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Suppose that the base isn t connected to anything, or is at the same potential as the emitter. This is zero base bias, sometimes simply called zero bias. How much current will flow through the transistor What will the milliammeter (mA) show The answer is that there will be no current. The meter will register zero. Recall the discussion of diode behavior from the previous chapter. No current flows through a P-N junction unless the forward bias is at least equal to the forward breakover
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NPN biasing 403 voltage. (For silicon, this is about 0.6 V.) But here, the forward bias is zero. Therefore, the emitter-base current, often called simply base current and denoted IB, is zero, and the emitter-base junction does not conduct. This prevents any current from flowing between the emitter and collector, unless some signal is injected at the base to change the situation. This signal would have to be of positive polarity and would need to be at least equal to the forward breakover voltage of the junction.
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Now imagine that another battery is connected to the base at the point marked control, so that EB is negative with respect to the emitter. What will happen Will current flow through the transistor The answer is no. The addition of this new battery will cause the emitter-base (E-B) junction to be reverse-biased. It is assumed that this new battery is not of such a high voltage that avalanche breakdown takes place at the junction. A signal might be injected to overcome the reverse-bias battery and the forward breakover voltage of the E-B junction, but such a signal would have to be of a high, positive voltage.
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Now suppose that EB is made positive, starting at small voltages and gradually increasing. If this forward bias is less than the forward breakover voltage, no current will flow. But as the base voltage EB reaches breakover, the E-B junction will start to conduct. The base-collector (B-C) junction will remain reverse-biased as long as EB is less than the supply voltage (in this case 12 V). In practical transistor circuits, it is common for EB to be set at a fraction of the supply voltage. Despite the reverse bias of the B-C junction, the emitter-collector current, called collector current and denoted IC, will flow once the E-B junction conducts. In a real transistor (Fig. 22-3B), the meter reading will jump when the forward breakover voltage of the E-B junction is reached. Then even a small rise in EB, attended by a rise in IB, will cause a big increase in IC. This is shown graphically in Fig. 22-4.
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If EB continues to rise, a point will eventually be reached where IC increases less rapidly. Ultimately, the IC vs. EB curve will level off. The transistor is then saturated or in saturation. It is conducting as much as it possibly can; it s wide open. This property of three-layer semiconductors, in which reverse-biased junctions can sometimes pass current, was first noticed in the late forties by the engineers Bardeen, Brattain, and Shockley at the Bell Laboratories. When they saw how current variations were magnified by a three-layer device of this kind, they knew they were on to something. They envisioned that the effect could be exploited to amplify weak signals, or to use small currents to switch much larger ones. They must have been excited, but they surely had no idea how much their discovery would affect the world.
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404 The bipolar transistor
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22-4 Relative collector current (IC) as a function of base voltage (EB) for a hypothetical silicon transistor.
PNP biasing
For a PNP transistor, the situation is just a mirror image of the case for an NPN device. The diodes are turned around the opposite way, the arrow points inward rather than outward in the transistor symbol, and all the polarities are reversed. The dual-diode PNP model, along with the actual bipolar transistor circuit, are shown in Fig. 22-5. In the discussion above, simply replace every occurrence of the word positive with the word negative. You need not be concerned with what actually goes on inside the semiconductor materials in NPN and PNP transistors. The important thing is the fact that either type of device can serve as a sort of current valve. Small changes in the base voltage, EB, cause small changes in the base current, IB. This induces large fluctuations in the current IC through the transistor. In the following discussion, and in most circuits that appear later in this book, you ll see NPN transistors used almost exclusively. This doesn t mean that NPN is better than PNP; in almost every case, you can replace each NPN transistor with a PNP, reverse the polarity, and get the same results. The motivation is to save space and avoid redundancy.
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