AC Electrics - Semiconductors — Page 259, Lesson 275

Let’s pick up with the P-N-P transistor. I want to walk you through how it operates, and then we’ll look at the summary that ties everything together. First, the P-N-P transistor. Its operation is similar in all respects to that of an N-P-N transistor, which we covered earlier. The only difference is that the applied EMFs — the electromotive forces, or voltages — are reversed. So if you understand the N-P-N, you understand the P-N-P; you just swap the polarity of the supply voltages. Now, the summary really brings out the key ability of a transistor. The transistor can control a large Emitter-to-Collector current by means of a small Base-to-Emitter current. That’s the core idea: a small current at the base controls a much larger current flowing from emitter to collector. This means the transistor can act as either a switch or an amplifier. As a switch, you turn the Base-Emitter current on and off. When the small base current is present, the large collector current flows — that’s the switch closed. When you remove the base current, the collector current stops — switch open. As an amplifier, you superimpose a small alternating current signal on the bias voltage. That small AC signal at the base causes a much larger variation in the collector current, giving you amplification. Now, the transistor doesn’t work alone. In conjunction with the junction diode and other electronic components — such as resistors, capacitors, and inductors — the applications for the transistor are almost limitless. That’s a powerful statement: the transistor, combined with these basic building blocks, can be used to build almost any electronic circuit. Furthermore, the ability to control precisely those areas to which doping is applied, using photo-etching techniques, means that all of these components — transistors, diodes, resistors, capacitors, inductors — can be incorporated into a highly sophisticated and complex circuit within a single, small piece of silicon. That’s the integrated circuit, or chip. The ubiquitous computer chip is one such example. Looking to the future, as production techniques improve, faster, more powerful circuits will be contained in ever smaller packages. That leads in turn to more sophisticated technology being incorporated in the modern airliner. So the trend is toward more capability in less space, and that directly affects the avionics and systems you’ll work with as a professional pilot.