Q. Explain a common emitter configuration without a feedback.

A simple common emitter transistor amplifier--having no negative feedback--is not an ideal amplifier. This is because of the variability of gain from one transistor to another:

making uniform gain, from amplifier to amplifier is impossible. Also, without feedback some amplifiers--having transistors with excessive gain--might be unstable and prone to be oscillate, as well as, poor signal to noise and distortion ratios ;low input impedance (poor impedance matching between stages, etc), and susceptibility to temperature extremes. Without negative feedback, high ambient temperatures can raise the operating point, thus heating the device further; ending with this positive (thermal) feedback, bringing on the transistor's permanent failure.

When (negative) feedback is introduced, most of these problems diminish or disappear, resulting in improved performance and reliability. The amount of feedback is dependent on the relative signal level dropped across this resistor, e.g., if the resistor value approached that of the collector load resistor, the gain would approach unity (Gv ~ 1).