Common Emitter Amplifier:
A common emitter amplifier is a type of transistor amplifier configuration where the transistor's emitter terminal is connected to a resistor, forming the emitter resistor. This configuration provides a high voltage gain and can be used for both amplification and signal inversion.

Feedback in Amplifiers:
Feedback in amplifiers is a technique used to control the gain, bandwidth, and stability of the amplifier. It involves feeding a portion of the output signal back to the input of the amplifier. Feedback can be either positive or negative, depending on whether the feedback signal is in-phase or out-of-phase with the input signal.

Unbypassed Emitter Resistance:
In a common emitter amplifier, the emitter resistor is often connected between the emitter terminal and the ground. This resistor is referred to as the unbypassed emitter resistance because it is not bypassed by a capacitor to the ground. The value of this resistor determines the amount of feedback in the amplifier circuit.

Current Series Feedback:
The unbypassed emitter resistance in a common emitter amplifier provides current series feedback. This means that a portion of the output current is fed back to the input of the amplifier in series with the input signal.

Effect of Unbypassed Emitter Resistance:
The unbypassed emitter resistance affects the input and output characteristics of the common emitter amplifier. It introduces negative feedback, which has the following effects:

1. Reduces Gain: The unbypassed emitter resistance reduces the overall voltage gain of the amplifier. This is because the feedback current through the emitter resistor reduces the input signal voltage across the base-emitter junction, resulting in a smaller output voltage.

2. Increases Input Impedance: The presence of the emitter resistor increases the input impedance of the amplifier. This is because the feedback current through the resistor adds to the input current, effectively increasing the input impedance seen by the source.

3. Improves Linearity: The negative feedback provided by the unbypassed emitter resistance improves the linearity of the amplifier. It reduces distortion and helps maintain a linear relationship between the input and output signals.

4. Increases Stability: The feedback provided by the unbypassed emitter resistance improves the stability of the amplifier. It reduces the effect of variations in transistor parameters and external components, making the amplifier less sensitive to changes.

Overall, the unbypassed emitter resistance in a common emitter amplifier provides current series feedback, which helps improve the performance, stability, and linearity of the amplifier circuit.