Using transistors
We’ve seen how transistors work but we don’t yet know how they can be used. After all, there are millions and millions of transistors around in the world today — you’d be forgiven for thinking that there must be hundreds, if not thousands of ways that a transistor may be used.
Well, you’d be forgiven, but you’d still be wrong!
In fact, to prove the point we’re now going to go through every possible way a transistor can be made to operate, and as this chapter isn’t five miles thick you’ve probably realised that there aren’t many ways a transistor may be used at all. Incredibly, at the last count, the number of ways a transistor may be used is — dah, da, dah, dah, dah, da, dah, dah — two!
Hint:
Yes, that’s right, only two basic uses of a transistor exist, and every transistorised circuit, every piece of electronic equipment, every television, every radio, every computer, every digital watch and so on, contains transistors in one form or another which do only one of two things.
We’ve already seen the first of these two uses — an electronic switch, where a tiny base current turns on a comparatively large collector current. This may appear insignificant in itself, but if you consider that the collector current of one transistor may be used as the base current of a following transistor or transistors, then you should be able to imagine an enormous number of transistors inside one appliance, all switching and hence controlling the appliance’s operation.
Incredible? Science Fiction? Well, let me tell you that the appliance described here in extremely simple terms already exists, in millions. We call the appliance a computer. And every computer contains thousands if not millions of transistor electronic switches.
We can see the second use of a transistor in the circuit of Figure 8.12. From this you should be able to see that we’re going to use a variable resistor to provide a variable base current for the transistor in the circuit. The breadboard layout of the circuit is in Figure 8.13. Before you connect the battery, make sure the preset variable resistor is turned fully anticlockwise.
Figure 8.12 A circuit to test a transistor with a varible base current
Figure 8.13 A breadboard layout for the circuit in Figure 8.12. The preset should be turned slowly with a fine screwdriver
Now, connect the battery and slowly (with a small screwdriver) turn the preset clockwise. Gradually, as you turn the preset, the LED should light up: dim at first, then brighter, then fully bright. What you’ve built is a very simple lamp dimmer.
So, the other use of a transistor is as a variable control element. By controlling the transistor’s tiny base current we can control the much larger collector current, which can be the driving current of, say, a LED, an ordinary lamp, a motor, a loudspeaker — in fact just about anything which is variable.
These two operational modes of transistors have been given names. The first, as it switches between two states, one where the collector current is on or high, the other where it is off or low, is called digital. Any circuit which uses transistors operating in digital mode is therefore called a digital circuit.
The other mode, where transistors control, is known as the analogue mode, because the collector current of the transistor is simply an analogue of the base current. Any circuit which uses transistors operating in the analogue mode is known as an analogue circuit.
Take note
Sometimes analogue circuits are mistakenly called linear circuits, However, this is wrong, because although it might appear that a linear law is followed, this is not so. If transistors were properly linear they would follow Ohm’s law, but (like diodes) — remember, they do not follow Ohm’s law and so are non-linear devices. In an analogue circuit, however, transistors are operated over a part of their characteristic curve (remember what a characteristic curve is from Chapters 6 & 7?) which approximates a straight line — hence the mistaken name linear. Of course, you won’t ever make the mistake of calling an analogue circuit linear, will you?
Yes, like diodes, transistors have characteristic curves too, but as they have three terminals they have correspondingly more curves.
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