Questions: Common-Collector Amplifier

Question 1

A common-collector (emitter follower) stage is inserted between a sensor with 50 kΩ source impedance and a speaker with 8 Ω impedance. What is the primary purpose of this stage?

• A. To amplify the voltage from the sensor before it reaches the speaker
• B. To invert the signal phase so the speaker receives the correct polarity
• C. To prevent the low-impedance speaker from loading the high-impedance sensor and causing catastrophic signal loss
• D. To convert the analog signal to a digital signal compatible with the speaker driver

Answer: (C)

Without the emitter follower, connecting a 50 kΩ source to an 8 Ω load creates a massive voltage divider: virtually all signal voltage drops across the 50 kΩ source resistance, and almost none appears across the 8 Ω speaker. The emitter follower solves this by presenting a high input impedance (so it doesn't load the sensor) and a low output impedance (so it can drive the speaker). Voltage gain near unity is irrelevant — the circuit's value is impedance transformation, not voltage amplification. The emitter follower does not invert phase.

Question 2

An emitter follower with β = 150 and an emitter resistor R_E = 200 Ω is driven by a source with 30 kΩ internal resistance. Approximately what output impedance does the stage present to a load at the emitter?

• A. 30 kΩ — the output impedance equals the source resistance
• B. 200 Ω — the output impedance equals R_E
• C. ~200 Ω in parallel with approximately 200 Ω, giving ~100 Ω
• D. Approximately 200 Ω from the emitter resistance appearing in the denominator of the impedance reflection formula

Answer: (C)

The output impedance looking into the emitter is approximately r_e + R_source/β, where r_e is the small-signal emitter resistance (~26mV/I_C) and R_source is the Thevenin source impedance at the base. Here R_source/β = 30,000/150 = 200 Ω, plus a small r_e. This is much less than the 30 kΩ source impedance — the emitter follower has divided it by β. This dramatic reduction in output impedance (from 30 kΩ to ~200 Ω) is what allows the stage to drive low-impedance loads without signal loss.

Question 3

The emitter follower has near-unity voltage gain and no phase inversion between input and output.

• T. True
• F. False

Answer: True

Both statements are correct. The voltage gain A_v = R_E/(R_E + r_e) is slightly less than 1 because the signal at the emitter tracks the base signal minus the small voltage drop across r_e (the intrinsic emitter resistance). There is no phase inversion because the emitter voltage rises and falls in step with the base voltage — the transistor is not inverting the signal as it would in a common-emitter configuration where the output is taken from the collector. The output is offset by a fixed V_BE (~0.7 V), but this DC offset does not affect the AC signal's phase.

Question 4

An emitter follower is a poor design choice when voltage gain matters because its gain is less than 1, and it should be replaced by a common-emitter stage in most applications.

• T. True
• F. False

Answer: False

This is the primary misconception the emitter follower is designed to dispel. The emitter follower is not chosen for voltage gain — it is chosen for impedance transformation. Its near-unity voltage gain is a feature, not a bug: it transfers nearly all the signal voltage to the output while transforming impedances by a factor of β. Current gain remains approximately β, providing substantial power gain. In multi-stage amplifiers, emitter followers are inserted wherever impedance mismatch would degrade performance, protecting each stage from the loading effects of subsequent stages.

Question 5

Explain how the impedance reflection rule produces both very high input impedance and very low output impedance in the emitter follower, using the same underlying mechanism.

Model answer: The impedance reflection rule says that impedances in the emitter circuit, when viewed from the base, appear multiplied by (β + 1) ≈ β. Conversely, impedances in the base circuit, when viewed from the emitter, appear divided by β. For input impedance: looking into the base terminal, R_E (in the emitter circuit) appears as β × R_E — a massive increase. For output impedance: looking into the emitter terminal, the source resistance R_source (in the base circuit) appears as R_source/β — a massive decrease. The same reflection factor β operates in opposite directions depending on which terminal you're looking into.

The underlying physics is the transistor's current amplification: β milliamps of emitter current result from 1 milliamp of base current. This means a small base current change causes a large emitter current change, so a large impedance at the emitter presents a small effective load to the base — and the reverse. Understanding this bidirectional reflection is the key to analyzing any BJT circuit from the base or emitter perspective.