Common-Collector Configuration
In common-collector configuration, the collector terminal of a transistor is connected as a common terminal between input and output as shown in fig. The base and collector terminals are used to apply input signal whereas the output signal is obtained. The CC configuration is also commonly known as the emitter follower or the voltage follower. This is because the output signal across the emitter is almost the replica of the input signal with little loss. In other words, the output voltage follows the input voltage and hence the voltage gain will be a maximum of one.
There is no phase inversion between input and output in the emitter follower circuit. The output voltage is in phase with the input signal voltage.
The input impedance of this circuit tends to be high sometimes greater than 100 kΩ at frequencies less than 100 kHz. But the output impedance is very low because it is limited to the value of the emitter resistor, which can be as low as 100Ω. This situation leads us to one of the primary applications of the emitter follower: impedance transformation. The circuit often is used to connect a high-impedance source to an amplifier with a low-impedance amplifier.
The common-collector configuration is used primarily for impedance-matching purposes since it has a high input impedance and low output impedance, opposite to that of the common-base and common- emitter configuration.
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