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Fig. 12-11.
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N-P junction (forward biased).
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When you hook up the battery backwards (Fig. 12-12) the situation is reversed. The applied charge makes the depletion zone larger and even harder to cross. Pushing electrons into the P side of the diode provides more free electrons in the P side. This increases the negative charge at the P side of the P-N junction, which expands the positive charge on the N side. Some electrons still make it through, but they are ghting the binding force in the silicon s valence so not many make it. This is the leak current you get when a diode is hooked up backwards.
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In 7 we made a distinction between analog and digital signals. Everything we have looked at so far has been analog in nature. This chapter introduces some digital concepts. A digital signal represents the state on and o , 1 and 0. Whenever a voltage is above a de ned value it is on. Whenever it is below a di erent, lower, value it is considered to be o .
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Fig. 12-12. N-P junction (reverse biased).
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Fig. 12-13.
Analog and digital signal.
Traditional digital electronics operate between 0 V and 5 V, though many modern systems work at much lower voltages. In the 5 V case, on may de ned as being a voltage greater than 4 V and o may be any voltage below 1 V (Fig. 12-13). Voltages between these high and low thresholds of 1 V and 4 V are unde ned. Note that this is only an example. Each component speci es its own minimum, maximum, and threshold voltages. The switching behavior as the
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voltage changes from high to low also varies based on the component in question. We ve already looked at one kind of electronic switch, the relay. Relays are what are known as electromechanical devices. They use electrical energy to drive a mechanical mechanism, in this case, a switch. A true electronic switch has no moving parts. One such switch is the transistor. Another is the MOSFET.
TRANSISTOR
A transistor behaves like a variable resistor or, using the water analogy, a water faucet. The basic transistor is the bipolar junction transistor, or BJT. Transistors come in two avors, npn and pnp. Note that transistors are not digital, but are analog switches. As the name suggests, an npn transistor consists of three pieces of material, a thin p-type semiconductor with n-type semiconductors on either side (Fig. 12-14). This is like two diodes placed together back to back (Fig. 12-15). The transistor s schematic symbol even re ects this two-diode shape. Using
Fig. 12-14.
NPN transistor.
Fig. 12-15.
NPN transistor symbol (two diodes).
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Fig. 12-16.
NPN transistor operation.
just the block diagrams that show the depletion zone, let s do a quick tour through the transistor s operation. The sequence is shown in Fig. 12-16. Fig. 12-16A shows the raw N-P-N con guration. Note the depletion zones on both sides of the P section. In transistor terminology, the center of the transistor is the base and is marked with a B. One of the ends is the emitter (E) and the other is the collector (C). If you connect the emitter and collector into a circuit right now, no current would pass through the transistor because the depletion zones insulate these sides from each other. What if you were to apply a small, say half-volt, charge between the base and the emitter (Fig. 12-16B) These two sections, by themselves, are a diode and you would have forward biased it. The insulating layer between the emitter and base is reduced just enough so that it can conduct. Now, what if you connect a larger charge, say 9 V, bewteen the emitter and collector (Fig. 12-16C) The emitter-base diode is not directly a ected, since we aren t letting more than a half volt of charge out of the base. The collector develops a large positive charge as its electrons escape, and there is plenty of negative charge at the emitter from the large battery. There is, however, still that insulating layer between the base and the collector. Doesn t that stop the current Interestingly enough, no. The base layer is thin enough, and the charge di erence between the base and collector is large enough, that most of the electrons zoom through the emitter and jump across the gap into the collector. Current ows from the emitter to the collector as long as the voltage di erential across the emitter-base junction keeps that insulating layer thin. If you remove the base voltage, the emittercollector current stops. If you increase the base voltage, the emitter-base insulator is even thinner and more electrons make it across to the collector. The emitter-collector current increases.
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