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CC amplifier Rin, Rout and why it is used

Short answer

Rin = R1||R2||(β+1)(re + RE||RL); Rout = RE || [re + R's/(β+1)] ≈ re + R's/(β+1). The CC configuration is used as a buffer due to its high input resistance, low output resistance, voltage gain near unity, and high current gain.

Determine the input resistance and output resistance of CC BJT amplifier circuit. Why common collector configuration is used in amplifier circuit design.

Input resistance

Common collector (emitter follower) with divider bias, load RL, R'E = RE || RL, T-model:

Rib = vb/ib = (β+1)(re + R'E)
Rin = R1 || R2 || (β+1)(re + RE || RL)

It is high (tens to hundreds of kΩ) since the emitter resistance is multiplied by (β+1).

Output resistance

With the source off, the base sees R's = Rs || R1 || R2; reflected to the emitter divided by (β+1):

Rout = RE || [ re + R's/(β+1) ] ≈ re + R's/(β+1)

It is low (tens of ohms). Example: IE = 1 mA, R's = 5 kΩ, β = 100: Rout ≈ 25 + 50 = 75 Ω.

Why common collector is used

  • Buffer / impedance matching: it has high Rin and low Rout, so it takes almost no current from a high-impedance source and drives a low-impedance load without loss of signal.
  • Voltage gain ≈ 1 (R'E/(re + R'E)), with no phase inversion.
  • High current gain (β+1), which is useful for power output stages.
  • Less distortion and a stable gain, because of 100 % series negative feedback (the emitter resistance), and a wide bandwidth.
  • Used as the last stage of multistage amplifiers, in voltage regulators and in Darlington pairs.

✓ Answer checked by Claude Opus 5.5 (high effort)

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