Which of the following statements is FALSE for a common-collector (CC) amplifier using a BJT?

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  1. It is commonly used for impedance matching and not for voltage gain.
  2. It has very high input impedance and low output impedance. 
  3. It has the lowest output impedance, the highest current gain and the lowest voltage gain.
  4. It is a current-controlled current source

Answer (Detailed Solution Below)

Option 4 : It is a current-controlled current source
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Detailed Solution

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The FALSE statement for a common-collector (CC) amplifier (emitter follower) using a BJT is: 4) It is a current-controlled current source.
Explanation:

  • A common-collector amplifier is a voltage buffer (voltage gain ≈ 1), not a current source. It has low output impedance, making it unsuitable for acting as a current source (which requires high output impedance).

  • While a BJT itself is a current-controlled device, the CC configuration does not function as a current source; it is designed for impedance matching and signal buffering.

Additional Information

  1. TRUE: CC amplifiers are used for impedance matching (high input impedance, low output impedance) with near-unity voltage gain (not voltage amplification).

  2. TRUE: CC amplifiers exhibit very high input impedance (due to series feedback) and low output impedance (emitter terminal behavior).

  3. TRUE: Among BJT configurations, CC has:

    • Lowest output impedance (emitter-follower effect),

    • Highest current gain (β + 1 ≈ β),

    • Lowest voltage gain (≤1).

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