Interview questions & answers
Q1. What is the voltage gain of a common collector amplifier and why is it always less than 1?
The voltage gain of a common collector amplifier is always slightly less than 1 because the output is taken across the emitter resistor, which forms a voltage divider with the transistor's internal emitter resistance re. For a BC547 circuit with RE = 1kΩ, gain typically comes out to 0.98–0.99. This near-unity gain is actually the feature, not a flaw — it means the output faithfully tracks the input without inversion, making it ideal for impedance transformation.
Follow-up: If the gain is less than 1, why would you use this configuration at all?
Q2. Why is the common collector amplifier also called an emitter follower?
It is called an emitter follower because the emitter voltage follows the base input voltage almost exactly, with only the VBE drop (≈0.7V for silicon) offset. When you drive the base of a 2N3904 with a 2V peak sine wave, the emitter output is a 2V peak sine shifted down by 0.7V. The name captures the circuit's behavior precisely — the emitter output tracks, not amplifies, the input signal.
Follow-up: Does the emitter follower introduce any phase shift between input and output?
Q3. What is the input impedance of a common collector amplifier and why is it high?
The input impedance is high, typically in the range of tens to hundreds of kilohms, because the base sees the emitter load multiplied by (β+1). For a transistor with β=100 and RE=1kΩ, the input impedance looking into the base is approximately 101kΩ. This multiplication effect is what makes the emitter follower valuable as a buffer — it loads the previous stage very lightly.
Follow-up: How does β variation across transistor batches affect the input impedance in practice?
Q4. What is the output impedance of a common collector amplifier?
The output impedance is low, typically in the range of tens to a few hundred ohms, because the emitter resistance is divided by (β+1). If the source resistance RS is 10kΩ and β=100, the output impedance is approximately RS/(β+1) ≈ 99Ω in parallel with RE. This low output impedance is why the emitter follower can drive low-impedance loads like speakers or transmission lines without signal loss.
Follow-up: How would you measure the output impedance of an emitter follower on a bench?
Q5. Where is the common collector amplifier used in real circuit design?
The common collector amplifier is used as a buffer stage between a high-impedance source like a sensor or op-amp output and a low-impedance load like a cable driver or speaker. In audio amplifiers such as the LM386 application circuit, a Darlington emitter follower stage drives the 8Ω speaker load. It is also used in level-shifting circuits where you need to shift a signal by exactly one VBE drop.
Follow-up: Can you replace an emitter follower buffer with an op-amp voltage follower? What are the trade-offs?
Q6. Does the common collector amplifier invert the signal?
No, the common collector amplifier is non-inverting — the output at the emitter is in phase with the input at the base. This is because as the base voltage rises, the transistor conducts more, pulling the emitter voltage up through the emitter resistor. Compare this to the common emitter configuration where output is taken at the collector and is inverted.
Follow-up: In a multi-stage amplifier, how does knowing the phase of each stage help in feedback design?
Q7. How does the DC operating point affect the AC performance of an emitter follower?
The DC bias sets the quiescent collector current, which determines the small-signal emitter resistance re = 26mV/IC. If IC is too low, re becomes large and reduces gain and bandwidth; if too high, power dissipation increases. For a design using BC547 biased at IC = 2mA, re = 13Ω, which is negligible compared to RE = 1kΩ, giving near-unity gain across the audio band.
Follow-up: What biasing scheme would you use for the emitter follower if the supply voltage varies between 9V and 12V?
Q8. What happens to the output of an emitter follower if the load resistance is very small?
When the load resistance becomes very small, it effectively reduces the total emitter resistance seen by the transistor, pulling the gain further below unity and increasing the current demand from the transistor. If you connect a 4Ω speaker directly to an emitter follower with a 2N3055 biased at 100mA, the transistor must supply large current and can overheat if not properly heatsinked. The gain drops because the effective RE is now the parallel combination of the emitter resistor and load.
Follow-up: How would you protect the transistor from thermal runaway in a power emitter follower stage?
Q9. What is a Darlington pair and how does it improve the emitter follower?
A Darlington pair connects two transistors so the emitter of the first drives the base of the second, effectively multiplying their current gains (β1 × β2). The TIP120 is a monolithic Darlington with a combined β of around 1000, giving input impedance in the megaohm range. The trade-off is a higher VBE drop of about 1.2–1.4V instead of 0.7V, which must be accounted for in bias design.
