Common collector amplifier In electronics, a common collector circuit, also known as an emitter follower circuit, refers to one type of circuit arrangement in which a bipolar transistor drives a load circuit such as a resistor or the next stage in an electronic amplifier. In this circuit arrangement, the collector node of the transistor is tied to a power rail or a common node, the emitter node is connected to the output load to be driven, and the base node acts as an input. Due to the physics of the bipolar transistor, the emitter node closely tracks ('follows') the voltage applied to the input node, which is useful in many applications.
The common collector circuit is found to have a voltage gain of unity, meaning AC signals appearing on the input will be nearly identically replicated on the output, assuming the output load is not too difficult to drive. The circuit has a typical current gain in the range of 10X - 1000X, meaning a small change to the input current results in a 10X to 1000X change to the output current supplied to the output load. Thus a weakly driven input node can be used to drive a powerful current on an output node.
Compare the common collector circuit configuration with the common emitter and common base configurations.
Characteristics
Inherent voltage gain:
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{(1 + \beta_0)(R_E || R_{load}) \over r_\pi + (1 + \beta_0) (R_E || R_{load})}
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Input resistance:
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R_1 || R_2 || (r_\pi + (1 + \beta_0) (R_E || R_{load}))
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Current gain:
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A_{vm} {r_{in} \over R_{load}}
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Output resistance:
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R_E || {r_\pi + R_1 || R_2 || R_{source} \over 1 + \beta_0}
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Transistor amplifiers
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