Articles

Transistor Amplifiers

Solid state (transistor - semiconductor) amplifiers are manufactured in the millions, being assembled by electronic robots in many cases. The result is that you can purchase amplifiers with outputs in the range of hundreds of watts per channel for hundreds of dollars instead of thousands of dollars in the case of tube amplifiers. Of course, you can also spend that $10,000 tube amplifier price mentioned above for a solid state amplifier, but you don't have to unless you really want the absolute top of the line equipment. Superb sound can be purchased with budgets that most of us can handle.

Types of Transistors

Transistors are like tubes in a way. They act as valves for the flow of electrical current, and can be used to amplify by using one of the connections on the transistor to control the flow, like the grid is used on the tube as described above. There are other connections on transistors which are the equivalent to the cathode and anode of tubes. Three popular types of transistors used in amplifiers are called "Bipolar", "J-FET" (Junctional Field-Effect Transistor), and "MOSFET" (Metal Oxide Semiconductor Field-Effect Transistor). FET type transistors perform more like tubes (musical quality) than bipolar transistors, but bipolar transistors are more capable of high current output than FETs, so they both have their specific uses. B,C, and E are Base, Collector, and Emitter, respectively. G, D, and S are Gate, Drain, and Source, respectively. Transistors have several layers sandwiched together. Impurities are added to the silicon which makes it conduct under specific circumstances. This is why they are called "semiconductors". If the impurity, e.g., gallium, causes the layer to be positively charged, the layer is called "P", while impurities that result in the layer being negatively charged, e.g., antimony, make the layer "N". The sandwich defines whether the transistor is PNP or NPN. In the above illustration, P types are shown. In the case of N types, the arrow would be in the opposite direction for schematic diagrams. Darlington circuits are often used, which consists of a transistor pair, with the collectors connected together, and the emitter of one transistor connected to the base of the other. This produces an amplifier stage with current gain that is the product of the individual gains of the two transistors.