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The Workings of a Transistor as an Amplifier in a Common Emitter Configuration

Scientists developed transistors in the early 1950s to replace vacuum tubes. They had the advantage that they were small, didn't need to warm up and didn't leak. One of the first applications was the transistor radio, which had transistor-amplifiers instead of vacuum tubes.

Such circuits consisted of several transistors connected together to amplify the sound signal. One of the basic building blocks of this kind of amplifier is the common emitter configuration, in which the base and emitter connections of the transistor share a common power supply. Even modern electronic devices contain transistors in the common emitter configuration, but they are part of integrated circuits with thousands of transistors in each circuit board.

Basic Characteristics
- A transistor is a semi-conductor device that has three layers. The two outside layers are called the collector and the emitter, while the middle layer is called the base. The transistor can act as an amplifier because the current through the collector/emitter connections is a multiple of the current through the base. A transistor with a small signal flowing into the base generates an output signal many times larger through the emitter, effectively amplifying the signal.
Biasing the Transistor
- Before a transistor can function, the voltages of the three connections must have the correct values and polarities. Transistors have operating characteristics shown as voltage curves. At the high voltage end of the curve, the transistor saturates and no longer amplifies. At the low voltage end, it cuts off and doesn't conduct. Circuit designers choose resistors to connect to each of the three transistor terminals. They calculate the voltage at each terminal for a voltage connected across the circuit, and make sure that the voltages are near the middle of the transistor characteristic curves, away from either end.
Connecting the Signal
- Once the appropriate resistors bias a transistor so that it operates near the middle of its characteristic voltage curves, the transistor can amplify a signal. A small signal voltage connected across the resistor at the base results in a current into the base. An amplified current many times the size of the signal then flows through the collector and emitter and through the resistors connected there. The voltage across the collector resistor is an amplified version of the voltage signal applied to the base.
Isolating the DC Component
- The amplified signal at the collector resistor is the same shape as the original signal voltage applied to the base, but it includes the DC voltage introduced by the biasing resistors. The total voltage consists of the AC signal component, which is the amplified signal, and a steady base DC voltage needed to operate the transistor. To isolate the DC voltage, and get a pure signal corresponding to the original input, circuit designers add a series capacitor to the output. For a DC current, the capacitor is an open circuit, and it acts to completely block the current. For the AC signal, the capacitor acts as a resistance, and it lets the signal pass.

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