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:—

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.

to92.gif - 13Kb

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.

diag5.gif - 13Kb

camera.gif - 4060 bytesAs 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 or , 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!

excla.gif - 1141 bytes 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 2A. Setting of the 2.5k potentiometer to 5 Volts. Make a note of the values of and . Use the 1M pot to increase in 2A 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.

excla.gif - 1141 bytes Reduce back down to 2A and repeat the process but with set to 10 volts.

excla.gif - 1141 bytes 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.

excla.gif - 1141 bytes 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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