Current gain. At the cost of no voltage gain.
common emitter amplifier can be where we want phase shift of 180 and as a current amplifier
what is common collector
Common collector amplifier can be used as a voltage buffer and in impedance matching
emitter follwer
current gain
becoz it gives good gain which is necessary for an amplifier.
No voltage gain
it is a mechanism applied in an amplifier so as to raise it's input impedance.Mostly this mechanism is applied in a common collector(emitter follower) amplifier.
the common collector can use as voltage buffer
Common-emitter gives more voltage gain because a common-collector amplifier has a voltage gain of 1. But a common-collector can have a power gain because the input impedance is much more than the output impedance.
no phase shift
The question does not quite make sense. It sounds like you are asking why does changing the emitter resistor in a class C common collector amplifier not affect the output voltage? If so, the answer is that the common collector is an emitter follower, meaning that the emitter will follow the base, less the base-emitter junction voltage, within the limits of hFe. The resistor is simply there to ensure output biasing when the base voltage goes low.
In a common emitter amplifier, the base-emitter current causes a corresponding collector-emitter current, in the ratio of hFe (beta gain) or collector resistance over emitter resistance, which ever is less. Since this ratio is usually greater than one, the differential collector current is greater than the differential base current. This results in amplification of the base signal. As you increase the base-emitter current, the collector-emitter current also increases. This results in the collector being pulled towards the emitter, with the result that the differential collector voltage decreases. This results in inversion of the base signal.