Capacitors are limited in what they can do. And it is this limitation that makes them so valuable in many applications. A capacitor is a rather simple device, and has nothing to do with flux; Doctor Emmett Brown's time traveling DeLorean notwithstanding. The easiest way to describe its construction is to picture a sandwich rolled up into a tube. The two pieces of bread are conductors, the meat is an insulator. Here's a picture of a film capacitor. These were made by rolling our conductor/insulator/conductor sandwich into a tube and attaching a lead at each end of the roll. There are other types of capacitors, used mainly in power supplies, that are called electrolytic, but they are based on the same principals of the conductor/insulator sandwich, executed with chemicals rather than metal and film. A capacitor only works with AC. Put a capacitor in series with a battery and nothing passes through. But put an AC signal, like that found in music, and it passes right through as if the capacitor were a wire. How does this help designers? There are many cases where DC and AC are present at the same time, like the output of a tube or transistor gain stage. In most designs the DC is a problem for the next stage; the AC is all that we are after. Passing the signal through a capacitor at the output of a gain stage allows the AC to get through, but the DC is locked out. Just what we want! But, of course, life's rarely so simple as all that. Capacitors have limitations. Depending on their size and the load they are working into, only music above a certain frequency will pass through the capacitor. Lower frequencies, like bass notes, are left behind, along with the DC. This can be good if you want to have a rumble filter, bad if you want all the bass present in the music. But fixable depending on how big the capacitor is. This frequency limitation can be put to good use, because if we want to make a filter–to eliminate frequencies above or below a certain point–the mighty capacitor is just what the doctor ordered. Its limitations are its strength. Tomorrow, how capacitors were used before direct coupled circuits.
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