Side-by-side comparison
| Parameter | CE | CB |
|---|---|---|
| Output terminal | Collector | Base |
| Voltage gain (A_v) | High (−β × R_C / r_e), typically −50 to −300 | High, ≈ α × R_C / r_e (no phase inversion) |
| Current gain (A_i) | β (≈ 100–300) | ≈ α ≈ 0.98 (< 1) |
| Input impedance | Medium (1–5 kΩ) | Very low (25–50 Ω) |
| Output impedance | High (≈ R_C, typically 4.7–10 kΩ) | Very high (≈ r_o, MΩ range) |
| Phase shift (voltage) | 180° (inverting) | 0° (non-inverting) |
| Bandwidth | Limited by Miller capacitance (C_bc amplified) | Best bandwidth; no Miller effect |
| Power gain | Highest of the three | High voltage gain, low current gain |
| Typical application | Audio amplifiers, general signal amplification | RF amplifiers, cascode stage input |
| Example circuit | BC547 audio preamp with 4.7 kΩ collector load | 2N3904 CB stage in VHF receiver |
Key differences
Common-emitter is the workhorse: a BC547 with a 4.7 kΩ collector resistor gives voltage gain of ~180 but introduces 180° phase inversion and suffers bandwidth roll-off above ~1 MHz due to Miller multiplication of C_bc. Common-base inverts nothing, avoids Miller effect entirely, and is the go-to for RF front-ends up to 100 MHz — but its ~50 Ω input impedance means it only works when driven by a matched 50 Ω source. Common-collector (emitter follower) gives less than unity voltage gain but current gain of β+1 and an output impedance below 100 Ω, making it the right buffer between a high-gain CE stage and a low-impedance load like an 8 Ω speaker.
When to use CE
Use the common-emitter configuration when you need voltage amplification in an audio or low-frequency analog signal chain, and phase inversion is acceptable or correctable. A BC547 CE stage with 4.7 kΩ R_C and 1 kΩ R_E biased at V_CC = 12 V gives A_v ≈ −47, enough to drive a second stage.
When to use CB
Use the common-collector (emitter follower) configuration when the signal source has high output impedance and needs to drive a low-impedance load without loading the previous stage. A BC547 emitter follower with R_E = 1 kΩ can drive an 8 Ω speaker with an output impedance of ~25 Ω, preventing gain collapse.
Recommendation
For general-purpose signal amplification, choose common-emitter — it gives the highest power gain and is the most flexible. Choose common-collector when impedance matching between stages is needed. Use common-base only in RF or wideband circuits above 10 MHz where bandwidth and 50 Ω matching matter.
Exam tip: In university exams and GATE, always be ready to derive the voltage gain, input impedance, and output impedance for each configuration using the small-signal model — mistakes in sign (especially the −β for CE) cost marks consistently.
Interview tip: Interviewers ask why common-base is used in RF circuits despite its low input impedance — the correct answer is elimination of Miller effect and matching to 50 Ω transmission lines, not just "higher bandwidth."