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Common Emitter Amplifier With Emitter Resistance

A common emitter amplifier is a high-gain transistor circuit that amplifies the voltage signal applied to the transistor base, and outputs it to the transistor collector. The base voltage, the emitter and the collector voltage are either all grounded or connected to the same reference voltage The amplifier gets its name from that common connection. A resistance in the emitter branch of the circuit can improve the performance of the amplifier and increase its reliability.

Biasing
- A transistor is an active system component requiring a voltage across the base-emitter junction before it can operate as an amplifier. The common emitter circuit generates this voltage by applying a DC voltage to the collector and using a voltage divider to deliver a reduced voltage to the base. One resistor connects the base to the DC voltage, and the other resistor links the base to the common emitter connection. The DC voltage appears across the collector-emitter, and the reduced voltage across the base-emitter, biasing the transistor so it can amplify a signal fed into the base.
Operation
- The common emitter amplifier has such a high gain that small differences in the transistor can cause changes in the amplification of the signal. Ambient temperature changes in the circuit's environment can also influence the circuit's performance. While a high gain may be desirable, it is more important for a useful amplifier to have a consistent gain for a particular signal. Placing a resistor in the emitter branch of the circuit reduces the gain, but also reduces the circuit's sensitivity to external influences.
Stability
- A common emitter amplifier with an emitter resistance has such a resistor in the emitter branch. This resistor increases the resistance of the base emitter and collector emitter paths of the circuit, and decreases the base current that flows as a result of the input signal to be amplified. The result is a lower gain and reduced sensitivity. External changes and differences in the transistors become less important for the operation of the circuit, and its amplification remains more stable.
Linearity
- A useful amplifier has the same gain over a wide range of input signals. That way, it amplifies weak signals to the same extent as it does strong ones and signals of different frequencies, avoiding distortion. When the gain remains the same, the amplifier is operating in a linear fashion. When the gain is very high, the linear range of the amplifier is very small. Adding an emitter resistance reduces the gain and increases the linear range of the amplifier, increasing its area of application.

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