The common emission amplifier circuit has the effect of voltage and current amplification, the two can not be both, in order to output current io is large, Rc is small, Rc is small uo, so the power amplification ability is general.
The common set amplifier circuit has the current amplification function, but the voltage gain is less than 1 (close to 1), because the input resistance is large, the output resistance is small, and the load capacity is strong, often used in the output stage.
A common-base amplifier circuit that has a voltage gain (the same as a common-beam), but its input resistance is small, so the current amplification capacity is limited, not used as a power amplifier, but the circuit frequency response is best.