How it works
In a common emitter (CE) amplifier, the input signal is applied to the base through coupling capacitor C1, and the amplified output is taken from the collector through C2. The emitter is common to both input and output — hence the name. Voltage gain AV = −RC/re, where re = 26 mV/IC is the dynamic emitter resistance. For IC = 2 mA, re = 13 Ω; with RC = 3.3 kΩ, AV ≈ −254. The negative sign confirms phase inversion. Emitter bypass capacitor CE (typically 100 μF) short-circuits RE at signal frequencies, preventing the gain reduction caused by negative feedback through RE. Input impedance Zi = R1 ∥ R2 ∥ (β × re), and output impedance Zo ≈ RC.
Key points to remember
Voltage gain with bypassed emitter: AV = −RC/re where re = 26 mV/IC. Without bypass capacitor, AV = −RC/(RE + re) — much lower but more stable. The 180° phase inversion is intrinsic to the CE configuration. Input impedance is moderate (a few kilohms), output impedance ≈ RC. Current gain AI = β, and power gain is the product AV × AI. Bandwidth is limited by internal transistor capacitances at high frequency (Miller effect on CBC) and by coupling and bypass capacitors at low frequency; the product bandwidth × gain is approximately constant (gain-bandwidth product). For a BC547 with fT = 300 MHz, a gain of 100 gives roughly 3 MHz bandwidth.
Exam tip
The examiner always asks you to derive voltage gain of a CE amplifier with and without the emitter bypass capacitor — state AV = −RC/re with CE and AV = −RC/(RE + re) without CE, and explain why gain drops without it.