Amplifier Configurations: CE, CB and CC With easy examples.

A transistor amplifier can be wired in three basic configurations: common emitter (CE), common base (CB) and common collector (CC). The choice sets voltage gain, input and output impedance, and whether the output is inverted.

No single configuration is best at everything: CE gives large voltage gain, CC drives heavy loads as a buffer, and CB excels at high frequencies. Picking the right one is the first decision in discrete amplifier design.

Key Concepts

• Common emitter (CE): base input, collector output; high voltage and current gain, medium input impedance, 180° inversion.
• Common base (CB): emitter input, collector output; CE-like voltage gain without inversion, very low input impedance, good at high frequencies.
• Common collector (CC): base input, emitter output; voltage gain ≈ 1, very high input impedance, low output impedance; also called an emitter follower.
• AC emitter resistance: r_e' = 25 mV/I_E, the base-emitter junction's small-signal resistance; it sets CE and CB gain.
• Beta (current gain): I_C/I_B, typically 50–300, linking base current to collector current.

Formulas

• CE voltage gain: A_v = -R_C/r_e' with R_C the collector resistor; the minus sign denotes 180° inversion.
• CB voltage gain: A_v = +R_C/r_e': CE-like magnitude, no inversion.
• CC voltage gain: A_v ≈ 1 (typically 0.95–0.99); the emitter follows the base.
• Input impedance: CE: β·r_e' at the base; CB: ≈ r_e' at the emitter; CC: β·(r_e'+R_E) at the base.
• AC emitter resistance: r_e' = 25 mV/I_E (I_E in mA gives ohms); all small-signal gains scale with 1/I_E.

Worked Example

A CE stage: I_C ≈ 1 mA, β = 100, R_C = 4.7 kΩ. r_e' = 25 mV/1 mA = 25 Ω. A_v = -4700/25 = -188: a 10 mV peak input becomes 1.88 V peak, inverted. Base input impedance: 100 × 25 = 2.5 kΩ. As a CC stage, gain would be ≈ 0.99 with far higher base input impedance — hence CE for voltage gain, CC for buffering.

Common Misconceptions

• The common collector is useless because its gain is 1. Its value is impedance transformation: light load on the previous stage, heavy drive capability, full current gain.
• Higher beta always means a better amplifier. Gain depends on R_C/r_e', not β; β sets input impedance and bias stability, and very high β is noisier.
• CB is just a worse CE. CB's low input impedance suits transmission lines, and no Miller multiplication gives far better high-frequency response — hence RF and cascode use.

Real-World Applications

• Audio preamplifiers: CE stages lift microphone signals to line level.
• Buffers and output drivers: CC followers sit between stages and speakers or cables, preventing loading.
• RF amplifiers: CB and cascode stages amplify radio signals where bandwidth matters.
• Oscillators and current mirrors: the three configurations form op-amp and waveform-generator building blocks.

CE amplifies voltage, CC buffers it, CB carries it to high frequencies. Ask which terminal is common, and any schematic tells you what the stage is for.