Common emitter

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as the voltage amplifier. In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as the 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 FET circuit is the common-source amplifier, and the analogous tube circuit is the common-cathode amplifier. Common-emitter amplifiers give the amplifier an inverted output and can have a very high gain that may vary widely from one transistor to the next. The gain is a strong function of both temperature and bias current, and so the actual gain is somewhat unpredictable. Stability is another problem associated with such high-gain circuits due to any unintentional positive feedback that may be present. Other problems associated with the circuit are the low input dynamic range imposed by the small-signal limit; there is high distortion if this limit is exceeded and the transistor ceases to behave like its small-signal model.One common way of alleviating these issues is with emitter degeneration. This refers to the addition of a small resistor between the emitter and the common signal source (e.g., the ground reference or a power supply rail). This impedance R E {displaystyle R_{ ext{E}}} reduces the overall transconductance G m = g m {displaystyle G_{m}=g_{m}} of the circuit by a factor of g m R E + 1 {displaystyle g_{m}R_{ ext{E}}+1} , which makes the voltage gain where g m R E ≫ 1 {displaystyle g_{m}R_{ ext{E}}gg 1} . The voltage gain depends almost exclusively on the ratio of the resistors R C / R E {displaystyle R_{ ext{C}}/R_{ ext{E}}} rather than the transistor's intrinsic and unpredictable characteristics. The distortion and stability characteristics of the circuit are thus improved at the expense of a reduction in gain. (While this is often described as 'negative feedback', as it reduces gain, raises input impedance, and reduces distortion, it predates the invention of negative feedback and does not reduce output impedance or increase bandwidth, as true negative feedback would do.) At low frequencies and using a simplified hybrid-pi model, the following small-signal characteristics can be derived. If the emitter degeneration resistor is not present, then R E = 0 Ω {displaystyle R_{ ext{E}}=0,Omega } , and the expressions effectively simplify to the ones given by the rightmost column (note that the voltage gain is an ideal value; the actual gain is somewhat unpredictable). As expected, when R E {displaystyle R_{ ext{E}},} is increased, the input impedance is increased and the voltage gain A v {displaystyle A_{ ext{v}},} is reduced.