Your finger is conducting positive voltage to the base of the transistor. Even though your skin has a high resistance, the transistor still responds. It isn’t just switching the LED on and off; it is amplifying the current applied to its base. This is an essential concept: a transistor amplifies any changes in current that you apply to its base.
Check Figure 2-88 to see more clearly what’s happening.
If you studied the section “Background: Positive and negative” in Chapter 1, you learned that there is really no such thing as positive voltage. All we really have is negative voltage (created by the pressure of free electrons) and an absence of negative voltage (where there are fewer free electrons). But because the idea of a flow of electricity from positive to negative was so widely believed before the electron was discovered, and because the inner workings of a transistor involve “holes” which are an absence of electrons and can be thought of as positive, we can still pretend that electricity flows from positive to negative. See the following section, “Essentials: All about NPN and PNP transistors,” for more details.
Figure 2-87.
Figure 2-88. These two diagrams show the same components as before, with a fingertip substituted for R2. Although only a trickle of voltage now reaches the base of the transistor, it’s enough to make the transistor respond.
Essentials
All about NPN and PNP transistors
A transistor is a semiconductor, meaning that sometimes it conducts electricity, and sometimes it doesn’t. Its internal resistance varies, depending on the power that you apply to its base.
NPN and PNP transistors are bipolar semiconductors. They contain two slightly different variants of silicon, and conduct using both polarities of carriers—holes and electrons.
The NPN type is a sandwich with P-type silicon in the middle, and the PNP type is a sandwich with N-type silicon in the middle. If you want to know more about this terminology, and the behavior of electrons when they try to cross an NP junction or a PN junction, you’ll have to read a separate source on this subject. It’s too technical for this book. All you need to remember is:
All bipolar transistors have three connections: Collector, Base, and Emitter, abbreviated as C, B, and E on the manufacturer’s data sheet, which will identify the pins for you.
NPN transistors are activated by positive voltage on the base relative to the emitter.
PNP transistors are activated by negative voltage on the base relative to the emitter.
In their passive state, both types block the flow of electricity between the collector and emitter, just like an SPST relay in which the contacts are normally open. (Actually a transistor allows a tiny bit of current known as “leakage.”)
You can think of a bipolar transistor as if it contains a little button inside, as shown in Figures 2-89 and 2-90. When the button is pressed, it allows a large current to flow. To press the button, you inject a much smaller current into the base by applying a small voltage to the base. In an NPN transistor, the control voltage is positive. In a PNP transistor, the control voltage is negative.
Figure 2-89. You can think of a bipolar transistor as if it contains a button that can connect the collector and the emitter. In an NPN transistor, a small positive potential presses the button.
Figure 2-90. In a PNP transistor, a small negative potential has the same effect. The arrows point in the direction of “positive current flow.”
NPN transistor basics
To start the flow of current from collector to emitter, apply a relatively positive voltage to the base.
In the schematic symbol, the arrow points from base to emitter and shows the direction of positive current.
The base must be at least 0.6 volts “more positive” than the emitter, to start the flow.
The collector must be “more positive” than the emitter.
PNP transistor basics
To start the flow of current from emitter to collector, apply a relatively negative voltage to the base.
In the schematic symbol, the arrow points from emitter to base and shows the direction of positive current.
The base must be at least 0.6 volts “more negative” than the emitter, to start the flow.
The emitter must be “more positive” than the collector.
Essentials
All about NPN and PNP transistors (continued)
All-transistor basics
Never apply a power supply directly across a transistor. You can burn it out with too much current.
Protect a transistor with a resistor, in the same way you would protect an LED.
Avoid reversing the connection of a transistor between positive and negative voltages.
Sometimes an NPN transistor is more convenient in a circuit; sometimes a PNP happens to fit more easily. They both function as switches and amplifiers, the only difference being that you apply a relatively positive voltage to the base of an NPN transistor, and a relatively negative voltage to the base of a PNP transistor.
PNP transistors are used relatively seldom, mainly because they were more difficult to manufacture in the early days of semiconductors. People got into the habit of designing circuits around NPN transistors.
Remember that bipolar transistors amplify current, not voltage. A small fluctuation of current through the base enables a large change in current between emitter and collector.
Schematics sometimes show transistors with circles around them, and sometimes don’t. In this book, I’ll use circles to draw attention to them. See Figures 2-91 and 2-92.
Schematics may show the emitter at the top and the collector at the bottom, or vice versa. The base may be on the left, or on the right, depending on what was
