Common Emitter Amplifier
A common emitter amplifier is a type of transistor amplifier circuit that uses a bipolar junction transistor (BJT) in the common emitter configuration. It amplifies the input signal voltage and produces an amplified output voltage.

Unbypassed Emitter Resistance
In a common emitter amplifier, the emitter resistance can be either bypassed or unbypassed. When the emitter resistance is bypassed, a capacitor is connected in parallel with the emitter resistor. This capacitor allows the AC signal to bypass the emitter resistor, effectively shorting it for AC signals. In this case, the emitter resistance does not provide any feedback.

On the other hand, when the emitter resistance is unbypassed, there is no capacitor connected in parallel with it. This means that the emitter resistor is not bypassed and it affects the amplifier's performance. The unbypassed emitter resistance provides current series feedback.

Current Series Feedback
Current series feedback is a type of negative feedback in which the feedback signal is derived from the current flowing through the load resistor. In a common emitter amplifier with unbypassed emitter resistance, the feedback signal is the voltage drop across the emitter resistor (Ve).

When an input signal is applied to the amplifier, the transistor amplifies it and produces an amplified output signal. Some of the output current flows through the load resistor, creating a voltage drop across it. This voltage drop is fed back to the emitter resistor (Ve), which results in a negative feedback signal.

The negative feedback signal reduces the input current and increases the effective emitter resistance. This has the effect of increasing the gain of the amplifier and improving its linearity. The unbypassed emitter resistance provides current series feedback by controlling the current flowing through the load resistor and stabilizing the amplifier's operation.

Conclusion
In a common emitter amplifier, the unbypassed emitter resistance provides current series feedback. This feedback helps to stabilize the amplifier's operation, improve linearity, and increase the gain. By controlling the current flowing through the load resistor, the emitter resistance plays a crucial role in the performance of the amplifier.