commonemitter

Common-emitter is a transistor amplifier configuration in which the emitter terminal is common to both the input and output circuits. The input is applied to the base, and the output is taken from the collector. The emitter is typically connected to ground or to a fixed bias network, while the base is biased to place the transistor in its active region. This configuration introduces a phase inversion between input and output and can provide substantial voltage gain.

Operation and use: Small changes in base current modulate the collector current, which, through the collector resistor, produce a larger output voltage swing. AC coupling capacitors isolate DC bias from surrounding stages. Emitter degeneration with an emitter resistor provides feedback that stabilizes bias and linearizes the transfer, while bypassing this resistor with a capacitor increases AC gain at the expense of some stability.

Performance characteristics: Voltage gain can be high; approximately −gm Rc for AC with the emitter bypassed, or −Rc / re′ for a simple small-signal model. Input impedance is typically moderate, influenced by the base circuit and any unbypassed Re, while output impedance is roughly Rc. The current gain is about the transistor’s beta, and the stage inherently provides a 180-degree phase shift between input and output.

Practical considerations and variants: Biasing schemes include fixed bias, collector feedback, and emitter self-bias, each with trade-offs for stability and drift. In RF and audio amplification, the common-emitter stage is a common first gain stage and is often combined with impedance matching networks and strategic coupling to other stages.

Compared with other configurations, the common-emitter offers high voltage gain and inversion, making it a foundational element in many amplifier designs.