In electronics, a common-emitter amplifier  ; ; ;is one of three basic single-stage bipolar-junction-transi stor (BJT) amplifier topologies, typically used as a voltage amplifier. In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name. The analogous field-effect transistor circuit is the common-source amplifier, and the analogous tube circuit is the common-cathode amplifier.