Output Impedance of a BJT under Common-Collector Configuration

The output impedance of a Bipolar Junction Transistor (BJT) in common-collector (CC) configuration is low. This means that the output impedance value is relatively small compared to other configurations.

Explanation:

In a common-collector configuration, the emitter terminal is common to both the input and output. The input is applied to the base terminal, and the output is taken from the collector terminal.

Characteristics of Common-Collector Configuration:

1. Voltage Gain: The common-collector configuration offers a voltage gain less than unity (less than 1). This configuration is also known as an emitter follower. The output voltage follows the input voltage, but with a slight voltage drop. This voltage gain is less than 1 due to the voltage division between the emitter resistor and the load resistor.

2. Current Gain: The common-collector configuration provides a current gain greater than unity (greater than 1). The base current and collector current are approximately equal, which results in a current gain greater than 1.

3. Input Impedance: The input impedance of the common-collector configuration is relatively high. This is because the base terminal is driven by a low-impedance source, and the emitter terminal is common to the input. The input impedance is primarily determined by the base-emitter junction.

4. Output Impedance: The output impedance of the common-collector configuration is low. This means that the output impedance value is relatively small compared to other configurations. The low output impedance is due to the emitter terminal being common to both the input and output. The output impedance is primarily determined by the collector-emitter junction.

5. Voltage Phase Relationship: The output voltage in the common-collector configuration is in-phase with the input voltage. This means that there is no phase inversion between the input and output signals.

Conclusion:

In conclusion, the output impedance of a BJT under common-collector configuration is low. This is due to the emitter terminal being common to both the input and output, resulting in a relatively small output impedance value compared to other configurations.