Transistors are the single most important component in electronics. They are at the heart of every digital circuit and enable modern technology. Transistors are active components which can increase the amplitude of a signal and can also be controlled by outside signals. This is in contrast to passive components like resistors, capacitors, and inductors that cannot amplify signals.
The transistor is an essential component for the modern age. In analog electronics, transistors are the building blocks of amplifiers, oscillators, and the venerable operational amplifier. In digital electronics, transistors are the basic component of every computer chip, processor, memory module, and an integrated circuit. This is why a good understanding of transistors is so critical for modern electronics. Integrated circuits are the core of every product, but sometimes the best way to interface circuits is to make a small circuit with discrete individual transistors which requires a good background in their behavior and operation.
A very rigorous transistor model is generally taught in undergraduate electronics courses, but this model is both unintuitive and overly complex when just starting out. Equations and models are essential, but often not the best way to start learning a complex topic or understand how components behave. The reality of how transistors really behave is hidden in equations where variables are assumed to be fixed when in reality manufacturing processes cause a great deal of variation that makes designing by equation alone very dangerous without a good background in how transistors work in real life.
The ability for a transistor to amplify a small signal is called gain and is a property called out on transistor datasheets. The gain is simply the amount of amplification that an input signal sees on the output, assuming the there is enough available power to supply the gain and the transistor can handle the power. Transistors are one of the most common devices for beginners to electronics to destroy by applying too much power or power to the wrong pin. When that happens, the magic blue smoke that runs the component causing the transistor to never operate again. One major thing to note about gain is that it varies greatly due to the manufacturing process and should not be counted on in a design.
A transistor is a three terminal device. The input terminal is called the base which input signals should be applied to, the collector terminal is when current enters the transistor and the emitter pin is where current leaves the transistor. Transistors are driven by current, so applying a small current to the base will result in a much larger current flowing from the collector through the emitter.
The basic transistor model is easiest to understand conceptually but does have some limitations. The basic model does work for many transistor circuits and does function as a great overall model as long as the limitations are known and respected. Going beyond the basic model of transistors is the Ebers-Moll transistor model. The Ebers-Moll equations and the basic model are two of the best ways to approach working with transistors and form a great foundation for good design practices that avoid many of the gotchas of working with transistors. Most common types of transistors are shown below
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By Matthew Burris