# Bipolar Transistors (npn)

### Overview

The npn bipolar transistor is considered as a transducer driver. It is used to allow low power circuits, such as comparator circuits, to control high power output devices that need a larger current or voltage. The lesson looks at the basic functionality of the npn bipolar transistor and how to calculate the value of a suitable base resistor before considering the transfer characteristics of develop a deeper understanding of what is happening.

### Learning Objectives

• Describe and analyse the operation and use of npn bipolar transistors in switching circuits and when used to control output transducers
• Use the following rule for an npn bipolar transistor circuit: VIN < 0.7 V, the transistor is off, VBE = VIN and VCE = the supply voltage
• Use the following rule for an npn bipolar transistor circuit: VIN ≥ 0.7 V, the transistor is on, VBE = 0.7 V and VCE = 0 V
• Select and apply IC = hFE × IB until saturation is reached
• Use data sheets to design circuits using npn bipolar transistors

### Lesson Content

Reading: Read the first four sections of the webpage - general introduction, overview, transistor basics and types of transistor. Make notes as necessary and make sure you can draw the symbol and name the three connections.

Video (6 min): Using a bipolar transistor as a transducer driver is demonstrated and then the video looks at how to calculate the most suitable value for the base resistor. The conclusion of the video is not completely valid, the lower value resistor should have been used.

Reading: Read the next four sections of the webpage which cover using the bipolar transistor as a transducer driver, the concept of current gain and the calculations required to determine the correct base resistor. Make sure you know how to do the calculations for gain and base resistor. Work carefully through the example given.

Video (7 min): An Interactive White Board demonstration about using a data sheet and then calculating the value of the base resistor for a bipolar transistor used as a transducer driver.

Reading: Read the final four sections of the webpage about the transfer characteristics of an npn bipolar transistor. Relate the transfer characteristics to the applications and calculations described in the previous sections.

Video (11 min): Demonstrating and discussing the transfer characteristics of an npn bipolar transistor. Relate each of the three examples to your understanding of how the transistor 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.

Notes: PDF download. The website in the form of 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 transistor gain, calculating the base resistor and using the bipolar transistor as a transducer driver.

Quiz: Online quiz about transistor gain, calculating the base resistor and using the bipolar transistor as a transducer driver.

### Self Evaluation

I can:

• Recall the names of the connections and the circuit symbol for a bipolar transistor
• Understand the bipolar transistor is an active component
• Connect a bipolar transistor as a transducer driver
• Appreciate that, for a bipolar transistor, a small base current can be used to control a larger collector current
• Appreciate the need for a protection diode in some cases
• Calculate the gain of a bipolar transistor
• Calculate the base resistor required for a bipolar transistor
• Use a data sheet to select an appropriate transistor for a given application
• Appreciate the transfer characteristics of a bipolar transistor and what these mean