4.1: Introduction

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The bipolar junction transistor, or BJT, is a foundational electronic component. It serves as the basis for a variety of applications ranging from simple amplifiers to device control to complex digital computing circuitry. Variations exist for applications spanning very low to very high frequency work; low, medium and high power; inexpensive general purpose through highly specialized niche items; and so forth. No matter what a BJT has been optimized for, all BJTs can be considered to be current boosting devices. Of course, if you can boost current, then you can also boost voltage and power, depending on the associated impedances. Further, all BJTs share the same basic structure: three alternating layers of N-type and P-type material with one external lead attached to each layer. In this manner, the BJT can be thought of as an extension of the basic diode: just add another segment of oppositely doped material to one end of the diode creating a second PN junction. The configuration could be either PNP or NPN. There are uses for both types and circuits often work best when the two types are used together.