The AC emitter resistance of an emitter follower

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Examoogle · Accepted Answer

Is usually less than the load resistance

Explanation: The AC emitter resistance $r_e$ in an emitter follower is a small dynamic resistance inherent to the transistor, typically equal to $\frac{26mV}{I_E}$. In practice, the total output resistance of an emitter follower is influenced by the load resistance $R_L$ reflected back to the base, making the overall emitter impedance significantly lower than common load values.

Working Principle: In a Common Collector (emitter follower) configuration, the output is taken from the emitter. Due to the high input impedance and low output impedance nature of the circuit, the effective output resistance is given by $R_{out} \approx r_e + \frac{R_S}{\beta + 1}$. Since $r_e$ is small (often a few ohms to a few tens of ohms) and $R_S/(\beta+1)$ is also small, the output resistance is much lower than typical load resistances ($R_L$), which are usually in the range of $1k\Omega$ to $10k\Omega$.

Key Formulas: $r_e = \frac{V_T}{I_E} \approx \frac{26mV}{I_E}$ — Dynamic emitter resistance; $Z_{out} \approx r_e + \frac{R_S}{\beta + 1}$ — Output resistance of emitter follower

Key Points: Emitter followers act as impedance transformers with high input and low output impedance.; The parameter $r_e$ is the dynamic resistance of the forward-biased emitter-base junction.; The total output resistance is effectively in parallel with the external load $R_L$.; Because $r_e$ is small, the circuit provides excellent voltage buffering.

Advantages: High input impedance; Low output impedance; Voltage gain near unity

Disadvantages: No voltage amplification; Susceptible to thermal variations affecting $I_E$

Applications: Impedance matching; Buffer amplifiers; Current amplifiers

Additional Info: The dynamic resistance $r_e$ is inversely proportional to the DC emitter current $I_E$.. Option C is incorrect because the load is reflected to the base divided by $\beta$, not the emitter resistance being $\beta$ times smaller.

Answer: D is correct — The AC emitter resistance and the overall output resistance of an emitter follower are inherently very small, typically placing them well below standard load resistance values.

Exam Tip: Always remember that the emitter follower (Common Collector) provides current gain but not voltage gain, making it ideal for driving low-impedance loads from high-impedance sources.

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