Common Collector Configuration:
The third and final transistor configuration is the common collector configuration, shown in the below figure with the proper current directions and voltage notation.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 configurations.
A common collector circuit configuration is provided in the above figure with the load resistor connected from emitter to ground. Note that the collector is tied to the ground even though the transistor is connected in a manner similar to the common emitter configuration. From a design viewpoint, there is no need for a set of common collector characteristics to choose the parameters of the circuit of the figure below.For all practical purposes, the output characteristics of the common collector configuration are the same as for the common emitter configuration.
Common Collector Configuration characteristics:
For the common collector configuration, the output characteristics are a plot of IE versus VEC for a range of values of IB.The input current, therefore, is the same for both the common emitter and common collector characteristics. The horizontal voltage axis for the common collector configuration is obtained by simply changing the sign of the collector-to-emitter voltage of the common emitter characteristics.
Finally, there is an almost unnoticeable change in the vertical scale of IC of the common emitter characteristics if IC is replaced by IE for the common collector characteristics (since alpha =1). For the input circuit of the common collector configuration, the common emitter base characteristics are sufficient for obtaining the required information.