What does a common emitter amplifier do?

Hint: A common-emitter amplifier (CEA) is one of three fundamental single-stage bipolar-junction-transistor (BJT) amplifier topologies used as a voltage amplifier in electronics. It has a high current gain (usually about 200), a medium input resistance, and a high output resistance. A common emitter amplifier's output signal is 180 degrees out of phase with the input signal.

Complete answer:
Common-emitter amplifiers have an inverted output and can have a very high gain that varies greatly from transistor to transistor. Because both temperature and bias current have a large influence on gain, the actual gain is rather uncertain. Because of any inadvertent positive feedback that may be there, stability is another issue with such high-gain circuits. The most common form of amplifier is the common emitter amplifier. The common emitter amplifier amplifies tiny or weak signals supplied to the base. It's a transistor with a common emitter for both the input and output.
Rather than voltage amplifiers, these amplifiers are mostly utilised as current amplifiers. Current gain is greater than voltage gain. Because it delivers significant values of current gain, voltage gain, and power gain, we generally utilise the CE design for transistor amplifiers. Furthermore, there is a 180-degree phase change between the input and output. It means that the output signal will be an inverted amplified replica of the input signal.
It's also known as a common-emitter amplifier since the transistor's emitter is shared by both the input and output circuits. The input signal is applied to the transistor's ground and base circuits. The output signal is visible across ground and the transistor's collector. The emitter is common to signals, input, and output since it is linked to the ground.

Note:
The most popular type of junction transistor amplifier is the common-emitter circuit. It has a greater input impedance and a lower output impedance than the common-base connection. Biasing can be done with a single power source. Furthermore, common-emitter (CE) operation often yields larger voltage and power increases. The base and collector circuit currents provide current gain in the common emitter circuit. Because a little change in base current causes a significant change in collector current, the current gain for the common-emitter circuit is always higher than unity, with a typical value of approximately 50.