Follow-up: What is the cross-over distortion issue in a push-pull emitter follower and how is it corrected?
Q10. How do you calculate the voltage gain of a CC amplifier using small-signal model?
Using the hybrid-π model, voltage gain Av = RE / (re + RE), where re = VT/IC = 26mV/IC at room temperature. For IC = 1mA and RE = 2.2kΩ, re = 26Ω and Av = 2200/(26+2200) ≈ 0.988. This model makes it clear why larger RE and higher IC both push gain closer to unity.
Follow-up: How does the small-signal model change when you add a bypass capacitor across part of RE?
Q11. What is the bandwidth of a common collector amplifier compared to a common emitter?
The common collector amplifier has a much wider bandwidth than the common emitter because the Miller effect is negligible — the collector-base capacitance CBC sees a voltage gain of nearly zero across it. A BC547 emitter follower can easily operate at 100MHz, while the same transistor in common emitter with gain of 100 would have its bandwidth choked by Miller multiplication of CBC. This makes the emitter follower preferred in high-frequency buffer applications.
Follow-up: What is the Miller effect and why does it not apply significantly to the emitter follower?
Q12. Can a common collector amplifier be used for voltage regulation? Explain.
Yes, a simple form of voltage regulation can be achieved by connecting a Zener diode at the base — the emitter will be regulated to VZ − 0.7V. A 5.6V Zener on the base of a 2N2222 gives a regulated 4.9V at the emitter with much lower output impedance than the Zener alone could provide. The transistor acts as a current amplifier, allowing the Zener to operate at a small stable current while the load draws more.
Follow-up: How does a common collector-based linear regulator compare to an IC regulator like the 7805?
Q13. How does temperature affect the performance of a common collector amplifier?
Temperature primarily affects VBE (−2mV/°C for silicon) and the transistor's β, both of which shift the DC operating point. In a BC547 emitter follower, a 50°C rise shifts the output DC level by about 100mV downward due to VBE reduction. A well-designed bias network with a voltage divider makes the operating point relatively temperature-stable by swamping VBE variations.
Follow-up: How would you design a temperature-compensated bias network for a CC amplifier used in an outdoor equipment application?
Q14. What is the significance of the coupling capacitor in a common collector amplifier?
The coupling capacitor blocks DC from passing between stages while allowing AC signals to pass, preventing the bias of one stage from disturbing another. For a CC amplifier operating down to 20Hz audio with a load of 10kΩ, a coupling capacitor of at least 1/(2π×20×10000) ≈ 0.8µF is needed; a 1µF or 10µF electrolytic is typically chosen. The capacitor also forms a high-pass filter, so its value directly sets the low-frequency −3dB point.
Follow-up: What problems can a leaky coupling capacitor cause in a multi-stage amplifier?
Q15. Why is the common collector configuration preferred over common emitter as a final output stage in audio amplifiers?
The common collector configuration is preferred as an output stage because its low output impedance matches well with low-impedance speaker loads, and its near-unity current gain allows large load currents without signal distortion. In a class-AB push-pull amplifier like those built around the BD139/BD140 pair, both transistors are wired as emitter followers to drive an 8Ω speaker. The common emitter stage would have high output impedance and lose most of its voltage across internal resistance into a low-impedance load.
Follow-up: How do you eliminate crossover distortion in a complementary symmetry emitter follower output stage?
Common misconceptions
Misconception: The common collector amplifier amplifies voltage just like the common emitter, but in a different way.
Correct: The common collector amplifier does not amplify voltage at all — its voltage gain is always less than 1; its primary function is current gain and impedance transformation.
Misconception: The output of the emitter follower is taken from the collector terminal.
Correct: In the common collector configuration, the collector is connected to the supply (AC ground) and the output is taken from the emitter terminal.
Misconception: Because the gain is less than 1, the emitter follower is a weak or useless amplifier stage.
Correct: The emitter follower's value lies in its very high input impedance and very low output impedance, making it an ideal buffer between circuit stages, not in voltage amplification.
Misconception: Adding a bypass capacitor across RE in a common collector circuit will increase voltage gain significantly.
Correct: Bypassing RE in a CC amplifier actually removes the emitter degeneration that stabilizes bias and provides the output voltage, collapsing the output signal rather than increasing gain.