An emitter-follower buffer is generally used when

Question

Examoogle · Accepted Answer

$R_L \ll R_G$

Explanation: An emitter-follower (Common Collector) amplifier is used for impedance matching because it possesses high input impedance and low output impedance. It acts as a buffer to connect a high-impedance source ($R_G$) to a low-impedance load ($R_L$) to maximize power transfer and minimize signal attenuation.

Working Principle: The emitter-follower circuit has a high input resistance, which avoids 'loading' the preceding signal source, preventing significant voltage drop across the internal resistance of the source ($R_G$). By providing a low output resistance, it ensures the circuit can drive a low-impedance load ($R_L$) effectively without excessive voltage loss. Mathematically, it works to isolate the source from the load by ensuring the output voltage follows the input voltage with near-unity gain ($A_v \approx 1$).

Key Formulas: $Z_{in} \approx R_B || \beta(r_e + R_L)$ — Input Impedance of CC amplifier; $Z_{out} \approx R_E || (r_e + \frac{R_S}{\beta})$ — Output Impedance of CC amplifier

Key Points: The emitter-follower provides high input impedance ($Z_{in}$) and low output impedance ($Z_{out}$).; It acts as an ideal buffer preventing source loading when $R_L \ll R_G$.; Voltage gain is always slightly less than unity ($A_v \approx 1$).; It is primarily used for current amplification and impedance transformation.

Advantages: High current gain; Prevents loading effects; Excellent for driving low impedance loads

Disadvantages: Voltage gain is less than one; Provides no power gain boost compared to CE configuration

Applications: Audio output stages; High-impedance sensor signal conditioning; Voltage regulators

Additional Info: When $R_L$ is very small compared to $R_G$, the source 'sees' the load as if it were connected directly, which leads to signal attenuation. The buffer resolves this.. Option A is incorrect because if $R_G$ is already much smaller than $R_L$, the source is already well-matched for voltage transfer.

Answer: C is correct — An emitter-follower acts as an impedance buffer that allows a high-impedance source ($R_G$) to drive a low-impedance load ($R_L$) without signal degradation.

Exam Tip: In competitive exams, remember the mnemonic 'Common Collector = Current Gain, No Voltage Gain, Impedance Transformer'.

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