Since its input impedance is much higher than its output impedance it is also termed a "BUFFER" for this reason it is also used in digital circuits with basic gates...
common emitter
An amplifier amplifies voltage or current. The most basic amplifier consists of a transistor where a signal is fed into the base and is output to the emitter or collector. For simple, low frequency applications (not RF), it may be simpler to use an operational amplifier (op amp) than designing your own amplifier. Also, the data sheets provide very helpful schematics for creating an amplifier. A very common one is the 741; these are also fairly cheap.
You can use an npn or a pnp bjt in a common emitter amplifier circuit. The decision of which one to use is based on whether you want the collector and base to be more positive (npn) or more negative (pnp) than the emitter.
current amplifier
The emitter bypass capacitor in a common emitter amplifier will have less resistance as the frequency increases. Since gain in this configuration is collector resistance divided by emitter resistance (within limits of hFe), the gain will thus increase for higher frequencies, making this into a high pass filter.
Common collector amplifier can be used as a voltage buffer and in impedance matching
the common collector can use as voltage buffer
what is common collector
emitter follwer
current gain
becoz it gives good gain which is necessary for an amplifier.
No voltage gain
Current gain. At the cost of 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.
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.