Experiment #4 Characteristics of a Silicon Transistor.

Experiment 4 — Characteristics of a Silicon Transistor.

Given that a typical home computer contains around a hundred million transistors (or more!) and ‘ordinary’ things like TV's and radios can contain hundreds it's likely that there are many more transistors on the Earth than people! It's probably a good idea to understand them...

There are all sorts, shapes, and sizes of transistor. In this lab we will only consider one basic type, the bipolar transistor. This comes in two ‘flavours’ called PNP and NPN. For the following experiments you should use the BC184L NPN transistors which are available.

When a theoretician presents a series of lectures about bipolar transistors he or she can usually make them sound very complex! The good news is that in practice you usually only have to know a few of the many properties of a transistor. All the other details only become necessary in that “one time in a hundred” when you build an unusual circuit. The basic properties of a BC184L are:—

Maximum allowed power dissipation, P = 350 mW
Max. allowed collector current, = 100 mA
Max. allowed collector-emitter voltage, = 30 V
Typical current gain, = 250 to 800

In practice, the transistor has many more properties. Worse still, many of them vary from transistor to transistor, and may change with temperature, the applied voltages, etc. Fortunately, we can often ignore these complications!

The BC184L is built into a standard TO-92 package with three leads. The diagram below shows what the package looks like and identifies the leads where B = Base, C = Collector, and E = Emitter.

Connect up the circuit shown in diagram 5 and use it for the following experiment. For this experiment, just put the transistor on the circuit board and use the resistors as part of the leads as shown in the photograph. Once this experiment is complete, you will use the same transistor and board and add new components to make an amplifier.

As with previous experiments there are some photos to show you what your circuit should look like. Click on the image of a camera to see the photos.

Electronic engineers often adopt the convention that upper case letters, like

, are used to signify steady or d.c. values and lower case ones, like

and

, are used to represent small changes or a.c. quantities. This convention will be used for the following explanations.

e.g.

signifies the DC voltage as measured between the base and the emitter of the transistor, whereas

signifies the AC voltage fluctuations between the collector and the emitter.

Note. When you have finished all these measurements keep your transistor on its board. You will need it for the next section!

Use your 'scope to measure

and

. Use the Avometer and DVM (Digital Volt Meter) to measure the currents,

and

Adjust the 1M

pot to set the base current,

, to 2

A. Setting of the 2.5k

potentiometer

to 5 Volts. Make a note of the values of

and

. Use the 1M

pot to increase

in 2

A steps, each time using the 2·5k

pot to set

back to five volts and then noting the new values of

and

. Stop when you either can't make

equal 5 volts or when

mA.

Reduce

back down to 2

A and repeat the process but with

set to 10 volts.

Plot two graphs of your results. One showing how

varies with

for both choices of collector-emitter voltage. The other showing how

varies with

for both collector-emitter voltages.

You should find that the

V and

V curves are fairly similar on each graph.

Most textbooks bang on at tedious length about “h-parameters”. The good news is that you can usually avoid knowing too much about these and still get circuits to work. One parameter is relatively important, this is the transistor's

value. We can define the

from the equation

Where

represents a small change in collector current, and

represents a small change in base current. i.e.

represents the ratio of a change in the base current to the corresponding change in collector current. The bipolar transistor is a current amplifier. If we change the base current by an amount,

the collector current will change by an amount

. Here we can think of the input and output as the magnitudes of alternating signals. Hence

is essentially the AC current gain that the transistor can provide. The transistor provides an output current fluctuation which is

times bigger than the input current fluctuation. The larger the value of

, the more the transistor can amplify a signal.

Use the graphs you have plotted to determine your transistor's

value at

2 mA when

volts.

(Remember that

tells you how quickly

changes with

so you can work out

from the slope of your graph at

2 mA.) Compare this with the value at

2 mA on the

volts curve. You should find that they are fairly similar. Note that the graphs you have plotted aren't straight lines through the origin. Hence the transistor's gain does vary with voltage, etc, although it should only vary gradually at moderate voltages and currents. Note also that the

versus

plots look similar to those you'd get from a diode. This is because the base-emitter part of a bipolar transistor is a diode!

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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.