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Transistor Configurations: CB, CE and CC Compared Simply

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A transistor has three terminals, so one terminal is made common to both input and output. This gives three configurations: common base (CB), common emitter (CE) and common collector (CC). Each has different voltage gain, current gain and input and output resistance, so the circuit designer chooses the one that fits the job.

Comparing CB, CE and CC helps in reading real circuits quickly. The CE stage is the most common amplifier, the CC stage is used as a buffer, and the CB stage is useful at high frequencies. Knowing their properties avoids trial and error in design.

Key Concepts

  • CB: Input at emitter, output at collector, base common. Current gain alpha = Ic / Ie, slightly less than 1, about 0.98.
  • CE: Input at base, output at collector, emitter common. Current gain beta = Ic / Ib, large, 50 to 300.
  • CC: Input at base, output at emitter, collector common. Also called emitter follower. Voltage gain close to 1.
  • Input resistance: CB is very low (tens of ohm), CE is medium (about 1 kΩ), CC is very high (tens to hundreds of kΩ).
  • Output resistance: CB and CE are high, CC is low.
  • Phase: CE output is 180 degrees out of phase with input; CB and CC outputs are in phase.

How It Works

  1. In CB, the emitter injects current and almost all reaches the collector, so current gain is near unity but voltage gain can be high.
  2. In CE, a small base current controls large collector current, giving both current and voltage gain and the highest power gain.
  3. In CC, the emitter follows the base voltage about 0.7 V below for silicon, so voltage gain is slightly less than one.
  4. CC gives current gain and lowers output impedance, matching a high-impedance source to a low-impedance load.
  5. CB has low input capacitance effect and works well for radio frequency signals.

Worked Example

A transistor has alpha = 0.98. Find beta. Relation beta = alpha / (1 − alpha). So beta = 0.98 / (1 − 0.98) = 0.98 / 0.02 = 49. If Ie = 10 mA in CB, Ic = alpha × Ie = 0.98 × 10 = 9.8 mA. The missing 0.2 mA is the base current.

Common Misconceptions

  • CC has no use because voltage gain is less than 1: It provides current gain and impedance buffering, which is very valuable.
  • CB current gain greater than 1: Alpha is always less than 1 because Ic is slightly less than Ie.
  • CE is best for every purpose: For high-frequency or impedance matching, CB or CC may be better.

Real-World Applications

  • CE stages in audio preamplifiers and general amplifiers.
  • CC emitter follower as output buffer for speakers and sensors through a suitable power stage.
  • CB stages in RF front ends and cascode amplifiers.
  • Combinations of CE and CC in integrated amplifier circuits.

CB gives low input resistance and good high-frequency response, CE gives the largest power gain with phase reversal, and CC gives high input resistance with low output resistance. Choosing correctly makes transistor circuits work as intended.

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