET is actually doing.

Learning Objectives

• Describe the use of n-channel enhancement mode MOSFETs in switching circuits, including those used to drive output transducers
• Analyse numerically the operation of MOSFETs in switching circuits when used as a transducer driver
• Select and apply the MOSFET equation I_D = g_M × (V_GS − V_th)
• Use data sheets to design switching circuits using MOSFETs

Lesson Content

Reading: Read the first four sections of the webpage - general introduction, overview, MOSFET basics and types of MOSFET. Make sure you can draw the symbol and name the three connections. The general introduction is the same as for the Bipolar Transistor lesson.

Video (7 min): Using a MOSFET as a transducer driver. The video shows how to use a MOSFET as a transducer driver, why a pull down resistor is necessary and demonstrates the threshold voltage.

Reading: Read the next five sections of the webpage which cover using the MOSFET as a transducer driver and the MOSFET equation. Make sure you know how to do the calculations using the MOSFET equation and work carefully through the example given.

Video (6 min): An Interactive White Board demonstration about using a data sheet to design a circuit using a MOSFET as a transducer driver.

Reading: Read the final sections of the webpage about the transfer characteristics of a MOSFET. Relate the transfer characteristics to the applications and calculations described in the previous sections.

Video (8 min): Demonstrating and discussing the transfer characteristics of a MOSFET. Relate each of the three examples to your understanding of how the MOSFET works and is used as a transducer driver.

Exercises: Complete the questions and use the answers to check your understanding.

Lesson Review

Review your learning by working through the presentations or notes which summarise the website content.

Presentation: Powerpoint download. MOSFET basics and use as a transducer driver, examples of the MOSFET equation and questions with answers.

Notes: PDF download. The website page as a PDF document.

Self Assessment

Complete either the questions (pdf download) OR the on-line quiz. They are the same questions.

Questions: PDF download. Questions about MOSFETs as transducer drivers and use of the MOSFET equation.

Quiz: On-line quiz with the same questions as the PDF download.

Self Evaluation

I can:

• Recall the names of the connections and the circuit symbol for an n-channel MOSFET
• Understand that a MOSFET is an active component
• Connect a MOSFET as a transducer driver
• Appreciate that, for a MOSFET, a Gate voltage can be used to control a large Drain current
• Appreciate the need for a protection diode in some cases
• Calculate the transconductance of a MOSFET
• Use the MOSFET equation to calculate the Drain current or Gate-Source voltage for a MOSFET circuit
• Use a data sheet to select an appropriate transistor for a given application
• Appreciate the transfer characteristics of a MOSFET and what these mean

Extra

Alternative YouTube links for the videos

MOSFET Basics on YouTube

Design Exercise on YouTube

Transfer Characteristics on YouTube